FOOD HABITS AND DIETARY ADAPTATION OF THE
ENGLISH SOLE (PAROPHRYS VETULUS) IN A
RECENTLY DISTURBED HABITAT
by
DENNIS KEITH MARKS
A THESIS
Presented to the Department of Biology
and the Graduate School of the University of Oregon
in partial fulfillment of the requirements
for the degree of
Master of Science
December 1986
o1MB LI~RARY NOTICE: THiS MA: tPiAl ~.'!:. ; . ,Ll,li~ COPYRlGHl LAW lm~~ 17, U.S. COU4·
APPROVED:
D Peter W. Frankr.
11
iii
An Abstract of the Thesis of
Dennis Keith Marks for the degree of Master of Science
in the Department of Biology to be taken December 1986
Title: DIET OF THE ENGLISH SOLE (PAROPHRYS VETULUS) IN A RECENTLY
DISTURBED HABITAT
Approved?
Dr. Peter W. Frank
Environmental changes affected the abundance of benthic
invertebrates and fish in the vicinity of a newly constructed jetty.
Stomach contents of 108 juvenile English sole were analyzed from the
site over a 3-year period to examine food preference and dietary
adaptation. The observed diet included a diversity of benthic
invertebrates and appeared to be transitional between that of O-age and
adult English sole reported by others. Po1ychaeta annelids were the
largest prey and the most important food each year (about 50% by
volume). Bivalves, primarily clam siphons, were second most important.
Other major prey included harpacticoid copepods and cumaceans.
Excepting amphipods and nematodes, which were largely absent probably
because of their small size, all invertebrate taxa were represented in
the sole stomachs in roughly the same proportions as observed in the
changing sediment samples, demonstrating a low degree of food
selectivity and a corresponding high capacity for dietary adaptation.
VITA
NAME OF AUTHOR: Dennis Keith Marks
PLACE OF BIRTH: Tulsa, Oklahoma
DATE OF BIRTH: September 8, 1955
UNDERGRADUATE AND GRADUATE SCHOOLS ATTENDED:
University of Oregon
University of California, Irvine
DEGREES AWARDED:
Master of Science, 1986, University of Oregon
Bachelor of Science, 1979, University of California, Irvine
AREAS OF SPECIAL INTEREST:
Biology of Aquatic Vertebrates
PROFESSIONAL EXPERIENCE:
Resident Naturalist, Tambopata Wildlife Refuge, Madre de Dios,
Peru, 1984
Research Assistant, Department of Environmental Biology,
University of Colorado, Boulder, 1983-84
Research Assistant, Department of Biology, University of Oregon,
Charleston, 1981-82
Research Assistant, Department of Ecology and Evolutionary
Biology, University of California, Irvine, 1978-79
AWARDS AND HONORS:
Sigma Xi Grant-in-Aid of Research, 1986
iv
ACKNOWLEDGMENTS
For their help with this project, I wish to thank Daniel
Varoujean, Michael Graybill, Jim Marks, Charlotte Coester, Michael
Roberts, and especially Peter Frank.
v
DEDICATION
This paper is dedicated to my father, Clark Marks, because the
thought of having to hear "Are you planning to finish your Masters?"
once every year for the rest of my life finally induced me to finish
it, and to my mother, Margaret Marks, who has, on various occasions,
tolerated fish stomachs and other of nature's sacrifices in her
refrigerator.
vi
,..--e
~;
vii
TABLE OF CONTENTS
c
Chapter
I. INTRODUCTION •••••••••••••••••••••••••••••••••••••••••••••
Page
1
Basic Description of Factors Influencing Diet
and Predation........................................ 1
The Effect of Disturbance on Diet and Predation........ 7
Flatfish Ecology....................................... 8
The English Sole....................................... 11
Description of Present Study........................... 15
II. MATERIALS AND METHODS •••••••••••••••••••.•••••••••••••••• 19
Sampling Methods....................................... 19
Stomach Content Analysis............................... 21
Data Analysis.......................................... 23
III. BACKGROUND DATA: THE UMPQUA TRAINING JETTY STUDy ••••••••• 25
Physical Changes in the Study Area..................... 25
Changes in the Occurrence of Benthic Invertebrates..... 29
Changes in Fish Abundance.............................. 43
IV. RESULTS OF STOMACH CONTENTS ANALySIS ••••••••••••••••••••• 48
English Sole Diet, General............................. 48
Relative Importance of Prey Types...................... 55
V. DISCUSSION••••••••••••••••••••••••••••••••••••••••••••••• 62
Conclusions. . . . . . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Interactions Between the Predator and Prey
Populations: Implications and Suggestions............ 65
Summary. • • • • • • . • • • . . • • • • • . . • • • • . • • • • • • . . • • • • . • . . . • . • . • • 69
LITERATIJRE CITED.................................................. 72
viii
LIST OF TABLES
Table Page
1. Mean Densities/m2 of Major Taxa of Benthic Organisms
Recorded in Samples Taken in Zones B, C, and D in
the Study Area.·.......................................... 30
2. Benthic Organisms: Cumulative Yearly Densities and
Percentage Contribution of Major Taxa..................... 31
3. Fish Species Found in Umpqua River Estuary During the
Study Period 1980-1982.................................... 44
4. Fish, Dungeness Crab, and Bay Shrimp Caught in Otter
Trawls Conducted in Zones B, C, and D in the Study
Areas (S) and Zones II, III, and IV in the Reference
Site (R) by Year (Abundances Expressed as Mean
Number/Trawl)............................................. 45
5. Taxa Identified From Stomachs of English Sole Taken
from the Study Site....................................... 49
6. Summary of Stomach Content Data Collected for English
Sole, Parophrys vetulus................................... 52
ix
LIST OF FIGURES
Figure Page
1. Study Site at the Mouth of the Umpqua River................ 17
l
I
2. Changes in Sediment Grain Size in Study Area Between
1981 and 1984 ..•.•••........•••.•••....•.••.•.•.....•...•
3. Monthly and Annual Mean Densities (in nos./m2) of
Dominant Benthic Invertebrates •••••••••••••••••••••••••••
4. Changes in the Frequency of Polychaete Families,
1980-1982: Changes in the Relative Frequency (%) of
all Families Sampled in the Study Area (Top); Means
of Monthly Densities (Number/m2) of Spionid and
Capitellid Polychaetes in Zones B, C, and D of the
Study Area (Bottom) ..••••..•.•••.••.••••••.••••.••••.••.•
28
34
38
5. Catch/Effort of English Sole in the Study Area............. 40
6. Length/Frequency Data for English Sole Caught in
the Study Area........................................... 47
7. Some of the Dominant Invertebrate Taxa Found in the
Stomachs of ~ vetulus from the Study Area •••••••••••••••
8. Right: Mean Percentage Composition for Major Prey Taxa
Found in P. vetulus from the Study Site, 1980-1982
Annual Means Based on Numerical Abundance ••••••••••••••••
9. Monthly Mean Percentage Composition for Major Prey Taxa
Found in P. vetulus from Study Site, 1980-1982 Based
on Numerical Abundance ••••••••.••••••••.•••••••••••••••••
50
53
56
10. Volume of Prey Consumed (in mm3/fish) by P. vetulus for
Each of the 3 years ••••••••••••••••••••~................ 60
11. Percentage Numerical Abundance for Major Prey Taxa Found
in P. vetulus from the Study Site (Top); Indices of
Relative Importance (IRI) for All Major Prey (Bottom).... 66
1CHAPTER I
INTRODUCTION
Basic Description of Factors
Influencing Diet and
Predation
Foraging and feeding almost certainly account for most of the
active time of an organism's nonresting life. However, since only
finite amounts of time and energy can be devoted to these activities,
efforts must be sufficiently productive to meet the organism's
nutritional needs for maintenance and reproduction. To obtain the
optimal quantity of high-quality food, predators have evolved
behavioral and morphological adaptations to increase the efficiency of
locating, capturing, and eating desirable prey (which, in turn, have
become correspondingly adapted to the avoidance of being located,
captured, and eaten). Efficient predators forage for the prey that
yields the greatest amount of nutrition for the least time and effort.
Predators with efficient strategies for obtaining food have a selective
,reproductive advantage and ultimately become dominant food competitors
(see Emlen, 1984).
The rate of predation on any given prey population has been shown
to be a function of prey abundance and distribution, and although it
has been reported for fishes and other animals under laboratory
2conditions (Ivlev, 1961), direct evidence from field situations is
scanty. Steele, McIntyre, Edwards, and Trevallion (1970) examined the
diets of a population of plaice1 on the west coast of Scotland and
concluded that the level of predation on clam (Tellina) siphons was a
function of the relative abundance of the clam in the benthos.
Similarly, the occurrence of harpacticoid copepods in the diet of
certain Q-age English sole exhibited a positive correlation with the
period of maximum density of copepods in the substrate (Hogue, 1982).
Neither researcher was able to find any distinct annual pattern and
Steele et al. (1970) attributed this lack of seasonality in diet to the
lack of any overwhelming seasonal pattern in the invertebrate
distribution.
Hogue (1982) found the diets of English sole taken from trawls
conducted in his study area to be very similar to each other in the
winter, while sole from different trawls had differing diets in the
summer. The seasonal distribution of prey in the area appeared to
account for the difference--randomization of the meiobenthos through
the mixing of sediments by winter storms was the likely cause for
widely similar diets, while the benthic fauna became increasingly
aggregated in the summer, leading to diet variability between clumps of
fish.
The pattern of dispersion or the degree of aggregation of the prey
can be more important to the predator than the absolute numbers of
1An Atlantic pleuronectid flatfish, Pleuronectes platessa.
3prey. Ivlev (1961) has demonstrated, for fish in an experimental
treatment, that an increase in prey concentration, apart from any other
change, has the same effect on predation as increasing the absolute
number of prey. Although the effect of feeding rate of variously
distributed prey populations is still being examined and certainly
varies for different organisms and environments, studies on sedentary
prey populations suggest a randomly dispersed population with an
overall lower degree of concentration is fed upon less efficiently than
an aggregated, patchy one (Ivlev, 1961).
Along with distributional effects, other morphological
characteristics of prey species can cause them to be more or less
desirable to predators. "There are probably few, if any, animals which
have a completely indifferent attitude toward food types, and
selectivity is probably inherent, to some degree, in most predators"
(Ivlev, 1961). Observed diet, or that subset of the available prey
that the predator has actually consumed, is modified by these food
preferences.
The most obvious factor influencing prey selection is a
proportional size relationship. For many predators, there is an
optimal prey size from which the predator will deviate only when prey
within this size range are scarce (Nikolsky, 1963). Ivlev (1961) has
shown experimentally with fish that there are size-related
preferences--when a fish was presented with various sizes of the same
type of prey, larger items were preferred, and the upper limitation on
prey size appeared to be only a function of mouth size. In diet
4studies of northeastern demersal fish, Levings (1974) and Tyler (1972)
found that selection of various sizes of prey was a function of
predator size; and although the predation on large species was density
dependent (and positively correlated), the consumption of smaller prey
was not. Smaller prey were largely disregarded probably because of
their low nutritional value. Selection of prey that are easier to
locate, capture, and consume will benefit the predator by yielding a
higher caloric intake per unit effort.
Finally, predators exert their own influence on the
characteristics of the prey populations. Steele et a1. (1970)
concluded that a decrease in predation on Tellina siphons by plaice was
due to the decreased abundance of the clams as a function of this same
predation. Iv1ev (1961) has also shown that the selectivity
(e1ectivity) for a given prey changes as a function of predator
density, due to competitive interaction. Fluctuations in the relative
abundance of prey in flatfish diets are usual and have been observed
for other demersal fish.
These factors--the abundance and distribution, along with the
relative size, feeding morphology, and physio10gy--of the prey and the
predator are reflected in the search strategy and prey selection of the
predator. These factors will define the range of prey types that will
be utilized in a particular habitat. While some animals feed on a very
restricted range of prey organisms, others use a high diversity of prey
types. The ability of predators to generalize on a variety of food
types is usually a result of their flexible behavior, accommodating
5feeding structures, and digestive physiology, or to a spectrum of prey
species with somewhat similar morphologies and habits. Although food
specialists have fewer prey from which to select, generalized predators
feeding on a variety of prey must be adapted to cope with a greater
range of predator escape tactics than more specialized predators that
deal with fewer types of prey.
A substantial amount of work has demonstrated the importance of
food availability on the evolution of feeding strategies and
morphologies of predators, and from this have come many ideas that
attempt to explain the range of diet specialization seen in nature.
According to MacArthur and Pianka (1966), a more productive and stable
environment should lead to a more restricted diet in terms of numbers
of species or prey types eaten. Organisms will specialize, given a
productive habitat, because a greater feeding efficiency can be
achieved by becoming especially proficient at utilizing a narrow range
of prey.
On an ecological, rather than an evolutionary time scale, the
degree of predator specialization has also been found to be a function
of the productivity and stability of the environment and the
distribution of prey organisms (Werner &Hall, 1979). Studies on
flatfish (Tyler, 1972) have shown that the diet overlap between
different species of fish in the same area increased as the environment
became increasingly unstable. In an environment with a scant food
supply, a predator cannot afford to pass up many acceptable food items
because the average search time is relatively high and the expectation
6of prey encounter is low. In such an environment, the consumption of a
wide range of prey will probably be the most energy efficient strategy
(Pianka, 1978). In a productive environment, search time is low and
substandard prey items can be bypassed because the expectation of
finding a superior item in the near future is high. More selective
foraging might result, leading to a narrower range of food being
selected. An exception is probably seen in very patchy environments
where predators spend much time searching. In such a habitat a
generalist strategy may be common even in a very productive area
(Pianka, 1978). Zaret and Rand (1971), from a study of tropical
freshwater fishes, observed a decrease in food overlap values between
species when food was less abundant in the dry season. The authors
suggested that when the food resource diminished, the fish adopted a
more specialized feeding behavior, which resulted in the avoidance of
intense competition. The reason for an organism, in a food-diminished
environment, to pass over less optimal food to search for prey of a
higher quality may relate more to highly visual predators in a clear
freshwater habitat than to any general trend. When the abundance of
prey decreases in the dry season, a predator that can identify food
items quickly and at a distance may be less apt to expend critical
energy capturing a low quality food, when it can identify it as such,
than to wait for a more easily captured prey of a higher food value.
Organisms foraging in areas of limited visibility, or searching with
tactile organs, should be much more likely to adopt generalist strategy
because all potential prey must be closely approached for assessment,
7and any acceptable prey to which the predator comes into close
proximity will be eaten.
Predators foraging in overlapping feeding ranges have evolved
uniquely, largely to avoid competing for very similar foods. Intra-
and interspecific competition among co-occurring organisms results from
this overlap and most certainly affects the quality and quantity of
available food. In their observations on the effect of clam (Te11ina)
abundance on the predation by plaice, Steele et a1. (1970) determined
that the plaice predators were themselves responsible for the periodic
reduction in clam abundance. In a natural system, competitive
interaction is extremely important in directing the evolution and
population dynamics of a community. There exists a limit to the rate
at which a fish population can remove food from the available habitat
and the growth rate of fish is limited by this.
The Effect of Disturbance on
Diet and Predation
The effect of an environmental disturbance and subsequent change
in food availability on a predator or group of predators has been
studied to a very limited degree. Apart from a numerical decline due
to a decline in prey abundance, several workers have determined that
the dietary overlap between predators often increases as a result of
perturbations in the habitat and fluctuations in prey abundance. The
work of Tyler (1972) and others suggested that environmental
disturbance often resulted in weakened prey partitioning within a
8predator community and a decrease in the degree of specialization among
predators. In a tank experiment (Ivlev, 1961), the aquarium
environment was subjected to an apparent stress in which the
invertebrate distribution was altered but the composition was left in
the original proportions. The range of prey consumed by the fish
increased significantly. Likewise, over a period of time, and given
environmental stability, food overlap diminishes and resource
partitioning increases. This will occur as two predators reduce the
abundance of their common prey and become better adapted at finding and
eating the unshared items. In an examination of past studies on fish
food partitioning, Tyler (1972) ranked each study by the amount of
environmental disturbance and discovered the same basic trend: As
environmental stability decreases, food partitioning also decreases and
diet overlap increases. As unstable environments have been shown to
decrease the partitioning of food resources, stable environments with
constant conditions have revealed assemblages of organisms with a
minimal amount of overlap in resource utilization.
Flatfish Ecology
The occurrence and distribution of fishes within Oregon estuaries
has not been extensively studied or documented and little research has
been conducted on the feeding habits in such areas. Food habit studies
have recently been completed on flatfishes in the nearshore habitat off
central Oregon, and many aspects of flatfish biology have been
9determined (Forrester, 1969; Allen, 1974; Hulberg &Oliver, 1979;
Wakefield &Pearcy, 1980).
Food has been shown to be a common limiting factor to the growth
of fishes in natural conditions, and the types, densities, and
distribution of food organisms in the benthic habitat of a nursery area
will have profound effects on the survival and growth of juvenile
flatfish. Rae (1956) suggested that differences in the types and
quantities of prey that were available between two sites in the same
vicinity resulted in differential growth rates of the lemon sole
(Microstomus kitt). Sedentary polychaete annelids were determined to
be the most common organisms in the areas of most rapid growth for
M. kitt. In estuaries and other nearshore habitats influenced by
tides, many organisms fluctuate widely in abundance, due largely to
patchy recruitment and sporadic mortalities. Because of the likelihood
of short-term fluctuations in the abundance and availability of
estuarine food items, most of the fishes which spend time here are not
specialized feeders (Moyle, 1982).
While recently it has been determined that flatfish have a variety
of feeding strategies, in the recent past northern demersal fishes were
considered to be trophic generalists (Bigelow &Schroeder, 1953). This
might have been due to the fact that early workers were not concerned
with the relative abundance of prey items and that the bulk of the·diet
may likely have come from only a few dominant species. Some of the
pleuronectids are known to be truly opportunistic feeders, regularly
using a wide range of prey (Tyler, 1972; Pearcy &Hancock, 1978).
10
However, many species of flatfishes have since been found to specialize
on a rather narrow range of food types (Edwards &Steele, 1968;
Kravitz, Pearcy, &Guin, 1977; Cailliet, Antrim, &Ambrose, 1979) such
that assemblages of these predatory species are able to partition the
prey community, each species specializing upon only a portion of the
food spectrum.
Studies of demersal flatfish have turned up a predominance of
benthic feeders, although species using midwater invertebrates and
fishes for food have also been studied (Kravitz et al., 1977;
Richardson &Pearcy, 1977; Steele et al., 1980). Bottom feeding
flatfish have been observed to utilize a variety of foraging strategies
which include excavating small amounts of sediment to unearth prey,
clipping off appendages just above the bottom (e.g., clam siphons,
polychaete feeding appendages), or capturing free-swimming prey
(Kravitz et al., 1977; Hogue, 1982).
Hatanaka, Kosaka, Sato, Yamaki, and Fuyui (1954) and DeGroot
(1971) discovered specific morphological adaptations in the mouth and
jaw structure and digestive morphology of demersal fishes in the
Pacific Northwest which allowed different species of fish to utilize
specific prey. Flatfishes that feed on benthic organisms (e.g., Rex
sole, Rock sole) most often have asymmetrical jaws; gill rakers with
short, blunt teeth; small stomachs; and long intestines. Fish that
feed on more pelagic animals (e.g., Petrale sole, Pacific sanddab) have
long, SYmmetrical jaws with sharp teeth, serrated gills rakers, and
large stomachs. These adaptations enable the fish to feed more
..
11
efficiently upon their prey organisms while providing each species with
a relatively peculiar spectrum of prey organisms.
Pearcy and Hancock (1978) discovered two general feeding types
among four species of syntopic flatfish off central Oregon. The Dover
sole, Microstomus pacificus, and the Rex sole, G1yptocepha1us zachirus,
fed primarily on infauna1 and epifaunal invertebrates, the major
fraction composed of annelids (64%), bivalves, and crustaceans (mainly
gammarid amphipods). Both the Pacific sanddab, Citharichthys sordidus,
and the Slender sole, Lyopsetta exi1us, preyed principally on pelagic
crustaceans such as euphausids and amphipods. Fish were occasionally
an important food for the sanddab. Two other locally important
flatfishes, the Rock sole (Lepidopsetta bi1ineata) and the Petra1e sole
(Eopsetta jordani), were determined to be benthophagous (~ bi1ineata)
and piscivorous (also pelagic invertebrates), respectively.
Partitioning of the food resource among these co-occurring fishes
was obvious from the data with the possible exception of the English
sole, Parophrys vetu1us which, possibly due to the patchy nature of the
bottom areas it occupied, may be a "scavenging generalist" predator
(Kravitz et a1., 1977).
The English Sole
The English sole, Parophrys vetu1us Girard, is a common flatfish
of the family P1euronectidae, found to at least 200 meters in the
eastern and western Pacific coastal waters of the northern hemisphere.
Ranging between Baja California and Alaska, it is one of the most
12
abundant of the flatfishes along the Oregon coast and has been
relatively well studied there, largely owing to its importance in
Oregon's commercial fishing industry (Westrheim, 1954).
Spawning of English sole off the Oregon coastal commences in the
fall and winter (Hewitt, 1980); the gonadal condition of the female
sole and the variability of the spawning season have been observed to
be inversely related to bottom temperature (Kruse &Tyler, 1983). The
concentrating of females in well-defined areas and their subsequent
movement offshore makes them more available to the commercial fishery
and has been correlated with spawning cycles (Hewitt, 1980).
Planktonic eggs hatch in the winter and spring (Rosenberg, 1981) and,
like all pleuronectids, English sole have planktonic larvae that drift
with currents until metamorphosis into juvenile fish. More than 90% of
all larvae collected in an offshore collection off Oregon were netted
between February and July (Richardson &Pearcy, 1977). Young are
pelagic for 6 to 10 weeks (Forrester, 1969) and are continuously
recruited to the bottom over approximately a 9-month period, mostly
from January to June (Pearcy &Krygier, 1980). Reproductive success
varies from year to year and is possibly a function of transport, by
water movement, from the spawning grounds to nursery grounds (Ketchen,
1956). Juvenile English sole, as with the young of many flatfish, are
thought to settle in these well-defined "nursery areas" following
metamorphosis. Estuaries and other shallow, protected inland areas are
known to be important nursery grounds for English sole during the 1st
year of life, although recent collections of Q-age English sole in
13
shallow, unprotected nearshore waters suggest that open-coast nursery
areas may be as important in recruitment to adult populations as are
the estuarine nursery grounds (Laroche &Holton, 1979).
The growth period for English sole is March through September
(- 0.1 mm/day), is most rapid during May and June, and appears to be
inversely correlated with water temperature and directly with the
degree of upwelling (Kreuz, Tyler, Kruse, &Demory, 1981). From
monthly length-frequencies of ~ vetulus caught in Yaquina Bay, Oregon,
Westrheim (1954) observed a modal growth to 14 em during the first year
of life. Subsequent to becoming benthic, 1- and 2-year-old fish are
known to migrate by degrees to deeper waters where they concentrate in
numbers, becoming available to demersal fisheries at 3 to 4 years of
age. Juvenile P. vetulus mature to adulthood in 3 to 8 years and the
natural life span may reach 20 years, as observed for Dover sole.
English sole landed for filleting in Oregon range in size from 250 to
500 mm (Westrheim, 1954).
Food of English Sole
English sole, as do other flatfish, have aSYmmetrical jaws with a
few long teeth which are used to forage in the sediment, primarily for
surface-dwelling and infaunal invertebrates. With a variety of feeding
behaviors, ~ vetulus is able to find and locate many types of prey.
While many, if not most, of the prey organisms are taken from the
surface of the substrate, deep-living species are accessible to the
sole. P. vetulus has been observed to dig into the sediment to extract
14
burrowing species that are not active at the substrate surface (Hulberg
&Oliver, 1979). Hogue and Carey (1982) reported two distinctly
different types of foraging behaviors in ~ vetulus in laboratory
aquaria. The first strategy is a sit-and-wait tactic in which the fish
remain motionless for a period before lunging forward to strike at the
prey on the bottom. The second entailed a series of rapid, thrusting
movements causing a few millimeters of sediment to billow into
suspension, followed by a succession of rapid strikes on the suspended
particles--harpacticoid copepods in this case.
P. vetulus has been consistently reported as a food generalist,
typical individuals displaying a wide variety of food items in the gut.
Kravitz et al. (1977) sampled a population of adult English sole
(230-450 mm SL, ~ = 37) from a nearshore, sandy bottom location off
central Oregon and found polychaetes and amphipods to be the
numerically dominant food. Bivalves, cumaceans, and ophiuroids were
also frequent in the stomachs. Based on the wide variety of prey types
in individual stomachs, Kravitz et al. (1977) proposed an opportunistic
foraging strategy for English sole where the fish appear to eat, in
whatever quantities encountered, most food encountered in foraging. In
a diet survey on a-age English sole (17-87 mm SL, ~ = 235) from a
slightly shallower location, Hogue and Carey (1982) also reported a
very generalized diet for ~ vetulus although the food composition was
significantly different, consisting primarily of small prey: juvenile
bivalves and bivalve siphons, harpacticoid copepods, polychaete palps,
amphipods, and cumaceans. Larger prey such as polychaetes and decapods
15
were taken in lesser quantities as would be expected in such small
fish.
Although the number of prey types found in the stomachs was high,
typically one group was numerically dominant throughout the sample
(e.g., polychaetes for adult fish). As documented for other
pleuronectid species (Edwards &Steele, 1968), Hogue (1982) reported
profound differences for English sole diets for both within and between
year samples as well as between individual tows, in certain months, and
suggested changes in the temporal and spatial density of prey organisms
as the probable reasons for the changes in diet.
Description of Present Study
The Site
The Umpqua River has the largest drainage basin and average
discharge of coastal rivers in Oregon (Hancock, 1985) and terminates in
a shallow bay 180 miles south of the Columbia River. This coastal
estuary is particularly influenced by freshwater effluent in the winter
when it is a two-layered system, and by tidal influx and upwelling in
the summer when the estuary grades from a partly-mixed system in March
to a well-rnixed system in July (Burt &McAllister, 1959).
Net transport of sediment material along the coast is to the
south. At the mouth of the Umpqua River, the seasonal movement of sand
around the north jetty and into the estuary has been observed through
aerial photographs. Heavy winter runoff carries silt and mud sediments
16
through the estuary and out of the river mouth. The mean grain size of
sediments in the bay is that of fine sand (Burt &McAllister, 1959).
The study area is located at the mouth of the Umpqua River, is
adjacent to a large sand beach and dune complex on the south side of
the river, and at present is enclosed on both the ocean and river sides
by stone jetties (Figure 1). Before October 1980, only a fragment of
the northern training jetty existed and the study area was a high
energy beach exposed to large swell, wave action, and both river and
tidal currents.
In October 1980, an existing training jetty was extended to the
beach from the main jetty, enclosing 57 hectares of water and tidal
beach. The jetty extension was constructed with a 200-foot porous
section and four culverts, each 4 feet in diameter, placed at mean
lower low water. Swell and wave action within the impounded study site
are now negligible, and circulation is limited to exchange through the
jetties and culverts. Depth in the study site ranges from 0 to 15 m
with mean depth for the deep water zones (B, C, and D, see Figure 1)
around 7 to 8 meters.
Background of the Study
The Umpqua Training Jetty Extension Monitoring Study (Hancock,
1985) was initiated before implacement of the jetty to assess the
future impact of this jetty configuration on overall water quality,
bottom sediments, and biological communities. The study was developed
by the Portland District Army Corps of Engineers in conjunction with
17
IOCOFT
I
1000 FT 0
I I , I I I I I I II
~ Water Quality Sampling Station.
6. Sedimet Cor. Sampl••
Letters = Benthic Invertebrat. a Inside
Water Quality Sampllnll Slation.
• Outside Water Sampling Slatian.
~ ~
~H
'-l
t.
! •North Jetty~ ==--:: ~~
~ f>.~ V<:) au M P
~
-
'--
...... ApprOJ(lmaf~~
~ HIgh Waf.. LIn.Q.
FIGURE 1. Study site at the mouth of the Umpqua River. The study area
was divided into four zones (A, B, C, and D) of equal area.
Transects (dashed lines) bisecting each zone were
established, and reference marks were painted on the south
jetty (the location of only marks 1-4 are shown). Extension
of the training jetty (slash marked portion) was completed
by October 1980.
Note. From Varoujean, D. H. (1984, April). Umpqua Training Jetty
Extension Monitoring Study (p. 3), (Final Report to Portland District),
Portland: U.S. Army Corps of Engineers.
18
other state and federal resource agencies. The biological monitoring
was contracted to the University of Oregon Institute of Marine Biology,
Charleston.
This study utilizes the data from the monitoring study with
additional work in an effort to correlate changes in the site with
changes in the diet of the English sole.
Purpose of Present Study
Data were analyzed from an area of a well-defined physical
disturbance. An effort was made to quantify the magnitude of the
change on the habitat and upon the benthic organisms used as food by
the dominant demersal fish, the English sole (Parophrys vetu1us).
English sole diets were analyzed over a 3-year period and, in
addition to the goal of adding to the general information on the food
habits of ~ vetu1us, the nature of the disturbance .shou1d allow for an
examination of the dietary adaptability of the sole to a changing
environment and food supply.
19
CHAPTER II
MATERIALS AND METHODS
Methods for the analysis of English sole stomach contents and for
the Umpqua River Training Jetty study (physical-chemical analysis and
benthic invertebrates community survey) are given below.
Sampling Methods
Samples were taken between the months of April and October 1980
through 1982. Collecting dates were scheduled to correspond to the
neap tide series each month in order to maintain consistency and avoid
possible complications caused by extreme tides.
The study site was divided into four zones (Figure 1) for the
sampling of benthic organisms and the measurement of physical
parameters, with a transect running the length of each for the repeated
sampling of demersal fishes. Fish were collected monthly; benthic
samples were taken every other month, beginning in May 1980.
Physical measurements were taken every month between April and
October (except in 1980, when sampling began in February) and included
measurements of water quality (water temperature, salinity, pH,
dissolved 02), arid an analysis of water movement, along with the
composition and dynamics of the sediments within the site.
20
Benthic samples were obtained from 15 stations (with five
replicates each) using diver-held can-type corers (surface area, 125
cm2). Samples were preserved immediately in the field and were washed
over a 0.5 mm mesh screen in the laboratory. Po1ychaetes were
identified to family, all else to more broad groups. With the single
exception of harpacticoid copepods, the 0.5 mm mesh collected all types
of organisms also found in the stomachs of English sole.
Beach seines and otter trawls were used monthly to collect fish.
Gill nets were set during the 1st year only and were used to ascertain
the effectiveness of the other two fishing methods.
Parophrys vetu1us were collected from the site with a 3-meter
otter trawl (with a 7-mm stretch mesh liner), towed at about 2 knots
for approximately 15 minutes, along each transect (once in each zone--
B, C, and D). Tows were done between 0830 and 1600 hr. which was
appropriate for obtaining recently feeding fishes, based on their
diurnal feeding habit (Hogue &Carey, 1982). After each set was
completed, the fish to be kept for stomach content analysis were
immediately placed in a 10% formalin solution buffered with sodium
borate (NaH2B03). The fish remained in formalin for about 48 hours,
were rinsed for an equal amount of time, then measured (total and
standard length) and weighed. The entire digestive tract was removed
from each fish and placed in a 70% isopropyl alcohol solution. From a
subsamp1e of stomachs taken from fish between 65 mm and 180 mm total
length, stomachs were randomly selected for analysis (except in very
small samples where all were analyzed). This size range corresponds to
21
1st- and 2nd-year fish (Rosenberg, 1981) and were segregated in an
attempt to minimize the variation in diet due to ontogenetic shifts in
prey selection (Hogue, 1982).
A reference site was selected nearby to act as a control for the
study site. The site was chosen for its similarity (in wave action,
water circulation, depth, and sediments) to the study site before it
was impounded. Fish were collected each month with otter trawl and
beach seine during 1981 and 1982 only. The benthos was sampled using
the same methodology as in the study area, bimonthly and in 1981 only.
Stomach Content Analysis
Stomachs were removed from the fish and estimates of fullness were
made, subjectively, on a scale from 1 (empty) to 6 (100%+). The
section of gut from the esophagus to the junction anterior to the
pyloric caecum, which includes the stomach, was dissected and analyzed
under a dissecting microscope equipped with an ocular micrometer. Due
to the advanced state of digestion, intestinal content was examined
only to add to the list of prey items and no quantitative measurements
were made. Animals were identified to family when possible, or placed
in broad groups and enumerated.
Polychaete feeding palps, predominately from species of the family
Spionidae, and bivalve siphons are parts of prey organisms that
protrude from the substrate surface. They were commonly observed alone
in the sole stomachs. Based on previous findings (Tyler, 1972; Hogue &
Carey, 1982; Peterson &Quammen, 1982) and laboratory observations of
22
fish selectively cropping these parts (Steele et al., 1970), both
feeding palps and siphons were considered separate food items. It was
often unclear, however, as to whether the polychaete palps and
tentacles were taken exclusively, as stomach contents were often a
partly digested mixture of annelid segments and appendages. These
parts were only counted when found alone or in clearly greater numbers
than the corresponding worms. Polychaete feeding palps were counted
and divided by two, since each polychaete has two palps. Each bivalve
siphon was regarded as representing one individual eaten, since it has
been reported (Edwards &Steele, 1968) that when whole clam (Tellina
sp.) siphons are presented to plaice, the fish swallowed it whole, and
the siphon remained intact in the stomach. The siphons were the
toughest item found in the stomachs and, when manipulated with forceps,
were not easily torn. Finally, often pieces were recognizable as the
tips of siphons unambiguously distinguishing them from separate
animals.
Polychaetes were the most difficult group to estimate when the
stomach contained only fragments. If heads were recognizable, then
each was counted as a whole worm. If the state of digestion prohibited
the identification of heads, estimates were made of the combined
lengths for pooled size (diameter) classes of fragments and the total
number of worms was approximated from the length to width ratios of
intact polychaetes.
The surface area of each item was estimated as it was identified
by using a gridded ocular in the eyepiece of the dissecting scope. The
23
thickness was approximated in the same way and, based on these
estimates, a volume was assigned to each individual food item.
Unidentifiable food was segregated into one mass and volumetrically
quantified in the same way.
Finally, the degree of digestion for each stomach was estimated on
a scale from 1 (whole animals) to 6 (finely digested).
Data Analysis
Diversity of prey consumed was measured with the Shannon-Wiener
information index (H') using natural logarithms (Pielou, 1969).
Overlap between the composite diets, between years, was calculated with
Spearman's rank correlation coefficient, corrected for ties (Snedecor,
1956).
The mean percentage composition, a measure of the percentage a
particular food type contributes to the individual diet, was determined
by averaging this proportion for all the fish in a sample. It is a
measure of the average individual consumptions and is sensitive to
changes in the distribution of prey in the population but is not
greatly affected by aberrant individuals.
Both numerical and volumetric computations were done to consider
both large and small prey. Mean percentage composition was computed
for each monthly sample (both by number and volume) and individual
proportions were averaged for each year. The frequency of prey
occurrence (Fa) is a measure of how widely a prey item is used among a
24
sample population and is that proportion of stomachs containing the
particular food item.
One comprehensive measure of the relative importance of each prey
category to a group of predators is the Index of Relative Importance
(IRI) (Pinkas, Oliphant, &Iverson, 1971) given as:
IRI = FO (N + V)
where N = numerical percentage of given prey, or the average number of
prey per fish, V = volumetric percentage, and FO = frequency of
occurrence, or the proportion of fish containing this prey item.
•25
CHAPTER III
BACKGROUND DATA: THE UMPQUA TRAINING
JE'ITY STUDY
The following results are from data collected for the Umpqua
Training Jetty Monitoring Study during 1980-1982, as described. All
biological data were collected by a team from the University of Oregon
Institute of Marine Biology at Charleston, Oregon and are presented in
Varoujean (1984). Physical and water quality data were collected by
the Portland District Army Corps of Engineers (Hancock, 1985).
Physical Changes in the
Study Area
Although four culverts placed in the training jetty allow limited
flushing of the site (Hancock, 1985) and the large rocks that make up
the jetties permit water to pass through, the water quality and
sedimentation within the jetty configuration has been a1tered--the
riverine influence within the enclosed area has been severely reduced,
wave action has been virtually eliminated, and a considerable amount of
organic silt is accumulating on the bottom.
In 1980, before jetty construction, the salinity in the study area
reached the lowest concentrations during the winter, dropping from an
average of 30 parts per thousand (ppt) (range: 27-34 ppt) at high tide
26
to less than 9 ppt at low tide, indicating a large degree of mixing
with river water. The variation in mean salinity and daily salinity
ranges were very similar between stations outside and inside the
enclosure. In 1981 and 1982, the salinity regime at the outside
stations remained essentially the same, given normal temporal
variation, and exhibited a larger range in maximum and minimum values
from the waters inside the impoundment. Salinity measurements in the
study area after impoundment exhibited a diminished daily variation and
a narrower absolute range between maximum and minimum values.
Spring and summer upwelling usually brings a colder, more saline
and nutrient-enriched body of water to the surface from the offshore
depths. Thermal stratification in the study area, before impoundment,
suggests that these colder waters were allowed to flow freely into the
area where the higher density of the water kept it below the warmer
river water. Additionally, water temperatures outside the jetty were
generally lower during high tide than at ebb tide, at all depths,
reflecting the difference in the ambient temperatures of the ocean and
river water. After emplacement of the jetty, the temperature data for
the study side indicated a reduction in thermal stratification and a
reduction in the daily temperature ranges between high and low tide.
Both changes suggest a decrease in the mixing of riverine and marine
waters within the enclosed site (Hancock, 1985).
Variation in the study site of other components such as nutrient
concentration and dissolved oxygen (which reached greater extremes than
in oceanic waters) also appeared to be similarly affected by a drastic
27
reduction in the influence of the freshwater river water and more
closely resembled oceanic water.
The elimination of vigorous surface currents, swell, and wave
activity in the site after 1980 appears to have had definite effects on
the sedimentation in the enclosed area. Both core samples and
observations indicate that the study area is rapidly infilling. Sand
is being carried, via littoral transport, through the South Jetty.
Fine-grained sediments with high levels of organics are entering
through the culverts and settling throughout the study site. The cores
from these areas have shown a change from the poorly graded sand
present in 1981 to silts and clays (Figure 2); there has been a four-
to fivefold increase in the sediment organic content. A large portion
of the study site now acts as a settling basin where large amounts of
organic material are being deposited. Large clumps of brown algae were
often brought up with the trawl samples. In many areas, a fine silt
flocculus 1 to 2 feet thick was observed on the bottom.
In 1980, then, the study site existed as an interface habitat,
experiencing extremes brought about by the interaction of the ocean
tides and the freshwater riverine effluent and affected by the surf and
vigorous circulation. After impoundment, quiet waters with less
variable and more oceanic conditions prevailed, accompanied by drastic
changes in the bottom sediments.
28
CHANGES IN SEDIMENT GRAIN SIZE 1981-1984
o
Z
<t(J)
I-
Z 50UJ
U
0:::
UJ
a..
LEGEND
OCT
'~~
FIGURE 2. Changes in sediment grain size in study area
between 1981 and 1984.
Note. From Hancock, D. R. (1985, January). Umpqua Training
Jetty Extension Monitoring Study (p. 8) (Final Report, IPA
#E1844077), Portland: U.S. Army Corps of Engineers, North
Pacific Division.
29
Changes in the Occurrence of
Benthic Invertebrates
Qualitative differences were found in both the study and reference
sites between the intertidal benthic invertebrate community and the
subtidal community (Varoujean, 1984). To avoid the complication of
zonation and the influence of surf on benthic invertebrates, data from
Zone A, which includes the intertidal beach, were not included here and
this paper will be concerned only with the subtidal communities of
Zones B, C, and D, where the majority of English sole are found.
Due to the relatively rapid infilling by current-propelled
sediments, the future suitability of the site to sustain marine life is
uncertain. However, at the conclusion of the study, the productivity
of the area was far from decimated--the overall density of benthic
organisms had increased (Tables 1 and 2) and the site appeared to
contain a greater biomass on and within the bottom sediments.
Found in the benthic core samples were 11 groups of invertebrates
from six phyla (Tables 1 and 2), although only specimens large enough
to be caught in the O.5-mm2 mesh were able to be observed, and
harpacticoid copepods and other minute organisms were inadvertently
discarded during the sieving process.
Of the groups in Table 1, those organisms contributing less than
5% of the total number, for all 3 years, include nemerteans;
oligochaete annelids; mysid, isopod and decapod crustaceans; and
brittle stars--the only echinoderm found in the samples. Together,
these groups make up only 6%, by number, of the benthic invertebrates
TABLE 1. Mean Densities/m2 of Major Taxa of Benthic Organisms Recorded
in Samples Taken in Zones B, C, and D in the Study Area.
1980 1981 1982
Ta'xa April JU1e August October April June August October April JU18 August October
Nemertea 174 88 34 69 81 184 115 124 28 20 30 23
Nemetode 539 84 32 75 130 1932 435 4574 138 6Tl 168 236
Polychaeta 472 325 528 468 955 566 527 944 390 5Tl 1034 843
Ollg0ch8eta 50 16 8 13 3 54 12 56 7 41 31 31
I'Iysidacsa 10 11 6 0 6 8 26 5 0 0 0 0
CUII8csa 0 1 51 5 0 0 0 101 136 574 181 235
"""'hipode 389 178 33 7 3 1 7 177 3 24 53 126
1s0p0da 4 23 3 0 0 0 0 5 3 4 1 3
Oecapoda 4 1 0 263 3 2 4 2 0 4 1 0
Bivalva 0 5 472 0 413 640 375 181 189 525 224 145
Echinodermata 3 0 7 3 1 72 5 5 3 3 3 0
Nunber Sa~les 49 60 58 60 53 57 58 48 58 59 59 52
~. From Umpqua Training Jetty Extension ~itoring Study (Final Report to Portland District) (p. 32), by D. H.
Varoujean, 1984, Portland. U.S. Army Corps of Engineers. l.J,)
0
TABLE 2. Benthic Organisms; Cumulative Yearly
Densities and Percentage Contribution of Major Taxa.
1980* 1981** 1982***
Taxa D % D % D %
Polychaeta 448 37.3 760 23.2 772 43.6
Oligochaeta 21 1.7 31 1.0 20 1.1
Bivalva 185 15.4 402 12.2 271 15.3
Amphipoda 240 20.0 135 4.1 87 4.9
Cumacea 14 1.2 25 0.8 280 15.8
Nematoda 182 15.2 1768 53.8 314 17 .8
Nemertea 91 7.6 126 3.8 25 1.4
Mysidacea 7 0.6 11 0.3 0 0.0
Isopoda 8 0.6 1 0.0 2 0.1
Decapoda 1 0.1 3 0.1 1 0.1
Echinodermata 3 0.3 21 0.6 2 0.1
Total Density 1200 3283 1774
HI 1.67 1.29 1.53
*Number of core samples for year = 227.
**Number of core samples for year = 216.
***Number of core samples for year = 228.
31
32
retrieved during sampling. Bay shrimp (Crangon francisco rum) and
juvenile Dungeness crabs (Cancer magister) were frequently taken in the
otter trawls but were much too large to be considered prey organisms
for juvenile sole. The remaining organisms that are dominant in at
least 1 of 3 years (and > 5% total) include nematodes, polychaete
annelids, small bivalves, and cumacean and amphipod crustaceans.
Using Student's t-tests, Varoujean (1984) found no consistent
pattern of significant difference in benthic invertebrate abundances
between sampling stations within a sampling zone or between sampling
zones in each month and between months. No significant correlation was
found for the benthic community between sample years. The Spearman
rank correlation coefficient for five pairs of values requires 100%
agreement of the rankings between groups in order to meet the 5%
significance level requirement. No two years were identically ranked
(rs ' 1980-1981 = 0.75; r s ' 1981-1982 = 0.88; r s ' 1980-1982 = 0.70; r s
[0.5] = 1.00). All of the organisms that were numerically dominant in
1980 were also dominant in 1981 and 1982, and with the single exception
of the cumacea, no new organisms became dominant in 1981-1982 that were
not already in 1980.
The results of the faunal analysis for the 1980 preimpoundment
site describe a macroinvertebrate community typically found in the
sandy substrates of the open coast. Very few epifaunal organisms were
found in the samples, as would be expected where the substrate is
subjected to relatively rigorous swell and wave action.
33
Monthly and between-year changes are evident for all the major
invertebrate groups (Figure 3), and in many cases reflect obvious
environmental changes--organisms typically found in the sandy
substrates of the open coast (e.g., amphipods, errant polychaetes) were
replaced by more opportunistic organisms and those more tolerant of the
quiet waters and the fine, organic sediments present in the study site
in 1981 and 1982 (e.g., certain sedentary polychaetes, bivalves). A
major difficulty faced by benthic invertebrates less well adapted to
the fine sediments entering the study site is that of fouling--in
particular, the disruption of ventilation to the gills and brood
chamber. Although some organisms were adversely affected immediately
after completion of the jetty (e.g., amphipods and certain polychaete
families), the benthic community generally appeared to undergo a
gradual succession in species composition.
Oliver, Slattery, Hulberg, and Nybakken (1980) observed that
benthic marine invertebrate communities of a subtidal high-energy
beach, in Monterey Bay, California, were organized along a gradient of
wave-induced substrate motion, and that this factor was the single most
important factor affecting benthic invertebrate zonation. Distinct
zonation patterns were found to be relative to bottom depth with small,
commensal, actively burrowing deposit-feeding amphipods and ostracods
dominant in the shallow, disturbed zones, with the more sedentary
34
J.~
1.1
700
~ 0.1I,
•E 0.•
t" 0.7,.J!l
a a, . 0.'t~
;;15 0.&
.Ii"",
"'"~ 0.4
~ 0.3Q
0.2
0.1
0
APR 8) Jl.L AUO OCT APR 81 Jl.L AUO OCT APR 82 Jl.L AUO OCT
(a) POlYCIKTA
700 ;;
.,.
600
"00'i~ eooE
.;.
{ 400
•Ci,
t;; ;J:JOc
eo
~
III 200~Q
100
0 fl
0
APR eo ..u.. AUO OCT APR 81 JUL. AUG OCT APR e:z ..u.. AUO OCT
(b) BlY-.w.
FIGURE 3. Monthly and annual mean densities (in nos. 1m2) of dominant
benthic invertebrates. (a) Polychaeta. (b) Bivalvia.
(c) Cumacea. (d) Amphipoda. (e) Nematodes. (f) Nemertea.
Yearly mean densities are also in nos. 1m2 •
700 N
to
~
&Xl 574
- 200
,...
..
•.. 500
•E 0 ;;;
,;. to ~
• ~} <100
ii
::J
~
~
"
.:J)()~
~
l/I 200~
0
100
51
0 5 0 0 0
0
APR eo JU.. AUG OCT APR 81 JU.. AUG OCT APR 82 JU.. AUG OCT
Cc) CUlIAC£A
35
APR 81 Jll. AUG OCT
533
500
,...
..,
•E <100
cT
•
"-
•ii
:>
.:J)()~
>;;
~
~ 200
l/I
~
0
100
7
0
APR eo Jll. AUG OCT
Cd)
3 7
APR 82 Jll. AUG OCT
FIGURE 3. (Continued) •
36
5 ...
.!' 4574
4.5 800
4
,....
..,
• 3.5E
N
cr .,.,.... 3 ~
........
...
.. "
" 0 2.5o.
• ::J::J 0
~~
., 'J 2
'J
t
11\ 1.5~
0
539
0.5
84 32 75
0
APR eo Jl.L AUG OCT APR 81 Jl.L AUG OCT APR 52 JUL AUG OCT
(e) HDolATOD£S
:zoo
190 154
1SO
170
160
,....
.. 150
•~ 140E
cr 130
• 120
.......
~ 1100
::J 100J!,.
90;;
oS 1lO....,
t 70
" 60Zi 500
40
30
20
10
0
APR eo Jl.L AUO OCT APR 81 Jl.L AUG OCT APR 52 JUL AUO OCT
( f) HEIIERTEA
FIGURE 3. (Continued).
37
burrowing and tube-building polychaetes1 dominant in the deeper, more
stable substrate.
The polychaete community exhibited substantial changes in
concentration during the 3-year study (Figure 3a); sampled densities
for 1981 and 1982 were almost 70% higher than the yearly mean for the
1980 preimpounded samples. Of the polychaetes, 16 families were
identified (Figure 4) and whereas most polychaete groups diminished
drastically in number after 1980, 4 of the 16--the spionid, capitellid,
owenid, and nereid polychaetes--increased after 1980. The most
important change in the polychaete community was a significant increase
(P < 0.02, df = 3) in the density of spionid and capitellid polychaetes
after impoundment (Figure 4).
Spionid polychaetes, like the owenids and magelonids, are a
relatively sedentary, tubicolous species with a feeding strategy that
is transitional between the indirect deposit feeders and filter
feeders. In addition to feeding on the bottom deposits, they have two
grooved feeding palps which are used to collect suspended detritus.
The tube it constructs may, among other benefits, provide the animal
access to clean, oxygenated water above a 'muddy or sandy bottom
(Barnes, 1980) and may be a very important adaptation for the new
conditions in the study site. Capitellids are errant burrowers and are
found, almost exclusively, in fine sands and mud. Both of these
1Most of these were surface deposit-feeders and suspension feeders
whereas the polychaetes living exclusively in the shallow, crustacean
zone did not have permanent tubes or burrows.
38
80
o~60
>-o
z
UJ
:::>
o
UJ 40
a:
LL
UJ
>
i=
<t 20
..J
UJ
a:
LEGEND
1=198019811982
spionids
capite1lids
1980
44(46)
75(32)
*
1981
36(19)
498(217)
*
1982
203(97)
423(181)
* • adjacent means significantly different, P<0.02 (t-test, 3 d.f.)
FIGURE 4. Changes in the frequency of polychaete families, 1980-1982:
changes in the relative frequency (%) of all families
sampled in the study area (top); means of monthly densities
(number/m2) of spionid and capitellid polychaetes in zones
B, C, and D of the study area (bottom). Numbers in
parentheses are one standard deviation from the mean.
Note. From Varoujean, D. H. (1984, April). Umpqua Training Jetty
Extension Monitorin Stud (pp. 30, 33) (Final Report to Portland
District , Portland: U.S. Army Corps of Engineers.
39
families can be found, however, in a wide variety of habitats and have
been previously reported associated with highly organic or frequently
disturbed areas; some capitellid worms are considered pollution
indicators (Light, 1975). The increase in abundance of these two
families accounts for the overall increase in polychaete density for
1981 and 1982 (Figure 4).
Nemerteans showed a small, probably insignificant increase in
sample abundance for the 1981 sample season before becoming much more
rare in the 1982 samples (Figure 3f).
Four species of bivalves, mostly small clams between 0.5 and 3 mm
long, exhibited an overall increase in density for the postimpoundment
years (Figure 5), although the trend is not marked. In 1980, there
were almost no bivalves in the samples during the first two sampling
sessions, indicating either a barrier to recruitment into the site
before construction, or unfavorable conditions (water or sediment
quality) for survival in the preimpoundment environment. The reference
site exhibited a similar pattern (few bivalves until the month of
September) which might suggest that there is larval recruitment into
the shallows, in the summer, followed by the destruction of this
community or the migration to deeper waters during the winter. Large
surf due to winter storms might cause such a pattern in the shallow
areas.
In 1982, as the study site continued to fill in with organic
sediments, the abundance of bivalves decreased somewhat (Figure 3b).
With inefficient gills that double as feeding surfaces bivalves might
40
i
o5AJ J
M 0 NTH
M
237 r- ....
I ........I ....,
I ,
I \
J \
I \
J \
J \
J \
J \
I \
J \
J \
J \
J \
I '
J "J ,
I 'I '
J "I ,
I
J
I
I /
I /
"',,' "
'I ,--------,'
'" , -----'
, ,I "
." ..~ "
.".,,, '\ ",
., \,'
\ /'
"
----1982
--1980
---1981
STUDY AREA
A
O+---.....---r--.....~-'"""""IF---r--- .....--..
50
150
100
200
"-I
Vl
FIGURE 5. Catch/effort of English sole in the study area.
Note. From Varoujean, D. H. (1984, April). Umpqua Training
Jetty Extension Monitoring Stud (p. 43) (Final Report to
Portland District , Portland: U.S. Army Corps of Engineers.
41
have begun to experience fouling problems. Barnard (1963) observed
that the distribution of Tellina modesta was correlated with the
percentage of silt in shallow water sediments which indicated very
specific sediment-size preference in some bivalves. Species or size-
specific success in dealing with the sediments in the study area might
allow for a partial destruction of the bivalve population. Other
factors, such as predation, may also affect the bivalve population.
At least 12 species of gammarid amphipods exhibited a dramatic
decline near the end of 1980 and did not begin to reappear, in any
consistent numbers, until 1982 (Figure 3d). October 1981 samples
discovered a patch of amphipods (in Zone B) of extremely high
concentration, but the study area generally appeared void of any
substantial numbers.
The cumaceans, an order of small, bottom-inhabiting crustaceans,
exhibited the most dramatic change in numbers (of individuals sampled)
over the 3-year period. The data indicate nearly a IS-fold increase in
population density between the end of 1981 and 1982 (Figure 3c).
Although many cumaceans feed directly from the substrate surface,
most are suspension feeders and are capable of exploiting the water
column in search of food. Cumaceans live buried in the sand and mud,
often in tunnels, and in areas of rapidly shifting sediments; they are
able to relocate when becoming too deeply buried (Barnes, 1980). In a
study of benthic invertebrate zonation, Oliver et al. (1980) found that
cumaceans were not abundant on the bottom but instead were successfully
captured in funnel traps just off the bottom. A factor in their
42
success in the study area might largely stem from their not being
restricted to the substrate surface. That the cumaceans were so much
more abundant in 1982 than the previous years suggests they prefer
quiet waters. The organic sediments and detritus brought into the area
are likely an ideal food supply for these organisms.
Nematodes occurred in very large numbers in the samples between
July and October of 1981 (Figure 3e), with sample population densities
far exceeding those of 1980 and 1982.
Spearman rank correlation coefficients were compared pooled
results between years. Values indicate a significant correlation
between 1981 and 1982 only (P = 0.05). The diversity (H') is
relatively low in 1981 and is probably the result of the abundance of
nematodes in that year.
Monthly comparison of the study site with the reference site in
1982 (the only year the reference site was sampled for benthic
invertebrates) revealed little.
From trends observed in the sediment samples taken over the 3-year
period, cumaceans, certain spionid and capitellid polychaetes, and
diminutive bivalves appeared to thrive much better in the fine, organic
sediments of the study site in 1982 than the more exposed and coarse-
grained river-influenced environment of 1980. Of equal importance, the
gammarid amphipods and species of many other families of polychaetes
have declined dramatically in the 2 years following impoundment.
43
Changes in Fish Abundance
Collected from the study site in 1980 were 38 fish species,
including 6 species of flatfish (Table 3). Large numbers of midwater
and nonf1atfish demersal species were taken by otter trawl and beach
seine. Gill nets yielded several species not taken by the other
methods, but these additional fishes comprised a very small percentage
of the overall catch. Species composition and relative numbers of fish
captured in the study site in 1980 are in agreement with data reported
for the lower Umpqua River Estuary (Mullen, 1977) and Coos Bay
(Cummings &Schwartz, 1971).
In the otter trawl samples, the dominant fishes were juvenile
English sole (Parophrys vetu1us) and juvenile speckled sanddab
(Citharichthys stigmaeus). Sampling during the 1981 and 1982 seasons
showed a striking decrease in the number of midwater and nonf1atfish
demersal species, with a concurrent increase in the number of juvenile
flatfish, especially English sole, in the study area (Table 4).
Sampling at the adjacent reference site in 1981-1982 showed little
change in species composition from that of 1980 and closely resembled
the 1980 preimpoundment study site.
Relatively few English sole were taken from the study area before
August 1980, before jetty completion. Catch data for the
postimpoundment years (Table 4, Figure 5) show that English sole were
present in the study area during all months sampled and that there was
a substantial numerical increase from 1980 levels. Only fish between
TABLE 3. Fish Species Found in Umpqua River
Estuary During the Study Period 1980-1982.
44
Common Name
Spiny dogfish
Big skate
Green sturgeon
Pacific herring*
American shad
Northern anchovy*
Rainbow trout
King (Chinook) salmon*
Silver (Coho) salmon*
Surf smelt*
Whitebait smelt*
Night smelt*
Longfin smelt*
Pacific tomcod*
Topsmelt*
Threespine stickleback
Bay pipefish
Black rockfish
Juvenile rockfish
Lingcod*
Kelp greenling*
Cabezon
Staghorn sculpin*
Buffalo sculpin
Padded sculpin
Pricklebreast poacher
Striped bass
Redtail surfperch*
Spotfin surfperch
Walleye surfperch
Silver surfperch*
Shiner surfperch*
Striped surfperch
Pile surfperch
White surfperch
Penpoint gunnel
Saddleback gunnel
Pacific sandlance*
Sand sole*
English sole*
Starry flounder*
Rock sole
Speckled sanddab*
Pacific sanddab*
Scientific Name
Squalus acanthias
Raja binoculata
Acipenser medirostris
Clupea harengus
Alosa sapidissima
Engraulis~
Salmo gairdnerii
Oncorhynchus tshawytscha
Oncorhynchus kisutch
Hypomesus pretiosus
Allosmerus elongatus
Spirinchus starksi
Spirinchus thaleichthys
Microgadus proximus
Atherinops affinis
Gasterosteus aculeatus
Syngnathus leptorhynchus
Sebastes melanops
Sebastes sp.
Ophiodon elongatus
Hexagrammos decagrammus
Scorpaenichthys marmoratus
Leptocottus armatus
Enophrys bison
Artedius teneStralis
Stellerina xyosterna
Roccus saxatilis
AmPhLStichus rhodoterus
Hyperprosopon~
Hyperprosopon argenteum
Hyperprosopon ellipticum
Cymatogaster aggregata
Embiotoca lateralis
Damalichthys~
Phanerodon furcatus
Apodichthyes flavidus
Pholis ornata
AiiiiiiOifiteB1ii!i'apterus
Psettichthys melanostictus
Parophrys vetulus
Platichthys stellatus
Lepidopsetta bilineata
Citharichthys stigmaeus
Citharichthys sordidus
*Indicates fish caught in study site.
Extension
45
TABLE 4. Fish, Dungeness Crab, and Bay Shrimp Caught in Otter Trawls
Conducted in Zones B, C, and D in the Study Areas (S) and Zones
II, III, and IV in the Reference Site (R) by Year (Abundances
Expressed as Mean Number/Trawl).
1980 1981 1982
Species S S R S R
Surf smelt 5
Whitebait smelt 2 +
Pacific tomcod 5 + +
Topsmelt 1
Lingcod 1 +
Kelp greenling 1 1 + + +
Staghorn sculpin 4 + 1 + +
Shiner surfperch 6 + 1 + +
Sand sole + + 1 + +
English sole 9 100 36 34 21
Starry flounder + 1 1 + +
Pacific sanddab 1 + 1 1
Speckled sanddab 20 6 17 2 12
Dungeness crab 49 4 16 1 13
Bay shrimp 33 15 6 5 3
Number hauls 21 21 17 21 21
+Present, but number/haul less than 1.
Note. Species for which the catch/effort was less than one in each of
the three years of study are not listed.
Note. From Um ua Trainin Jett
Report to Portland District p. 32 , by
Portland: U.S. Army Corps of Engineers.
46
20 and 180 mm (SL) were captured in the study area. This size range is
probably representative of the English sole community at the site,
although some bias may exist as a result of large fish being better
able to avoid the sampling trawl. Length/frequency data for English
sole in the study site (Figure 6) indicate that smaller individuals
constituted a greater proportion of the total in 1981 and 1982 than was
the case in 1980, where fish smaller than 80 mm standard length were
rare or not found in the samples over the year. The change in English
sole size-frequency data over the 3-year study period may be owing to
an abundance of larval P. vetulus entering the study site through the
jetty as planktonic larvae, metamorphosing, setting, and maturing
within the study site. Although juvenile English sole commonly inhabit
relatively shallow, nearshore waters, they are probably uncommon in
very shallow areas subject to heavy wave action, as in the
preimpoundment study area. The stone jetty does not appear to prevent
the recruitment of the planktonic larvae of flatfish into the site, and
the quiet waters of the impoundment area seem to have made the site
more conducive to settlement and growth.
Because the jetty will impede emigration to deeper waters, the
study site might act as a trap for certain bottom fishes with
planktonic larvae by providing them with favorable conditions for
recruitment and early growth. Unless the study site proves to be too
unproductive to support an abundant bottom-fish community, the average
size of P. vetulus would be expected to increase in coming years.
47
20
X IS
'"
It 12
;;:
0 '0
n.'.J
'"...
dO
~ 5
~
Z
0
0
I. , g
_ .
. .-
STANDARD LENGTH (mm)
2S
20 It II
n.'2_
X 15
'";;:
...
0 '0
'"...
dO
~ 5
~
Z
0
0
,. ~~
-"'• •
STANDARD LENGTH (mm)
X
'";;:
10
...
0
'"
.. 10
w
dO
~
~
Z
0
•
,. 1 0; .. i~
STANDARD LENGTH (mm)
FIGURE 6. Length/frequency data for English sole caught
in the study area.
Note. From Varoujean, D. H. (1984, April). Umpqua Training
Jetty Extension Monitorin Stud (p. 44) (Final Report to
Portland District , Portland: U.S. Army Corps of Engineers.
48
CHAPTER IV
RESULTS OF STOMACH CONTENTS ANALYSIS
With the exception of the October samples, fish caught in 1980
were not saved. The diet analysis, then, for 1980 is based on fish
captured in October of that year, just after completion of the training
jetty. As with the benthic invertebrates collected from the study
site, only English sole taken from Zones B, C, and D were examined in
order to avoid possible complications in including the intertidal
habitat in Zone A.
English Sole Diet, General
Stomachs from 128 juvenile Parophrys vetulus were examined over
the 3-year study, of which only 12 were empty. Fish used in the diet
analysis were between 68 and 178 mm total length, corresponding to 1st-
and 2nd-year English sole. As is apparent from Table 5, ~ vetulus
feeds on a wide variety of benthic animals (see Figure 7). Prey from
seven broad taxonomic groupings were found but the majority of prey
came from only three classes of invertebrates: the Polychaeta,
Bivalvia, and Crustacea. Seven prey categories from these three
classes account for over 99% (numerically) of the total prey organisms
taken by ~ vetulus and are Polychaeta annelids, juvenile bivalves,
clam siphons, cumaceans, harpacticoid copepods, amphipods, and
TABLE 5. Taxa Identified From Stomachs of
English Sole Taken From the Study Site.
49
Major
Group
Nematoda
Nemertea
Polychaeta
Bivalva
Gastropoda
Crustacea
Ophiuroidea
Vertebrata
Dominant
Organisms
*
Clams and miscellaneous
bivalves, clam siphons
Harpacticoid copepods,
cumaceans, gammarid
amphipods
Incidental
Organisms
+
+
Veliger and egg case
Cladocerans, ostracods,
calanoid copepods,
cirripedians (nauplii)
isopods, amphipods,
decapods (zoea), mysids
Brittle stars
Fish eggs
+Indicates presence at site.
*Identified specimens include polychaetes from the families:
Capitellidae, Magellonidae, Nephtyidae, Nereidae, Opheliidae,
Owenidae, Spionidae, and Terebellidae.
50
(d)(c)
Capi/.,Ia
capitola(b)
Magtlona sacculata
(e)
(f)
Note. All figures except (e) from S. F. Light, 1975,
~t's Manual: Intertidal Invertebrates of the Central
California Coast pp. 165, 223, 249, 254, 329, 332 ,
Berkeley: University of California Press. (e) is from
C. P. Hickman, 1973, Biology of the Invertebrates
(p. 547), St. Louis: C. V. Mosby.
FIGURE 7. Some of the dominant invertebrate taxa found in the
stomachs of P. vetulus from the study area. (a) a
spionid polychaeta annelid, Polydora (note feeding
palps). (b) a magelonid, Magelona (note feeding palps).
(c) a capitellid polychaete. (d) a harpacticoid copepod,
Tisbe sp. (e) a cumacean, Diastylis, shown buried in
sediment. (f) gammarid amphipods. (g) a mysid, Mysis sp.
(h) an ostracod, a less frequently observed prey item.
(h)
~~
Cylindro/~Mrissp.(g)
51
nematodes. Quantitative data are summarized for these prey categories
in Table 6, and the yearly mean abundances (numerical) are compared in
Figure 8. Animals constituting less than 1% of the total number of
prey items for all three years include c1adoceran, ostracod, isopod,
decapod, and mysid crustaceans; brittle stars (Echinodermata:
Ophiuroidea); and gastropod and fish eggs, along with other incidental
food (e.g., barnacle nauplii and gastropod veligers).
Most of the prey items were small, between 0.5 mm and 4.0 mm along
their greatest dimension. Harpacticoid copepods were the smallest
prey, between 0.2 and 0.3 mm in length. Gammarid amphipods, cumaceans,
intact juvenile bivalves, and bivalve siphons ranged from between
0.5 mm and 4.0 mm long, with a few individuals from each of these
groups being slightly larger. Po1ychaetes, predominantly of the
families Spionidae and Capite11idae, were by far the largest prey, with
pieces frequently between 6-10 mm and occasionally up to 30 mm in
length. Polychaete palps and nematode worms were commonly over 1 mm
long, but are very thin and constitute a much smaller volume of food
than other prey of equal length. The majority of nematodes were
determined to be free living, based on their appearance (often
,partially digested) and their position in the digestive tract
(frequently a part of the food bolus), which distinguished them from
the parasitic forms.
All the prey items in the stomach contents were also found in the
benthic faunal sediment cores, with the exception of ca1anoid copepods,
TABLE 6. Summary of Stomach Content Data Collected for English
Sale, Parophrys vetulus.
Polych8etes Juvenile Bivalves e18lll S1pI>ons Harp!
Jllean
No. Fish length (CIIl)
S~le Oets Exaadn8d and Range Ha vb FD" H V FD N V FO N
12 Dctcber 1900 12 12.2 (9.0-16.5) 0.44 0.60 (0.85) 0.07 0.08 (0.39) 0.10 0.09 (0.31) 0.26
9.60 47.40 1.80 7.90 1.50 2.60 28.70
23 July 1981 9 9.0 (6.~11.5) O.IB 0.45 (0.89) 0.20 0.29 (0.89) O.ZG 0.15 (0.78) 0.24
10.00 47.60 3.80 24.70 6.40 12.00 19.10
20 ~lJ9USt 1981 16 (2) 12.8 (9.6-17.8) 0.44 0.60 (1.00) 0.09 0.09 (0.57) 0.35 0.24 (0.93) 0.00
15.10 54.80 2.00 9.20 8.40 16.80
17 Septmlber 1981 9(1) 11.6 (9.S-13.0) 0.56 0.80 (1.00) 0.00 0.04 (0.50) 0.29 0.15 (0.50) 0.04
6.80 60.80 0.50 2.10 6.30 9.80 1.50
21 October 1981 9 (3) 11.2 (9.6-13.6) 0.44 0.68 (1.00) 0.16 0.18 (0.50) 0.16 0.12 (0.17) 0.24
4.30 34.70 1.00 9.70 6.50 10.00 1.70
Totals & ""'ens 43 (6) 11.4 (6.~17.8) 0.43 0.62 (o.s7) 0.12 0.14 (0.62) 0.27 0.18 (0.68) 0.10
10.30 51.10 2.00 11.50 7.10 13.00 5.20
27 July 1982 17 (3) 10.2 (8.6-13.0) 0.24 0.34 (0.93) 0.20 0.29 (0.79) 0.17 0.12 (0.86) 0.03
6.90 40.90 6.10 49.20 5.60 12.70 1.70
23 ~uglJSt 1982 16 12.8 (10.5-16.1) 0.39 0.57 (1.00) 0.05 0.05 (0.44) 0.38 0.05 (0.88) 0.01
12.60 101.80 1.10 5.10 1.10 6.60 0.30
29 September 1982 16 12.7 (9.5-15.3) 0.50 0.60 (1.00) 0.03 0.08 (0.19) 0.29 0.25 (0.75) 0.03
15.90 90.50 0.30 4.60 7.40 17.80 0.90
28 October 1982 IS (5) 10.9 (9.6-12.6) 0.26 0.37 (0.90) + 0.01 (0.20) 0.32 0.27 (1.00) 0.05
8.90 37.30 0.30 9.90 14.20 27.40 1.40
Totals & l'Ieans 64 (8) 11.2 (6.5-18.0) 0.36 0.48 (0.98) 0.07 0.11 (0.36) 0.30 0.22 (0.88)
11.80 11.80 2.00 15.80 10.20 22.00 0.40
.,.., percentage ~tion by rulber (N) (\4lPBr) and ...... ruabers of x:-: per stoalch (u-r). s.. for each N in ewry prey cata..lory.
bMean percentage cc.position by vatu. (V) (\4lPBr) and _11I1 val~ (in ) of organi_ per su-ch (looer). S- for each V in every prey catl!9>ry.
Cfrequency of occurrence (FO). Saons for ewry prey cata..lory.
+Indicates • value for lass than 0.01.
"9QC of the aillcellaneous orqani_ "",re II)'sids, all ft'Olll one stomach.
Nots. IlJobers in perlll1t1wses Lnder the heading "No. Fish E.xaadned" are ruaber of fish with empty stonoachs.
Prey Categories
Clam Siphons Harpecticoid Copepods CuDacaa AftlJhipods Nematodes Mscellaneous I
V FO N V FO N V FO N V FO N V FO N V Fl
0.09 (0.31) 0.26 0.15 (O.46) 0.00 0.00 0.00 0.00 0.00 0.00 0.05 + (0.23) 0.lJ8" 0.06 (1,
2.60 28.70 17.10 0.46 0.40 5.90 22.00
0.15 (0.78) 0.24 0.06 (O.57) 0.02 0.01 (0.44) 0.03 0.02 (M7) 0.01 (O.22) 0.01 0.01
12.00 19.10 3.90 0.80 0.90 1.10 0.22 0.20 0.01 0.40 1.10
0.24 (O.93) 0.00 0.00 0.00 0.01 + (0.14) 0.00 0.00 0.00 0.10 0.02 (O.36) 0.01 0.04
1S.80 0.30 0.30 2.70 1.00 0.10 8.S0
0.15 (O.50) 0.04 + (0.13) 0.00 0.00 0.00 0.01 (0.13) 0.06 + (0.25) + +
9.80 1.50 0.40 0.04 0.60 1.10 0.40 + +
0.12 (0.17) 0.24 0.03 (O.33) 0.00 0.00 0.00 0.02 0.02 (0.17) 0.00 0.00 0.00 +
10.00 1.70 0.50 0.20 0.30 +
0.18 (0.S8) 0.10 0.01 (0.24) + + (0.1S) 0.01 + (O.:!2) 0.05 0.01 (0.24) + 0.20 (1,
13.00 5.20 1.10 0.30 0.30 0.40 0.80 1.30 0.20 0.20 3.50
0.12 (0.86) 0.03 + (0.21 ) 0.34 0.23 (O.93) 0.01 0.02 (O.m 0.01 + (O.29) +
12.70 1.70 0.60 14.30 19.90 0.60 1.10 0.90 0.50 0.70 0.10
0.05 (0.88) 0.01 + (0.19) 0.12 0.06 (O.56) 0.03 0.03 (O.38) 0.01 + (0.13) 0.01 0.01
S.60 0.:Jl 0.10 4.90 5.90 1.20 3.10 0.30 0.20 0.40 2.90
0.25 (O.75) 0.03 + (0.19) 0.07 0.05 (O...) 0.01 + (0.25) 0.04 0.01 (0.13) +
17.80 0.90 0.30 0.90 1.10 0.50 0.90 0.10 0.20 0.10 0.10
0.27 (1.00) 0.05 + (0.50) 0.25 0.24 (O.80) 0.11 0.09 (0.80) 0.00 0.00 0.00 + 0.02
27.40 1.40 0.30 13.20 :Jl.90 4.:Jl 8.40 0.30 2.80
0.22 (O.88) + 0.03 (O.25) 0.19 0.13 (0.66) 0.04 0.03 (0.39) 0.02 + (0.66) + 0.01 (1.
22.00 0.40 1.20 7.60 12.70 1.40 2.00 0.30 0.20 0.20 0.91
cry .PrrI catsgory. "
V in every prey categlry.
53
COP (21l.0X)
POl (44.~)
POL (4J.O">
81\1 (7.0'1)
(O.~>
1980
mean length. 122 mm (22.5)
~. 13 (12 measured)
TO'DL l.ENCl1H ( ....)
mean (total) length. 114 mm (25.3)
~. 41 (4 empty)
1981
1982
TOTAL l.ENCl1H ( ....)
mean (total) length. 112 mm (22.5)
~. 64 (8 empty)
eo 'JD ID to 100 "0 120 UID 140 '10 'ea 171) ,.,
as lO • • 1e. 110 ,. ,. 140 110 ,. 17D , ..
• 1D .. • 'aa 110 120 UD uo tID lID 11'0 I.
.:~
H• iii iii IlJ i .1lJ. iii Iii i i •• iii i·
t.
li •~ .
S.
f •
, FIGURE 8. Right: Mean percentage composition for major prey taxa
found in P. vetulus from the study site, 1980-1982 annual
means based on numerical abundance. Polychaetes, POL;
bivalves, BIV; bivalve siphons, SIP; nematodes, NEM;
cumaceans, CUM; amphipods, AMP; and all other minor taxa
present in stomachs, MISC (Note: 90% of the MISC in 1980
were mysids). Left: Length-versus frequency histogram for
all P. vetulus examined (n = number of fish/sample).
Numbers in parentheses are one standard deviation from the
mean.
54
barnacle naup1ii, and gastropod ve1igers, which are pelagic. Mysids
also migrate vertically into the water column.
Typically high frequencies of occurrence in the stomach samples,
for most prey groups, indicate a wide range of prey types were consumed
by a majority of fish in the samples. Po1ychaetes were found in nearly
all stomachs (frequency of occurrence [FO] for each year, respectively,
was 84%, 97%, and 98%), and bivalves and/or bivalve siphons were found
in well over 50% of the stomachs examined.
The average diversity H' (Shannon-Weaver, 1949) of food consumed
per sampling date was computed using the same eight major groupings1
for each calculation and so is really a measure of evenness. H' was
0.704, 0.716, and 0.764 for 1980, 1981, and 1982, respectively. The
difference in the value of H' between years is probably not
significant.
The Spearman rank correlation coefficient, r s ' has been corrected
for ties (Fritz, 1974) and eight pairs of ranked values produced
significant correlation (at 5% significance) between diets in
successive years only (rs 1980-1982 = 0.48, r s 1980-1981 = 0.82, r s
1981-1982 = 0.77; r s [0.05] = 0.714;), indicating that the prey
consumed in 1981 were transitional between those of the 1st and 3rd
years.
1The seven major prey categories already mentioned, plus the
mysids which contributed 7% of the prey to the 1980 diet samples.
55
Relative Importance of Prey Types
The importance of amphipods and nematodes to the P. vetulus diet
was relatively low throughout the study, their combined numbers never
exceeding 6% of the total prey consumed in any year. Although the
numbers of some prey, particularly some of these less abundant
organisms, did not change much over the 3-year study, some of the
important prey in the diet of the English sole did change in a
consistent fashion, as can be seen in the yearly comparison (Figure 9).
Polychaetes, bivalves (whole and their siphons), copepods, and
cumaceans dominated the sampled diets over the 3-year period accounting
for 91%, by number, of the diet for all years: Polychaetes and bivalves
were major dietary organisms all 3 years; copepods were very important
in 1980 only, and cumaceans were a dominant food in 1982 only.
Polychaete annelids were the most important prey for all 3 years,
both in number and biomass (Figure 8). Roughly 30% to 60%2 of the diet
(by number) over the entire 3 years was composed of polychaetes.
The proportion of the diet comprised of polychaetes was the
highest in 1981, dropping somewhat in 1982. These differences are
probably not significant, however, and may in part be due to the
increased consumption of other prey, particularly the cumacea and
bivalves, which increased substantially. Although the majority of
polychaetes in the stomachs were of the families Capitellidae,
2Not including feeding palps eaten separately, a small but
consistent contribution to the diet.
56
OCT eo .IlL 81 AUG SEP OCT .IlL 82 AUG SEP OCT
(a)
40....------------------------...,
36
30
25
Hi
10
o
OCT 110 .IlL 81 AUG SEP OCT .IlL 82 AUG SEP OCT
(b) IZZI IWAL.\O _ IWAL.VE SIPHONS
FIGURE 9. Monthly mean percentage composition for major prey taxa
found in P. vetulus from study site, 1980-1982 based on
numerical-abundance. (a) Polychaeta annelids and feeding
palps taken separately. (b) Juvenile bivalves and clam
siphons taken separately. (c) Cumacean, amphipod, and
harpacticoid copepod crustaceans. (d) Nematodes and mysids.
57
JUL 82 AUG SEP OCT
JUL .t AUG 9EP OCTOCT 80
o
15
10
20
0
OCT 80 JUL 81 AUG SEP OCT
(C) IZZI ClIoUCEA ~ AM~DS
11
10
t
8
7
~ •W~
w 5L
4
J
2
(d)
FIGURE 9. (Continued).
58
Spionidae, and Owenidae, a large proportion of polychaete material was
not identifiable and the proportions of polychaete families consumed
each year could not be determined. However, the number of spionid
feeding pa1ps that were taken separately from whole specimens did
increase each year (Figure 9a). Feeding pa1ps of the mage10nid
polychaete, Mage10na saccu1ata, were also discovered in the sole
stomachs.
Most of the intact bivalves found in the stomachs were clams of
the genera Te11ina, a small clam commonly observed as prey for
P. vetu1us (Hogue, 1982), and Si1iqua. The yearly means of the
frequencies of occurrence indicate that almost twice as many fish had
intact clams in their stomachs in 1981 than in the other two years.
The numerical abundance of bivalves in the diet was highest in 1981
(12%) (Figure 8) and a consistent decline in their numbers was observed
for 1982, as seen in the monthly comparison (Figure 9b). Bivalve
siphons eaten separately were much more important as food than whole
clams and contributed about 20%, by number and volume, to the diets
over the 3-year study (3-year mean). Bivalve siphons, as a separate
food, became increasingly important in the diet (Figures 8 and 9b) as
the number and volume of siphon pieces found in the stomachs increased
each year (10%, 27%, and 37%, by number, for 1980, 1981, and 1982).
Even more significantly, the proportion of fish using siphons for food,
measured by the frequency of occurrence, increased substantially each
year: 31%, 68%, and 88% for each consecutive year (Table 6).
59
Cumaceans were an important food in 1982 only (19% of the total
diet, by number), probably reflecting the 1st year they were present in
substantial numbers in the benthic community. P. vetulus was eating
large numbers of cumaceans each month during the sampling period in
1982 (Figure 9c) but very few before that--less than 1% in each of the
previous years.
Mysids and harpacticoid copepods, both important prey in 1980 (7%
and 26%, by number, respectively), became rare food items in the
postimpoundment environment (less than 1% each in 1982). Apparently a
more important food than the mysids, the feeding on harpacticoid
copepods gradually declined (Figure 9c); in 1981 they still contributed
10% to P. vetulus diets. Mysids were only occasionally found in the
stomachs after the October 1980 sampling (Figure 9d).
The volume of food eaten by ~ vetulus over the 3-year period
(Figure 10) does not appear to have changed substantially; the total
volume of food per fish, at the time of collection was 173 mm3 and
159 mm3 for the first and last years (1980 and 1982) respectively
(Figure 10). The average was only 100 mm3 for 1981, but probably does
not represent a significant difference. The volume of identifiable
prey consumed was higher in the 1982 samples than for the previous 2
years and is probably attributable to a greater number of large,
durable prey (e.g., polychaetes, clam siphons, intact bivalves, and
cumaceans) in the diet. Numbers also increased, but the volumetric
gain was not accompanied by a comparable numerical increase in the
60
UNIDENTIFIABLE
t I
IDENTIFIABLE PREY
ISSS1
VOLUME,cu.mm./Ind.
200 ...------------------
115
150
125
100
15
50
25
0"""'--...-----.....11II..--......--...........----.--.....-------11
1980 1981
YEAR
1982
FIGURE 10. Volume of prey consumed (in mm3/fish) by P.
vetulus for each of the 3 years. --
polychaetes and the bivalves, very possibly indicating growth in the
size of these prey over the study period.
61
62
CHAPTER V
DISCUSSION
Conclusions
Although immediately following completion of the jetty some
organisms were very adversely affected (e.g., amphipods and certain
polychaete families), the benthic community appeared to undergo a
gradual succession in species composition. Certain dominant open-coast
species were replaced by others more tolerant of the new conditions,
while previously excluded organisms (e.g., bivalves) were able to
tolerate the new set of conditions and recruited into the site.
Because of the rapid infilling by current-propelled sediments, the
future suitability of the site to sustain marine life is not certain.
However, at the conclusion of the study the productivity of the area
was not greatly impaired--the overall concentration of benthic
organisms had in fact increased and the site appeared to contain a
greater density of organisms on and within the bottom sediments.
The resultant effect of these changes on the English sole
population at the site was not surprising given the feeding
capabilities of the sole. Although annelids remained the dominant food
over the entire 3-year period, other prey organisms commonly eaten by
P. vetulus in the open-coast environment of 1980 (e.g., copepods and
63
mysids) were replaced by prey that became more abundant in the
following years (e.g., cumaceans and bivalve siphons).
The contention that the English sole is a roving generalist,
feeding on a wide range of prey types, is supported by this study.
Prey organisms were almost exclusively benthic, varied greatly in size,
and consisted of fossorial organisms which were retrieved from the
sediment, epibenthic animals, and pelagic invertebrates.
P. vetulus taken from the study area had fed upon all the
organisms known from the benthic samples (within the size range
limitations for potential prey) including the relatively rare
invertebrates (decapods, brittle stars). The data revealed only a few
weak preferences during the 3-year study; nearly all prey items were
represented in the diet of ~ vetulus approximately in the proportions
found in the substrate. Exceptions were the nematodes and amphipods,
neither of which were consumed in any substantial quantity (regardless
of their abundance in the sediment) and which were largely ignored
probably because the energy expenditure for locating and consuming the
small prey is great compared to the relatively small benefit. Large
prey, specifically polychaetes and clam siphons, appear to have been
positively selected for, in the sense that these organisms were
probably rarely disregarded. Generally, however, the stomachs of
P. vetulus appeared to be proportional subsamples of the benthic
community, except for the minute organisms and animals too large to
capture.
64
Foraging at close range in a low-visibility medium, the English
sole probably consumes almost every prey it encounters. P. vetu1us did
not appear to be adversely affected by the almost complete removal of
an important food item, harpacticoid copepods, from the site
immediately after jetty completion; they simply consumed more of the
other prey organisms. They fed extremely efficiently on many prey
types by substituting one dominant organism for another as the prey
abundances fluctuated over the 3-year period (e.g., as the copepods
were a major food in 1980 and the cumaceans, in 1983). Additionally,
there was no apparent minimal threshold of prey concentration below
which the prey were ignored. P. vetu1us continued to feed on prey
(such as cumaceans, amphipods, mysids, and brittle stars) even when it
occurred in very low densities. The suggestion that the English sole
is an opportunistic feeder (Pearcy &Hancock, 1978) is supported here.
The diet of the 1st- and 2nd-year English sole (68-178 mm TL)
examined in this study was more similar to the diet observed for the
adult English sole (230-450 mm 5L) examined by Hogue (1982) than that
of the recently settled fish (17-87 rom 5L) studied by Kravitz et a1.
(1977). The juvenile fish studied here contained elements of both
these diets and probably represent a typical diet of intermediate-
sized English sole. Hogue (1982) found a significant difference in the
prey consumed by O-age English sole and those greater than 35 rom
standard length. Feeding data from the present study appear to
represent a transitional diet between the small fish which concentrate
on a high diversity of exclusively small prey types (approximately
65
0.5-1.5 mm long: copepods, nematodes, juvenile bivalves, and polychaete
palps) and juveniles and adults which utilize a few dominant and larger
prey (1.5-4 mm: amphipods, cumacea, and polychaete annelids).
Interactions Between the Predator and
Prey Populations: Implications
and Suggestions
The Index of Relative Importance (IRI) (Figure 11) (Pinkas et al.,
1971) for the polychaetes was higher in 1980 than might be expected,
considering their relatively low (apparent) density in the benthic
samples in that year (Figures 5 and 11). This discrepancy might
reflect a degree of selectivity for polychaetes so that a more constant
level of predation is maintained in spite of fluctuations in the
relative densities of polychaetes in the benthos. Almost as a rule,
Polychaeta annelids present the predator with a large, very nutritious
item with very little indigestible hard parts. As opposed to many
smaller prey, probably few predators, if any, even in a very productive
environment would disregard a polychaete to search for alternative
prey. Additionally, ~ vetulus may have adopted a new strategy to
forage for certain polychaetes that were numerous after 1980. In 1980,
when there was a more even distribution of polychaete families in the
study site, English sole may have had a more general foraging strategy,
feeding on a wide range of worms. As spionid and capitellid worms
began far to outnumber the rest, fish may have become efficient at
finding and eating these worms.
66
900 -r----------------------------~
53.8
-.:'
•1
.;.
•
".!!o
::J
":~
"l:Co
800
700
600
500
400
300
200
100
12500
10000
7500
5000
2~00
o
23.2 43.6
POLYCHAETES BIVALVES
CZZJ 1980
~ 1981
IlS883 1982
15.8
0.8
1.2
AMPHIPODS CUMACEANS NEMATODES
IRI
~l-~LJRI]
NEMATODES
1980 1981 1982
L-I_...JJ f';SSSSJ ••
FIGURE 11. Percentage numerical abundance for major prey taxa found in
P. vetulus from the study site (top); Indices of Relative
Importance (IRI) for all major prey (bottom).
67
Although the degree of significance is questionable, data for the
number of intact clams and clam siphons consumed suggest that the sole
likewise exhibited a preference for clam siphons. Despite an apparent
decrease in the density of bivalves in the sediment in 1982 (and a
concurrent decrease in the numbers of intact bivalves eaten by
~ vetu1us), the proportion of siphons observed in the diet appeared to
have increased during this time.
A similar but less marked trend was observed for polychaete
annelid pa1ps between 1981 and 1982.
Hogue (1982) suggested that the consumption of parts of
macrobenthic organisms (e.g., clam siphons, polychaete pa1ps) rather
than the whole animal is related to the maximum size of food capable of
being captured and consumed. Large po1ychaetes and bivalves might be
difficult to displace from the sediment due solely to their mass and
depth; ingestion of these large items may be inefficient or impossible
for juvenile fish. As the size of these animals increases, the
available appendages become not only a more nutritious item but also a
more noticeable one. As these items get larger and easier to locate,
the sole might also become more efficient behaviorally at locating and
clipping these prey.
It was suggested earlier that the mean size of bivalves in the
study site increased between 1980-1982. A size increase in the bivalve
population, making the siphons a more preferable food and the whole
clams increasingly unavailable, would help to explain the trend
observed for clam siphons and possibly the po1ychaeta pa1ps.
68
Furthermore, a learned foraging behavior which concentrated on this
abundant new food source may have allowed the sole to forage
efficiently for them even in the case of a decline in the bivalve
abundance. The apparently contradictory trends, then, may reflect the
increased use of the siphons in lieu of the larger, more difficult to
get, and less nutritious whole clams. If this was in fact the case,
then the increase in the proportion of siphons in the diet might
support the suggestion made by Hogue of size-related prey selection--a
feeding preference for food of a greater nutritional value per unit
effort expended by the predator (larger prey). Although it is
conceivable that polychaete palps were also eaten in greater numbers
for similar reasons, the increase might be more simply explained by the
increase of palp-bearing polychaetes (in particular, the spionids) in
1981-1982.
As mentioned briefly, the number of amphipods eaten by the sole
did not vary much from year to year, in spite of the substantial
decrease seen in their abundance between 1980 and 1982. Steele et al.
(1970) found that even in an environment in which amphipods were a
large proportion of the total biomass suitable to plaice as food,
amphipods formed only a small but relatively constant part of the diet.
He proposed that their availability is effectively low due to the
fish's difficulty in capturing the amphipods1, but that the stimulus to
1Supported by observations of plaice feeding in aquaria (Steele et
al., 1970).
69
feed on them always exists due to their constant and substantial
numbers.
Summary
Foraging along the bottom, ~ vetu1us probably encounters a
somewhat patchy, productive, but interspersed distribution of mixed
prey types and, with a variety of feeding behaviors, is able to locate
and capture many types of prey. The data indicate a substantial
increase in the density of benthic invertebrates in the disturbed study
area, suggesting that a relatively productive area now exists there.
Because a food-dense environment offers a lower mean search time per
item than does a food-sparse environment (and the study site appears to
be relatively food dense), Pianka (1978) predicted that an optimal
consumer should restrict its diet to only the better types of food
items in a food-dense environment. As no such obvious preferences were
apparent in the diet of ~ vetu1us, a number of concessions might be
made. English sole may not be optimal foragers under any conditions;
they may search randomly and capture anything they can as it is
encountered. Secondly, the benthic invertebrate community may not
present the density of organisms to the fish that the benthic samples
suggest. Although 90% of all benthic forms occur in the upper 1 em of
the substrate, only part of the constituents are probably available at
anyone time. Additionally, any given prey type is not likely to be
distributed evenly or randomly through the sediment, and the fish may
only encounter certain prey in patchy and unpredictable intervals.
70
Lastly, there may not be a great deal of variability in the quality of
prey; until such a level is reached where the energy spent capturing
and eating an item exceeds its worth, the fish will eat anything it
encounters, only foregoing one item to search for a second if the
second is abundant and available enough.
The data suggest, and observations support, the notion that
P. vetu1us feed at close proximity to the substrate and the associated
prey organisms. In the low-visibility environment of a bottom fish,
particularly in the fine sediments of the study site, prey organisms
would be encountered in a fairly random and density-proportional
fashion, especially given a patchy distribution of invertebrates, as in
the study site. In this low-visibility environment, the predaceous
fish will necessarily closely approach all prey items just to recognize
it. At this point, so little additional energy need be spent to
capture the prey that it generally will be. Therefore, P. vetu1us
would be expected to demonstrate a generalized diet, and food
preference will probably be minor and will not be commonly observed.
If a high enough density of predictable prey types (e.g., sedentary
po1ychaetes or bivalves) is available, then a simple modification in
the predator search and capture strategies will likely provide the
predator with a greater feeding efficiency on that abundant prey. A
preference might also be observed in the situation proposed for
~ vetu1us, if less nutritious and/or more difficult prey (e.g.,
amphipods) are occasionally disregarded, making the remainder of the
prey relatively preferred.
71
Finally, the effect that the benthic fish population will have on
the invertebrate communities within the site can only be open to
speculation. The feeding intensity of the juvenile English sole seems
great enough, and their densities high enough, to have a profound
effect on the abundance of the benthic invertebrates in the area.
Although no such trend has been observed, or can easily be
distinguished from the effects of the jetty alteration, the diet,
growth, and reproductive success of ~ vetulus and other bottom fishes 2
will surely be affected by the effect of predation and the subsequent
changes in prey abundance.
2Besides English sole, Starry flounder, Sand sole, Speckled and
Pacific sanddab, among other nonflatfish demersal species, utilize
benthic invertebrates to some extent or depend entirely on these
organisms.
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