NOAA Technical Memorandum NMFS NE 159
An Overview and History
of the Food Web Dynamics Program
of the Northeast Fisheries
Science
Center,
Woods Hole, Massachusetts
by Jason S. Link and Frank P. Almeida
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods
Hole, MA 02543
Print
publication date October 2000;
web version posted November 26, 2004
Citation: Link JS, Almeida FP. 2000. An Overview and History
of the Food Web Dynamics Program
of the Northeast Fisheries
Science
Center, Woods Hole, Massachusetts. US Dep Commer, NOAA Tech Memo NMFS NE 159; 60 p.
Download complete PDF/print version
Abstract
We
provide an overview of the National Marine Fisheries Service (NMFS),
Northeast Fisheries Science Center (NEFSC), Food Web Dynamics Program
(FWDP). The FWDP’s food habits database is one of the largest in
the world and extends from 1973 to present. This database covers
the entire Northeast U.S. Continental Shelf Ecosystem and has over 250,000
stomachs from more than 120 predators, with more than 1,200 different
types of prey collected. We discuss the differences in sampling
protocols and priorities over the history of the program, and address
issues of time-series continuity. For most species, diet can be
adequately characterized with an examination of 500-1,000 stomachs. The
basic diet composition for 38 predators, including economically and ecologically
important species, demonstrates that most members of the fish community
of this ecosystem are generalists, exhibiting a broad diet as either
a benthivore, planktivore, or piscivore. Many major ecosystem and
multispecies issues in fisheries management can only be addressed with
a knowledge derived from food habits data such as those described in
this document.
INTRODUCTION
As the history of the fishes themselves would not be complete
without a thorough knowledge of their associates in the sea, especially
such as prey upon them or in turn constitute their food, it was considered
necessary to prosecute searching inquiries on these points....
Spencer Baird (1873)
In his seminal report to Congress published in 1873, Baird called for a
research program to explore five potential causes of declines in fish stocks
in Southern New England waters, and thereby established the precursors
to the Woods Hole scientific
community and NMFS. Two of the five major causes proposed by Baird consider
trophic dynamics.
The research objectives of the FWDP are to: 1) assess predation mortality
relative to fishing mortality for commercially important fishes; 2) mechanistically
and predictively model species interactions that impact the status of these
stocks, particularly critical life stages; 3) relate changes in diet to changes
in population level growth rates; and 4) better understand the Northeast U.S.
Continental Shelf Ecosystem. In this document, we provide a history of
food web dynamics research, review the food habits data collection and analytical
methods, and give an overview of the FWDP food habits database at the NEFSC.
PROGRAM HISTORY
Baird’s (1873) report was in direct response to declining fish populations
in nearshore Southern New England waters. In contrast, there were consistently
productive offshore fisheries for Atlantic halibut, haddock, Atlantic cod,
and similar species for the next seven or eight decades. However, by
the 1950s, after the halibut fishery had effectively collapsed, there were
indications that haddock populations were also declining. Specific surveys
were initiated in the late 1950s and were expanded in the 1960s to track these
trends. Additionally, collection of basic biological information on haddock
had begun in 1953 to determine the relationship between the distribution and
abundance of this species and the availability of benthic fauna (USFWS 1954). Since
there was already an active Benthic Ecology Program (BEP) at the Woods Hole
Laboratory (Steimle et al. 1995) from Baird’s inclusive interest in
all aspects of the ecosystem, and since haddock are principally benthivores,
the BEP undertook examination of haddock stomachs (Table
1). From the BEP perspective, the examination of stomachs was an
alternative sampling method to categorize the benthic fauna of the region. In
1963, a standardized bottom trawl survey was initiated (see “Methods” section),
designed to provide quantitative abundance indices for virtually all finfish
species and to concurrently collect selected biological data as was feasible. Opportunistic
stomach sampling continued on these surveys until 1966.
By the late 1960s, there was a clear, documented decline in the gadid-flatfish
species complex of fish. In addition, Atlantic herring began a rapid
decline in the late 1960s, followed by Atlantic mackerel in the early 1970s. Given
this ubiquitous decline, a multispecies management scheme was formalized in
1974 to include a two-tiered quota system; the first tier placed single species
total allowable catch limits on the international fleet prosecuting fisheries
in the Northwest Atlantic, while the second placed a limit on the overall multispecies
catch (Hennemuth and Rockwell 1987). This approach reaffirmed the need
for broad biological information from as many species as possible.
Protocols for stomach sampling during 1969-72 varied and were generally categorical
due to a lack of resources. The efforts during this period identified
the major trophic interactions during a time of precipitous change in the fish
community, and characterized the diets of major groundfish species (Maurer
1975; Maurer and Bowman 1975; Bowman et al. 1976; Edwards and Bowman
1979; Langton and Bowman 1980; Bowman 1981).
In 1973, the Feeding Ecology Project (FEP) was formed to initiate more systematic
stomach collections. In the mid- to late 1970s, the issue of recruitment
became increasingly important due to a massive decline in fish biomass. At
this time, Atlantic herring and Atlantic mackerel were added to the stomach
sampling protocol due to their potential as larval fish predators (Jossi and
Marak 1983). Two new programs were formed in response to the change in
focus toward factors affecting recruitment. The Food Chain Dynamics Investigation
(FCDI) was formed to extend the work of the FEP, while an Ecosystem Modeling
Investigation (EMI) primarily studied the mortality of prerecruits. The
need to quantify species interactions was also recognized as an important issue,
and reports detailing refined and quantified characterizations of the diet
for many major species were produced as a first step to address this issue
(Michaels and Bowman 1982; Bowman and Michaels 1984).
In the late 1970s and early 1980s, the focus of the programs remained on attempting
to link recruitment variability with predation, particularly to match predator
consumption rates to observed larval fish mortalities (Grosslein et al.
1980; Sissenwine 1984; GLOBEC 1991). Additionally, attempts to model
the energy and production budgets of the region, especially Georges Bank, were
initiated. The emphasis on the importance of predation was extended to
all life stages for more species throughout the 1980s, with continual effort
given to refining estimators of diet and consumption. As the fish community
continued to undergo drastic changes, studies to evaluate these patterns were
also undertaken.
Through the early 1990s, an emphasis continued on the importance of predation,
particularly larval mortality (Almeida et al. 1999). In the mid-1990s,
the FWDP was formed to extend efforts of the FCDI and EMI, with a focus on
the fish community as an entity. Trophodynamic, aggregate biomass, and
multispecies modeling efforts were initiated to continue exploring the causal
mechanisms responsible for the observed changes in the fish community. The
current, broad focus of the FWDP is to examine all trophic aspects of ecosystem
dynamics in the U.S. waters of the Northwest Atlantic.
METHODS
DATABASES
The FWDP has two major sources of data. Both sources provide primarily
stomach content information, i.e., diet composition, total and individual
prey weights or volumes, and prey length. The more extensive source
is the standard, multispecies, NEFSC Bottom Trawl Survey Program. These
surveys were designed to monitor trends in abundance and distribution
and to provide data and samples to study the biology and ecology of the
fishes and invertebrates inhabiting the Northeast U.S. Continental Shelf
Ecosystem. During the surveys, food habits data are collected for
a variety of species.
Additionally, "process-oriented" cruises are conducted periodically
to address specific questions related to the trophic dynamics of the
fishes in the ecosystem. While an important component of the overall
trophic dynamics research program, the data from these cruises are not
included in this report. Other databases, not described in this
document, encompass the prey fields of these fishes and include zooplankton,
ichthyoplankton, and benthos.
THE SURVEYS
The series of standardized bottom trawl surveys conducted in the Northwest
Atlantic from Cape Hatteras, NC, to Nova Scotia (approximately 85,300
nm2, or 293,000 km2) is the central element in
a broadscale ecological and fishery research program at the NEFSC (Figure
1). Surveys have been conducted continuously in the fall since
1963, and in the spring since 1968; seasonal surveys have also
been conducted in summer and winter on an intermittent basis.
Trawl stations are selected using a stratified random design that provides
unbiased estimates of a species availability to the trawl gear in relation
to distribution. Strata were defined based on water depth, latitude,
and historical fishing patterns. Within each stratum, stations
are assigned randomly. The number of stations allotted to a stratum
are in proportion to its area (approximately one station per 200 nm2,
or 690 km2); however, a minimum of two stations are assigned
to small strata in order to calculate their means. The surveys
are conducted in depths of approximately 27-366 m; however, greater depths
are occasionally sampled in the deep canyons along the continental shelf
break. Once onboard, the catch is sorted and weighed (to the nearest
0.1 kg) by species, with individuals measured (to the nearest 1.0 cm)
and categorized by sex and maturity stage. Subsamples of key species
are eviscerated for feeding ecology and other studies. Geographic
location, depth, and hydrographic data are also collected at each station. A
complete description and evaluation of the Bottom Trawl Survey Program
can be found in Grosslein (1969), Azarovitz (1981), and SWG/NEFC (1988).
Processing the Catch
From 1963 to 1992, all species were weighed and measured prior to biological
sampling. If a species was also destined to be sampled for other
biological studies (e.g., feeding ecology, age and growth, maturity
staging), each individual fish processed was measured a second time. This "double
measuring" invariably resulted in some inconsistent, mismatched fish
lengths between the overall length frequency records for the species
and the individual lengths recorded for biological samples. In
addition, routine biological samples were generally collected using length
categories (usually 10 cm), with sampling targets based on a 6-hr watch
schedule. For example, the scientific crew were required to process
10 silver hake ranging in length from 0-10 cm, 10 ranging in length from
11-20 cm, etc., for special sampling per watch.
Beginning with the 1992 winter survey, the ondeck processing protocols
were modified to: 1) eliminate "double measuring", 2) give individual
identification numbers to each fish sampled for biological studies, 3)
sample on a one-individual-per-one-centimeter basis, and 4) require that
samples be collected at every station.
Food Habits Sampling
During 1963-66, the food habits sampling protocol required examination
of a random selection of fish species from each station for at-sea prey
identification and a qualitative estimate of diet composition. No
criteria were set for numbers of samples to collect; the number of stomachs
examined was determined by the length of time available between trawl
stations and the expertise of the onboard staff (Langton et al.
1980). From 1969 to 1972, samples were collected from up to 20
(but not less than 5) stomachs from each species caught at each station. Each
stomach sample was preserved by size class at sea in 10% buffered Formalin
for later processing in the laboratory. Because these data have
not undergone standard audit procedures and are currently unavailable
in digital format, they were not used in these analyses.
Beginning in 1973, a systematic approach to collecting stomachs was
initiated. The 1973-80 period was divided into two 4-yr blocks
with different groups of primary demersal and pelagic species sampled
during each block. (See Table 2 for
an overview of stomach samples requested by species during the 1973-99
time series. Appendix A contains detailed sampling protocols and
lists of species.) The survey area was divided into five broad
geographic regions, and a maximum of 100 stomachs per species per cruise,
with no more than 10 per station, were requested (Langton et al.
1980). In addition, from 1977 to 1980, samples from 42 other, less
commonly encountered species were also requested. Individual samples
were preserved at sea in 10% buffered Formalin for further processing. Prey
composition (percentage), weight (0.01 g), number, total stomach weight
(0.01g), and lengths (millimeters) of fish prey were determined upon
examination in the laboratory. Prey identification was to the lowest
taxon feasible.
In 1981, a significant change to the at-sea sampling protocol was made. While
the stomachs of major species such as Atlantic cod, haddock, silver hake,
yellowtail flounder, winter flounder, Atlantic herring, and Atlantic
mackerel continued to be individually preserved, all other species had
prey examined and identified at sea. In addition, a volumetric
measurement of stomach contents (0.1 cm3) was initiated. The protocol
also required sampling a specific number of fish for priority species
per length class at every station (see Appendix
A). Shipboard stomach processing also included percent diet
composition, prey number, and prey lengths. These changes were
implemented because laboratory processing of large numbers of samples
proved too costly, and fish prey identification is assumed to be more
accurate when stomach contents are fresh. This change in protocol
placed an additional burden on seagoing staff to identify both fish and
invertebrate prey while onboard ship. Many workshops were and are
still conducted, and a variety of identification aids have been placed
onboard ship to educate staff in the identification of prey species.
Since 1985, all stomach samples have been processed, and prey identified,
at sea. Because of the time limitation at each station, from 1985
to 1991, systematic sampling focused on eight principal species: white
hake, red hake, pollock, Atlantic herring, Atlantic mackerel, Atlantic
cod, silver hake, and spiny dogfish (with the latter three receiving
the highest priority). Eighteen other species were processed as
time permitted (see Appendix
A). Each
of the eight principal species had target sampling levels per 6-hr watch,
with an overall cap of 50 stomachs per station.
In 1992, with the implementation of new ondeck sample processing protocols
(described earlier), the focus shifted from per-watch targets to per-station,
length-based sampling. During 1992-93, stomach samples from 23
species were requested with maximum levels set by species and station. First
priority was assigned to Atlantic cod, silver hake, spiny dogfish, and
skates. In 1994, after concerns were eliminated that the per-station
protocols would overwhelm the at-sea technical staff, the maxima were
removed.
From 1995 to 1998, the primary objective of sampling continued to be
to provide data to estimate predation on larval and juvenile stages of
fish, particularly gadids, and sampling priorities were given to Atlantic
cod, spiny dogfish, silver hake, Atlantic herring, and fourspot flounder,
with the addition of little, winter, and thorny skates during the spring
surveys.
In 1999, after an examination of data obtained during the previous surveys
to determine an adequate sample size for characterizing predator diets
(see "Stomach Sampling Effort Coverage" section),
the sampling protocol was modified again to collect samples from a broad
variety of species. While a low level of monitoring of historically
priority species was maintained, the priority was shifted to commercially
or ecologically important species that appeared to be undersampled in
previous schemes. The sampling focus reemphasized benthivorous
fishes, and attempted to be more inclusive of the entire fish community. Sample
ranges (in fish length) were set for each species, generally one stomach
per 5-cm length range for most species, and one stomach per 10-cm range
for elasmobranchs. This protocol assumes that small per-station
sample sizes over a wide range of species across many hundreds of stations
will allow for adequate characterization of diets.
DATA PROCESSING
Once collected, whether on log sheets recorded at sea or in the laboratory,
the data are entered into a computerized database (currently Oracle),
audited, documented, and archived for analysis. This database is
maintained on the NEFSC network computer system. In addition to
the data collected from the Bottom Trawl Survey Program, data from the
many special, process-oriented, cruises examining species- or life stage-specific
interactions are also online, but are not included in this analysis due
to their more focused nature.
DATA CONTINUITY ACROSS THE TIME
SERIES
PREY TAXONOMIC RESOLUTION
Taxonomic resolution was more detailed for invertebrate species during
the earlier (pre-1981) period of the database. To correct for potential
differences in the resolution of prey taxonomy between in-lab and at-sea
sampling, we established four prey categories. These categories
span the lowest taxonomic level feasible (often genus and species) to
a very broad class- or phylum-level category (Table
3). For most analyses, invertebrate prey are grouped
into order or family level, while fish prey are maintained at the lowest
level feasible. If specific time periods or prey species are of
interest, a lower taxonomic resolution is appropriate. However,
for most purposes, a broader resolution is preferable given the differences
in protocol across the time series.
WEIGHT-VOLUME CONVERSION
In order to convert from stomach volume to stomach weight (or vice versa)
to account for differences in sampling protocols across the time series,
we executed a least-squares linear regression, with no intercept, to
convert stomach-content data from volume (0.1 cm3) to mass (0.1 g). This
regression was done using all species that had simultaneous weight and
volume measurements. Both a regression for all species combined
and regressions for individual major species were calculated.
A conversion factor for volume to weight of 1.1:1 was determined from
simple linear regression to be the most appropriate coefficient for all
predator species (Table 4). This
coefficient (i.e., 1.1) is similar to those obtained from other
studies (Bowman 1982; Tanasichuk et al. 1991). For those
fishes that are piscivores and molluscivores (e.g., red hake and
goosefish), the coefficient is slightly higher, whereas for those that
are planktivores (e.g., Atlantic mackerel) and other benthivores
(e.g., windowpane and fourspot flounder), the coefficient is slightly
lower, reflecting the different densities of different prey items. The
variation of coefficients among species does not significantly depart
from the overall coefficient of 1.1.
SUMMARY STATISTICS
STOMACH SAMPLING EFFORT
COVERAGE
We plotted the number of stomachs sampled versus the number of prey
species observed in the diet for each species. The stomach sample
size at which an asymptote was reached indicated adequate information
to characterize the diet of that predator. Similar to species-area
curves (Preston 1962), an asymptote indicates a low probability of revealing
novel prey items with examination of additional stomachs.
The number of stomachs examined versus the number of prey items observed
generally indicated that an asymptote was reached between 500 and 1,000
stomachs. Figures 2-4 show the relationships graphically. For
example, we can categorize the general diets of spiny dogfish, silver
hake, Atlantic cod, white hake, pollock, yellowtail flounder, winter
skate, and bluefish without expecting many more additional prey items
in new stomach observations (Figures 2A-F,
and 3A,E). Conversely, the plots indicate
that we do not have adequate sample sizes for such species as smooth
skate, witch flounder, Atlantic halibut, and alewife (Figures
3B-D,F). The species that are specialists such as the planktivorous
Atlantic herring and Atlantic mackerel and the benthivorous thorny skate
(Figures 4A-C) generally reach an asymptote
at a lower number of stomachs than do more omnivorous species such as
spiny dogfish, Atlantic cod, and silver hake.
DATA OVERVIEW
Predators
There are over 250,000 stomach records currently in the database. Predator
sizes range from 1 cm to 2.5 m (Table 5). More
than 120 species have been sampled, with 27 species having more than
2,000 stomachs sampled, and 42 species having more than 200 stomachs
sampled. Approximately 30-40% of the stomachs examined are empty,
varying across species.
Mean stomach contents generally reflect the diet composition (discussed
later) and maximum size of the predator (Table
5). As a group, elasmobranchs are the largest fishes we sample,
are generally piscivorous, and subsequently have the largest mean stomach
contents. Exceptions include some of the skates and rays that feed
primarily on benthos. Many pelagic piscivores also have large mean
stomach contents. Goosefish, Atlantic cod, white hake, pollock,
lumpfish, Atlantic halibut, Atlantic sturgeon, and groupers all have
large mean lengths and mean stomach weights. Other than Atlantic
sturgeon, most of these species are noted piscivores, particularly at
the larger sizes. Conversely, the planktivores have the smallest
mean stomach weights, reflecting their smaller size and zooplankton diet. Most
other species have intermediate stomach weights.
Prey
There are over 500,000 prey records in the database. Prey sizes
range from 0.1 mm to 1 m. There are 1,304 distinct prey items comprising
10 major taxa: arthropods, mollusks, fishes, polychaetes, echinoderms,
cnidarians, poriferans, ctenophores, bryozoans, and urochords. The
top 10 prey items by percentage occurrence for all predators include
decapod crustaceans (principally shrimps), gammarids and other amphipods,
unidentified and miscellaneous fishes (“other fishes”), unidentified
and miscellaneous crustaceans (“other crustaceans”), euphausiids, polychaetes,
ctenophores, cephalopods (principally squids), bivalves, and copepods
(Figure 5).
Tracking the abundance of these groups can indicate major changes in
ecosystem dynamics and foreshadow changes to upper trophic levels, particularly
commercially valuable fishes (Christensen 1996; Jennings and Kaiser 1998). Major
fish prey include northern sand lance, gadids (principally silver hake),
clupeids, anchovies, and Atlantic mackerel. In addition to the
large number of empty stomachs, unidentified fish, unclassified crustaceans,
and well digested prey were observed most frequently in the stomachs,
indicating that much of the observed prey is highly digested and difficult
to identify.
DIET SUMMARIES
Statistical Estimators
Various information can be obtained from stomach content examination
(Hyslop 1980; Bowen 1996; Cortes 1997) depending upon the question being
addressed. Although sampling priorities shifted between species
across the history of the program, most of the major species were sampled
continuously over the time series (Table 2). Given
the slightly different sampling protocols described previously, we treated
each stomach as a random sample in one of three possible statistical
designs: unweighted random, stratified, or two-stage clustered. From
these food habits data, percent frequency of occurrence of prey items,
total stomach contents as either volume or weight, and percent mean diet
composition of prey items can be estimated for a given species.
The percent frequency of occurrence can be calculated as
|
EQ 1 |
where nij is the number of stomachs of predator j in
which prey item i occurs, and Nj is the total number
of predator j stomachs examined.
The simple, unweighted, percent mean diet composition ()
can be calculated as either weight or volume. For weight, it can
be calculated as
|
EQ 2 |
where k represents an individual fish, wij is
the stomach
weight of prey i in predator j, and
|
EQ 3 |
is the total weight of all ni prey species in predator j. Percent
mean diet composition may also be calculated as a ratio of means (Malvestuto
1996),
|
EQ 4, |
inclusive or exclusive of empty stomachs, where
|
EQ 5 |
and
|
EQ 6. |
Although not calculated in this document, these diet parameters can be estimated
across several statistical groups or factors (s). Examples include: 1)
temporal factors such as decade, year (or year blocks), season, month, or
time of day; 2) spatial factors such as geographic region, stratum, or statistical
area; 3) abiotic factors such as depth, sediment type, wind speed and direction,
current speed and direction, temperature, or salinity; and 4) predator factors
such as length, weight, age, condition factor, or sex.
A weighted mean () to estimate mean weight
of prey i in predator j for statistical group s may be
calculated as
|
EQ 7 |
where t represents an individual bottom trawl tow, Njts is
the number of predator j stomachs in tow t for statistical group s, Nts is
the number of tows in statistical group s, and
|
EQ 8. |
If one sums across all statistical groups, the weighted mean of prey i in
predator j () becomes
|
EQ 9 |
where Ns is the number of statistical groups and Njs is
the total number of predator j in statistical group s. Mean stomach
weight of predator j for all prey combined ()
can similarly be estimated for a statistical group as
|
EQ 10 |
where
|
EQ 11, |
and across all statistical groups as
|
EQ 12 |
which, as in our case, if one evaluates all elements in a cluster such that Nts equals
the total number of all tows (Nt), then EQ 12 is a direct
simplification of a two-stage weighted cluster mean (Schaeffer et al.
1990). From EQ 9 and 12, a weighted mean diet composition can also be
estimated for any prey i in predator j:
|
EQ 13. |
We principally report the simple arithmetic (unweighted mean ratio inclusive
of empty stomachs, i.e., EQ 3 with statistical grouping across all
factors) mean diet composition for these predators in this document. Variance
estimators for each of these estimators can also be calculated, with caveats
from normal, Poisson, negative binomial, gamma, lognormal, delta, or similar
statistical distributions (e.g., Zar 1984; Pennington 1996; Tirasin
and Jorgensen 1999). Given the central limit theorem and generally
large sample sizes, we presume underlying normal distributions of the data,
although a delta or delta-gamma approach is appropriate given the large
number of zero values in the database.
Diet of Major Species
The two sharks regularly sampled for food habits have dissimilar
diets. Spiny
dogfish (Figure 6A) consume mostly pelagic prey,
with clupeids, squids, scombrids, ctenophores, shrimps, and other fishes being
major prey items. Smooth dogfish is a benthivore, feeding principally
on decapod crabs (Figure 6B).
The skates are principally benthivores (Figure 7 and Figure 8),
with amphipods, polychaetes, bivalves, and various decapods (crabs and shrimp)
being major prey items. Winter skate is more piscivorous than the other
skates, consuming high proportions of northern sand lance and Atlantic herring
in its diet. Smooth skate is more pelagic than the other skates, consuming
higher proportions of euphausiids and decapod shrimps than other prey in its
diet. All skates have a relatively catholic diet.
The planktivores (e.g., Atlantic herring, Atlantic mackerel, alewife,
northern sand lance, butterfish) have diets dominated by well digested prey,
reflecting both their faster digestion and our difficulty in identifying smaller
prey (Figure 9 and Figure 10). Copepods,
euphausiids, amphipods (primarily hyperiids), mysids, and northern sand lance
are the other major prey items of these fishes. Well digested prey were
likely one or more of these zooplankton or small fish prey items.
Similarly, the squids have a diet that is also dominated by well digested
prey, probably a result of prey mastication from the beak of these predators
(Figure 11). Squids are highly cannibalistic
and piscivorous.
The principal gadids Atlantic cod, haddock, and pollock are dietary generalists
(Figure 12). These three species’ diets form
a continuum from benthic to pelagic prey, with the haddock diet more benthic
(e.g., brittle stars, polychaetes, amphipods), the cod diet in between,
and pollock diet more pelagic (e.g., euphausiids, northern sand lance,
decapod shrimps). Clupeids, northern sand lance, and other gadids (mainly
silver hake, although there is some cannibalism) are the major fish prey of
these species.
The hakes are primarily pelagic predators, consuming mainly euphausiids, clupeids,
squids, decapods, and other gadids (Figure 13 and Figure 14). These
species exhibit a broad diet that is principally fish and/or shrimp in composition.
The flatfishes can be largely categorized as either squid-and-fish eaters
(i.e., Atlantic halibut and fourspot and summer flounders) or worm-and-amphipod
eaters (i.e., windowpane and yellowtail, winter, and witch flounders)
(Figure 15, Figure 16, and Figure 17). The
notable exception is American plaice which primarily consumes echinoderms (Figure
15B) -- more similar to haddock (Figure 12B)
than to other flatfishes. Windowpane and fourspot flounder exhibit a
broad diet. The morphology of these flatfishes suggests benthic feeding,
thus the high degree of piscivory exhibited by some species is noteworthy.
The other major piscivores (e.g., goosefish, weakfish, and bluefish)
consume a broad variety of fishes and squids (Figure
18). Pelagic prey such as anchovies, clupeids, northern sand lance,
longfin inshore squid, and gadids (principally silver hake) are the major dietary
items of these predators, with weakfish consuming prey whose ranges are centered
further to the south (e.g., sciaenids, butterfish), and with goosefish
consuming more benthic prey (e.g., sea robins, bothids, pleuronectids,
skates) than bluefish.
Scup are primarily benthivores, whereas black sea bass and Acadian redfish
consume shrimp and other fishes (Figure 19). Black
sea bass and scup both have a broad range of prey represented in their diets,
but specialize on decapods and polychaetes, respectively.
Sea raven are benthic piscivores, consuming several different fish prey (Figure
20). Longhorn sculpin are also benthic and piscivorous, but primarily
consume a broader mix of benthic invertebrates (i.e., decapod crabs,
amphipods). Similar to American plaice, the ocean pout diet consists
of a high proportion of echinoderms. All three of these fish exhibit
a relatively broad, benthic diet.
DISCUSSION
DIET OVERVIEW
The diet summaries presented here extend previous documentation for
many of these fish species (e.g., Wigley 1956; Sherman et al.
1978; Edwards and Bowman 1979; Grosslein et al. 1980; Bowman 1981,
1983; Cohen et al. 1981, 1982; Langton 1982, 1983; Durbin et
al. 1983; Bowman and Michaels 1984; Bowman et al. 1984; Hahm
and Langton 1984; Overholtz et al. 1991). For further details
on a particular species or species group, we refer the reader to these
more specific documents. In general, we can categorize the diets
of most species with more than 1,000 stomachs examined. For example,
we know that Atlantic cod typically consume a wide mix of benthic invertebrates,
herrings, silver hake, shrimps, and northern sand lance (Figure
12A), and that spiny dogfish typically consume ctenophores, shrimps,
and smaller pelagic fishes (Figure 6A). How
the diets of these species alter across seasons, location, size classes,
or decades is described elsewhere, although the major diet compositions
are generally consistent (Garrison and Link 2000).
Many of the species over the period of this time series have been undersampled
(Table 5, Figure 3B-D,F)
due to logistical constraints and changing priorities. One of the
challenges is to focus on species we know little about despite their
limited commercial value. For example, it would have been difficult
to predict 30 yr ago that goosefish would currently be the most valuable
finfish in the NMFS Northeast Region (Clark 1998), yet, fortunately,
information was collected for this species. These data demonstrated
the importance of this species as a piscivore. These undersampled
species, as well as protocols to better address the frequency of empty
stomachs and well digested prey, merit further examination.
Most species in this fish community are generalists, with a few pelagic
or benthic specialists. Garrison and Link (2000) have categorized
six major feeding groups, including benthivores (e.g., Figure 7, Figure 8, Figure 15B-C,
and 16C), planktivores (e.g., Figure
9 and Figure 10), piscivores (e.g., Figure
18), pelagic (small fish and shrimp) feeders (e.g., Figure 13 and Figure 14), demersal invertebrate feeders
(e.g., Figure 12), and crab specialists
(e.g., Figure 6B). Given the
broad diets, high degree of omnivory (Link 1999), and generalist feeding
nature of most species in this community, most predator-prey interaction
strengths are mild in this ecosystem (Link 1999; Sissenwine et al.
1982). Thus, the population-level impacts of changes in prey (potentially
impacting growth) or predators (potentially impacting survivability)
are likely less significant than if this were an ecosystem of specialists
with strong interactions.
HISTORICAL AND CURRENT
RELEVANCE
We have documented the changing priorities and protocols of the food
habits sampling over the history of this program (Table 1 and Table 2; Appendix
A). Despite these caveats, this is a unique data set to assist
with understanding the ecosystem dynamics of the Northwest Atlantic. To
our knowledge, there are no other data sets that span 25 yr for more
than 120 species over an entire continental shelf.
The importance of the food-habits time series is enhanced given its
potential role in at least partially explaining the notable dynamic nature
of the Northwest Atlantic fish community (Mayo et al. 1992; Boreman et
al. 1997; Clark 1998; Fogarty and Murawski 1998). Briefly,
in the past 30-40 yr, the abundance of commercially desirable gadids
(e.g., Atlantic cod and haddock) and flatfishes (e.g.,
yellowtail flounder) has declined, with a concurrent increase in the
abundance of less desirable elasmobranchs (e.g., spiny dogfish
and skates) and small pelagic species (e.g., Atlantic herring
and Atlantic mackerel). These changes were caused primarily by
a significant increase in fishing pressure exerted on gadids and flatfishes
beginning in the early 1960s with the arrival of distant-water fleets. Even
with the foreign fleets largely displaced from the U.S. Exclusive Economic
Zone in 1977, effective effort on the fish stocks has remained high,
and for many species, stock biomass dropped to historically low levels
in the 1990s due to the increased capacity and efficiency of the domestic
fleet (Clark 1998; Fogarty and Murawski 1998). These phenomena
are not limited to just this ecosystem; other ecosystems around the world
exhibit similar patterns (NRC 1999). There is high heuristic value
in exploring from a trophic ecology perspective why the multispecies
trajectory proceeded as it did in this ecosystem, particularly to assess
how the multispecies trajectory may proceed in the future.
Professor Baird’s (1873) concerns remain appropriate, and are still
relevant to ongoing ecosystem investigations (e.g., Sherman 1991;
Sherman et al. 1993; Christensen et al. 1996; Larkin 1996;
Jennings and Kaiser 1998; NRC 1999). Central to ecosystem considerations
are species interactions. In many food webs, predation can be a
major ecological process affecting fish populations (Sissenwine 1984;
Bax 1991, 1998; Christensen 1996) and the major source of mortality for
fish (e.g., Sissenwine et al. 1984; Keast 1985; Mittelbach
and Persson 1998). Multispecies, trophodynamic, food web, and ecosystem
models are tools to give insights into fish communities where classical
fisheries methods are unable to do so (e.g., Steele 1974; Andersen
and Ursin 1977; Helgason and Gislason 1979; May et al. 1979; Mercer
1982; Kerr and Ryder 1989; Daan and Sissenwine 1991). For example,
how important is natural mortality to a given fish stock (Sissenwine
1984), what are the system-level emergent properties from a fish community
and how are they altered with overfishing (Jennings and Kaiser 1998),
or what levels of biomass tradeoff are we willing to accept among a given
species mix? Quantifying the food habits of these species is at
the heart of these and similar questions.
ACKNOWLEDGMENTS
We
thank the hundreds of individuals who have collected food habits data
over the course of the Bottom Trawl Survey Program. We thank past
and present members associated with the FWDP for their foresight in creating,
auditing, maintaining, and supporting this database of unprecedented
scale and scope, especially (listed alphabetically): R. Bowman,
E. Broughton, R. Edwards, M. Fogarty, R. Fritz, B. Griswold, M. Grosslein,
J. Hauser, M. Hill, B. Kaminer R. Langton, R. Maurer, N. McHugh, C. Milliken,
T. Morris, R. Rountree, R. Theroux, and R. Wigley. We particularly
thank M. Grosslein, M. Fogarty, and W. Michaels for their discussions
to clarify earlier portions of the history of this program. We
thank M. Fogarty, M. Grosslein, C. Milliken, J. Gibson, and W. Gabriel
for their constructive comments on earlier versions of the manuscript.
REFERENCES
CITED
Almeida, F.; Fogarty, M.; Grosslein,
M.; Kaminer, B.; Link, J.; Michaels, W.; Rountree, R. 1999. Georges
Bank predation study: report of the 1994-96 field seasons. Northeast
Fish. Sci. Cent. Ref. Doc. 99-06; 58 p. Available from:
National Marine Fisheries Service, Woods Hole, MA 02543.
Andersen, K.P.; Ursin, E. 1977. A multispecies extension
to the Beverton and Holt theory, with accounts of phosphorus
circulation and primary production. Medd. Dan. Fisk.-Havunders.
7:319-435.
Azarovitz, T.R. 1981. A brief historical review
of the Woods Hole Laboratory trawl survey time series. In:
Doubleday, W.G.; Rivard, D., eds. Bottom trawl surveys. Can.
Spec. Publ. Fish. Aquat. Sci. 58:62-67.
Baird, S.F. 1873. Report on the condition of the
sea fisheries of the south coast of New England in 1871 and 1872.
Part I. Washington, DC: U.S. Commission of Fish and Fisheries;
852 p. Available from: National Marine Fisheries Service,
Woods Hole, MA 02543.
Bax, N.J. 1998. The significance and prediction
of predation in marine fisheries. ICES [Int. Counc.
Explor. Sea] J. Mar. Sci. 55:997-1030.
Bax, N.J. 1991. A comparison of the fish biomass
flow to fish, fisheries, and mammals on six marine ecosystems. ICES [Int.
Counc. Explor. Sea] Mar. Sci. Symp. 193:217-224.
Boreman, J.; Nakashima, B.S.; Wilson, J.A.; Kendall, R.L., editors. 1997. Northwest
Atlantic groundfish: perspectives on a fishery collapse. Bethesda,
MD: American Fisheries Society; 242 p.
Bowen, S.H. 1996. Quantitative description
of the diet. In: Murphy, B.R.; Willis, D.W., eds. Fisheries
techniques. Bethesda, MD: American Fisheries Society; p.
513-532.
Bowman, R.E. 1981. Food of ten species of Northwest Atlantic
juvenile groundfish. Fish. Bull. (U.S.) 79(1):220-226.
Bowman, R.E. 1982. Preliminary evaluation of the
results of analysis of the stomach contents of silver hake (Merluccius
bilinearis) aboard ship and in the laboratory ashore. Woods
Hole Lab. Ref. Doc. 82-25; 13 p. Available from: National
Marine Fisheries Service, Woods Hole, MA 02543.
Bowman, R.E. 1983. Food of silver hake, Merluccius
bilinearis. Fish. Bull. (U.S.) 82:21-35.
Bowman, R.E.; Michaels, W.L. 1984. Food of seventeen
species of Northwest Atlantic fish. NOAA Tech. Memo.
NMFS-F/NEC-28; 183 p.
Bowman, R.E.; Eppi, R.; Grosslein, M. 1984. Diet
and consumption of spiny dogfish in the Northwest Atlantic. ICES [Int.
Counc. Explor. Sea] C.M. 1984/G:27; 16 p.
Bowman, R.E.; Maurer, R.; Murphy, J. 1976. Stomach
contents of twenty-nine fish species from five regions in the
Northwest Atlantic. Northeast Fish. Sci. Cent. Ref.
Doc. 76-10; 37 p. Available from: National Marine Fisheries
Service, Woods Hole, MA 02543.
Christensen, V. 1996. Managing fisheries involving
predator and prey species. Rev. Fish Biol. Fish. 6:417-442.
Christensen, N.L.; Bartuska, A.M.; Brown, J.H.; Carpenter, S.;
D’Antonio, C.; Francis, R.; Franklin, J.F.; MacMahon, J.A.; Noss,
R.F.; Parsons, D.J.; Peterson, C.H.; Turner, M.G.; Woodmansee,
R.G. 1996. The report of the Ecological Society of
America Committee on the Scientific Basis for Ecosystem Management. Ecol.
Appl. 6:665-691.
Clark, S.H., editor. 1998. Status of fishery resources
off the northeastern United States for 1998. NOAA Tech.
Memo. NMFS-NE-115; 149 p.
Cohen, E.; Grosslein, M.; Sissenwine, M.; Serchuk, F.; Bowman,
R. 1981. Stomach content studies in relation to multispecies
fisheries analysis and modeling for the Northwest Atlantic. ICES [Int.
Counc. Explor. Sea] C.M. 1981/G:66; 14 p.
Cohen, E.; Grosslein, M.D.; Sissenwine, M.P.; Steimle, F.; Wright,
W.R. 1982. Energy budget of Georges Bank. In:
Mercer, M.C., ed. Multispecies approaches to fisheries
management advice. Can. Spec. Publ. Fish. Aquat. Sci.
59:95-107.
Cortes, E. 1997. A critical review of methods of
studying fish feeding based on analysis of stomach contents:
application to elasmobranch fishes. Can. J. Fish. Aquat.
Sci. 54:726-738.
Daan, N.; Sissenwine, M.P., editors. 1991. Multispecies
models relevant to management of living resources. ICES [Int.
Counc. Explor. Sea] Mar. Sci. Symp. 193; 358 p.
Durbin, E.R.; Durbin, A.G.; Langton, R.W.; Bowman, R.E. 1983. Analysis
of stomach contents of Atlantic cod (Gadus morhua) and
silver hake (Merluccius bilinearis) for the estimation
of daily rations. Fish. Bull. (U.S.) 81(3):437-454.
Edwards, R.L.; Bowman, R.E. 1979. Food consumed
by continental shelf fishes. In: Clepper, H., ed. Predator-prey
systems in fish communities and their role in fisheries management. Washington,
DC: Sports Fishing Institute; p. 337-406.
Fogarty, M.J.; Murawski, S.A. 1998. Large-scale
disturbance and the structure of marine systems: fishery impacts
on Georges Bank. Ecol. Appl. 8(S1):S6-S22.
Garrison, L.P.; Link, J.S. 2000. Fishing effects
on spatial distribution and trophic guild structure in the Georges
Bank fish community. ICES [Int. Counc. Explor. Sea] J.
Mar. Sci. 57:723-730.
GLOBEC [U.S. Global Ocean Ecosystems Dynamics Program]. 1991. GLOBEC
Northwest Atlantic Program: GLOBEC Canada/U.S. Meeting on N.W.
Atlantic Fisheries and Climate. U.S. Global Ecosyst. Dyn.
Program Rep. 2; 87 p. Available from: U.S. GLOBEC Scientific
Steering Committee Coordinating Office, c/o Woods Hole Oceanographic
Institution, Woods Hole, MA 02543.
Grosslein, M.D. 1969. Groundfish survey program
of BCF Woods Hole. Comm. Fish. Rev. 31(8-9):22-25.
Grosslein, M.D.; Langton, R.W.; Sissenwine, M.P. 1980. Recent
fluctuations in pelagic fish stocks in the Northwest Atlantic
Georges Bank region, in relation to species interactions. In:
Saville, A., ed. The assessment and management of pelagic
fish stocks. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 177:374-404.
Hahm, W.; Langton, R.W. 1984. Prey selection based
on predator/prey weight ratios for some Northwest Atlantic fish. Mar.
Ecol Prog. Ser. 19(1-2):1-5.
Helgason, T.; Gislason, H. 1979. VPA-analysis with
species interaction due to predation. ICES [Int.
Counc. Explor. Sea] C.M. 1979/G:52; 10 p.
Hennemuth, R.C.; Rockwell, S. 1987. History of fisheries
management and conservation. In: Backus, R. ed. Georges
Bank. Cambridge, MA: MIT Press; p. 430-446.
Hyslop, E.J. 1980. Stomach contents analysis --
a review of methods and their application. J. Fish Biol.
17:411-429.
Jennings, S.; Kaiser, M.J. 1998. The effects of
fishing on marine ecosystems. Adv. Mar. Biol. 34:201-352.
Jossi, J.W.; Marak, R.R. 1983. MARMAP plankton survey
manual. NOAA Tech. Memo. NMFS-F/NEC-21; 258 p.
Keast, A. 1985. The piscivore feeding guild of fishes
in small freshwater ecosystems. Environ. Biol. Fish.
12:119-129.
Kerr, S.R.; Ryder, R.A. 1989. Current approaches
to multispecies analysis of marine fisheries. Can. J.
Fish. Aquat. Sci. 46:528-534.
Langton, R.W. 1982. Diet overlap between Atlantic
cod, Gadus morhua, silver hake, Merluccius bilinearis,
and fifteen other Northwest Atlantic finfish. Fish.
Bull. (U.S.) 80:745-759.
Langton, R.W. 1983. Food of yellowtail flounder, Limanda
ferruginea (Storer), from off the northeastern United States. Fish.
Bull. (U.S.) 81(1):15-22.
Langton, R.W.; Bowman, R.E. 1980. Food of fifteen
Northwest Atlantic gadiform fishes. NOAA [Natl.
Ocean. Atmos. Admin.] Tech. Rep. NMFS [Natl. Mar. Fish.
Serv.] SSRF [Spec. Sci. Rep. Fish.] 740; 23 p.
Langton, R.W.; North, B.M.; Hayden, B.P.; Bowman, R.E. 1980. Fish
food habits studies -- sampling procedures and data processing
methods utilized by the Northeast Fisheries Center, Woods Hole
Laboratory, U.S.A. ICES [Int. Counc. Explor. Sea] C.M.
1980/L:61; 16 p.
Larkin, P.A. 1996. Concepts and issues in marine
ecosystem management. Rev. Fish Biol. Fish. 6:139-164.
Link, J. 1999. Reconstructing food webs and managing
fisheries. In: Baxter, B., ed. Ecosystem approaches
for fisheries management: proceedings of the Symposium on Ecosystem
Considerations in Fisheries Management, September 30 - October
3, 1998, Anchorage, Alaska. Alaska Sea Grant Rep. AK-SG-99-01:571-588. Available
from: University of Alaska Sea Grant, Fairbanks, AK 99775.
Malvestuto, S.P. 1996. Sampling the recreational
creel. In: Murphy, B.R.; Willis, D.W., eds. Fisheries
techniques. Bethesda, MD: American Fisheries Society; p.
591-624.
Maurer, R.O. 1975. A preliminary description of
some important feeding relationships. Int. Comm. Northw.
Atl. Fish. Res. Doc. 75/IX/130; 15 p.
Maurer, R.O.; Bowman, R.E. 1975. Food habits of
marine fishes of the Northwest Atlantic. Woods Hole
Lab. Ref. Doc. 75-03; 90 p. Available from: National
Marine Fisheries Service, Woods Hole, MA 02543.
May, R.M.; Beddington, J.R.; Clark, C.W.; Holt, S.J.; Laws,
R.M. 1979. Management of multispecies fisheries. Science 205:267-277.
Mayo, R.K.; Fogarty, M.J.; Serchuk, F.M. 1992. Aggregate
fish biomass and yield on Georges Bank, 1960-87. J.
Northw. Atl. Fish. Sci. 14:59-78.
Mercer, M.C., editor. 1982. Multispecies approaches
to fisheries management advice. Can. Spec. Publ. Fish.
Aquat. Sci. 59; 169 p.
Michaels, W.L.; Bowman, R.E. 1982. Food of seventeen
species of Northwest Atlantic fish. Part II. Examination by year
and listing of prey species. Woods Hole Lab. Ref. Doc.
82-17; 117 p. Available from: National Marine Fisheries
Service, Woods Hole, MA 02543.
Mittelbach, G.G.; Persson, L. 1998. The ontogeny
of piscivory and its ecological consequences. Can. J.
Fish. Aquat. Sci. 55:1454-1465.
NRC [National Research Council]. 1999. Sustaining
marine fisheries. Washington, DC: National Academy Press;
164 p.
Overholtz, W.J.; Murawski, S.A.; Foster, K.L. 1991. Impact
of predatory fish, marine mammals, and seabirds on the pelagic
fish ecosystem of the northeastern USA. ICES [Int.
Counc. Explor. Sea] Mar. Sci. Symp. 193:198-208.
Pennington, M. 1996. Estimating the mean and variance
from highly skewed marine data. Fish. Bull. (U.S.) 94:498-505.
Preston, F.W. 1962. The canonical distribution of
commonness and rarity. Ecology 43:185-215, 431-432.
Schaeffer, R.L.; Mendenhall, W.; Ott, L. 1990. Elementary
survey sampling. Boston, MA: PWS-Kent; 390 p.
Sherman, K. 1991. The large marine ecosystem concept:
research and management strategy for living marine resources. Ecol.
Appl. 1:349-360.
Sherman, K.; Alexander, L.M.; Gold, B.D., editors. 1993. Large
marine ecosystems: stress, mitigation and sustainability. Washington,
DC: AAAS Press; 376 p.
Sherman, K.; Cohen, E.; Sissenwine, M.; Grosslein, M.; Langton,
R.; Green, J. 1978. Food requirements of fish stocks
of the Gulf of Maine, Georges Bank, and adjacent waters. ICES [Int.
Counc. Explor. Sea] C.M. 1978/Gen:8 (Symp.); 14 p.
Sissenwine, M.P.; Brown, B.E.; Palmer, J.E.; Essig, R.J.; Smith,
W. 1982. Empirical examination of population interactions
for the fishery resources off the northeastern USA. In:
Mercer, M.C., ed. Multispecies approaches to fisheries
management advice. Can. Spec. Publ. Fish. Aquat. Sci.
59:82-94.
Sissenwine, M.P. 1984. Why do fish populations vary? In:
May, R.M., ed. Exploitation of marine communities. New
York, NY: Springer-Verlag; p. 59-94.
Sissenwine, M.P.; Cohen, E.B.; Grosslein, M.D. 1984. Structure
of the Georges Bank ecosystem. Rapp. P.-V. Reun. Cons.
Int. Explor. Mer 183:243-254.
Steele, J.H. 1974. The structure of marine ecosystems. Oxford,
England: Blackwell Scientific; 128 p.
Steimle, F.W.; Burnett, J.M.; Theroux, R.B. 1995. A
history of benthic research in the NMFS Northeast Fisheries Science
Center. Mar. Fish. Rev. 57(2):1-13.
SWG/NEFC [Survey Working Group/Northeast Fisheries Center]. 1988. An
evaluation of the bottom trawl survey program of the Northeast
Fisheries Center. NOAA Tech. Memo. NMFS-F/NEC-52;
83 p.
Tanasichuk, R.W.; Ware, D.M.; Shaw, W.; McFarlane, G.A. 1991. Variations
in diet, daily ration, and feeding periodicity of Pacific hake
(Merluccius productus) and spiny dogfish (Squalus acanthias)
off the lower west coast of Vancouver Island. Can. J.
Fish. Aquat. Sci. 48:2118-2128.
Tirasin, E.M.; Jorgensen, T. 1999. An evaluation
of the precision of diet description. Mar. Ecol. Prog.
Ser. 182:243-252.
USFWS [U.S. Fish and Wildlife Service]. 1954. Annual
report of the Woods Hole Laboratory for the year ending June
30, 1954. Washington, DC: U.S. Fish and Wildlife Service;
14 p.
Wigley, R.L. 1956. Food habits of Georges Bank haddock. U.S.
Fish Wildl. Serv. Spec. Sci. Rep. Fish. 165; 26 p.
Zar, J.H. 1984. Biostatistical analysis. Englewood
Cliffs, NJ: Prentice-Hall; 718 p.
Stomach Sampling Protocols
Used during NEFSC Bottom Trawl Surveys, 1963-99
STOMACH SAMPLING PROTOCOL DURING 1963-66
General procedures are as follows:
1. After completion of other duties, randomly select
fish from baskets for stomach content examination.
2. Record cruise number, station number, and species sampled
at top of log. Record length (cm), sex, and maturity stage of each fish
examined.
3. Dissect out stomach and empty contents onto measuring board. Sort,
identify, and record prey items (record as empty when applicable).
4. Measure all fish and crab prey (total or fork length for fish
and carapace width for crabs).
STOMACH SAMPLING PROTOCOL DURING
1967-72
Preserve in 10% neutral buffered Formalin up to 20 (but
not less than 5) stomachs per species per catch, selecting individuals
at random (i.e., without regard to size) from the sorted catch. If
time does not permit sampling all those species with >5 individuals
in each catch (usually there are 4-6 species with >5 individuals),
then give first priority to the most abundant species, but seek to
obtain some samples of all but the rare species for the entire cruise. Note
that certain species (e.g., Atlantic herring, silver hake)
may yield only small or occasional catches but they should get high
priority because they represent a large biomass.
In a few cases when there is a very wide range in size
for a given species (e.g., “small” versus “whale” size category
of Atlantic cod), random selection will not be suitable, and separate
samples from each size category will be desirable.
Label jar tops with cruise number, station number, species,
and number of stomachs, and repeat data on label inside jar. Do
not overfill jars; allow about 2-inch air space at top.
STOMACH SAMPLING PROTOCOL DURING
1973-76
Stomachs will be collected only from species listed
in Table
A1. Fifty stomachs should be collected for each
species in each strata set per cruise. No more than 10 stomachs
per species should be collected for any one station, and the same
species should not be sampled at two consecutive stations.
When the catch of a species is large, the stomach samples
should be taken randomly with respect to fish length. When
fish are small, they should be preserved whole after puncturing the
gut cavity. Only one species shall be placed in any one jar. After
removing the stomach from a large fish, make a label showing cruise
number, station number, maturity stage, species, sex, and length
(cm). Wrap the stomach and the label in cheesecloth, and secure
with a cable tie. Stomachs from the same species collected
at the same station may be preserved together in one jar. NEVER
PUT MORE THAN ONE SPECIES OR FISH FROM DIFFERENT STATIONS IN THE
SAME JAR. Label the jar cap with cruise number, station number,
species, and the number of stomachs in the jar. See Table
A2 for examples of stomach and jar cap labels. Use
10% Formalin as a preservative.
Collect 50 young of the year (YOY) of the species listed
in Table
A1 for each strata set. They should be preserved
whole, and the jar cap labeled as aforementioned. The lengths
which are less than or equal to those listed in Table A1 signify
YOY. Be sure to slit the gut cavity.
If it appears that the necessary stomachs from each
strata set will not be collected as the cruise progresses, disregard
the above directions and take all stomachs possible until 50 have
been collected for each species, then collect using the normal procedure.
STOMACH SAMPLING PROTOCOL DURING
1977-80
SAMPLING ROUTINE
Fish and squid stomachs will be collected from the species
listed in Table A3, Table A4,
and Table A5. The
collection is based on three separate needs as follows: 1)
offshore priority species, 2) inshore priority species, and 3) miscellaneous
species. Stomach sampling takes priority over any other non-NEFC
sampling.
On all groundfish cruises sampling inshore strata, the
inshore priority species listed in Table
A4 will be sampled for food habits studies. Collect
150 fish representing the entire size range of each species listed.
The species chosen for the miscellaneous collection
were selected because little or no food habit data have previously
been collected, or additional information about their food habits
is needed. Table
A5 is a guide to those miscellaneous species needed from
specific areas, and to those miscellaneous species needed from any
area. Many of the fish included in this collection are not
very abundant, and few will probably be encountered.
SAMPLING METHODS
Stomach samples should be taken from fish representing
the length range of a given species caught at any station. It
is important that the largest and smallest fish be included in the
collection, along with the predominant size group (i.e., stratified
by length). No more than 10 stomachs per given species need
to be collected at any one station, and the same species should not
be sampled at two consecutive stations. If it appears that
the necessary number of stomachs will not be collected as the cruise
progresses, disregard the above directions and take all stomachs
necessary to complete the collection, then sample normally.
Stomachs from large fish will be excised, and a label
indicating cruise number, station number, species, length (cm), sex,
and maturity stage will be completed (see Table
A6), and together they shall be wrapped in cheesecloth
and secured with a cable tie. Smaller fish may be preserved
whole after puncturing the gut cavity and completing a label denoting
cruise number, station number, and species. Stomachs of the
same species collected at the same station may be preserved together
in one jar. NEVER PUT STOMACHS OF MORE THAN ONE SPECIES OR
ONE STATION IN THE SAME JAR. Use 10% Formalin as a preservative. Label
the jar cap with cruise number, station number, species, and number
of stomachs in the jar (see Table A6). List stomachs collected on
the tally sheet provided by the Food Habits Project for your
convenience.
Juvenile fish may be identified by utilizing the lengths
listed with each species in Table
A3. The lengths of fish less than or equal to those
indicated signify juvenile fish.
Additional samples may be collected if time permits.
STOMACH SAMPLING PROTOCOL DURING
1981-84
The Feeding Ecology Project requires stomach content
samples from the species listed in Table
A7 whenever they occur in trawl catches. It is especially
important that we document the size and species of fish prey being
consumed by piscivorous predators in the Georges Bank - Nantucket
Shoals area (Strata 9-25). Predators of immediate concern are
silver hake, Atlantic cod, and spiny dogfish. Sampling of these
and other species, which are listed in their order of priority in
Table A7, should be conducted as follows.
PRESERVED SAMPLES
Stomach contents (and stomach tissue) of silver hake,
Atlantic cod, yellowtail flounder, haddock, winter flounder, Atlantic
mackerel, and Atlantic herring are to be preserved according to the
length categories and numbers given in Table
A7, throughout the entire survey area. Stomachs
from different species, length categories, or stations should never
be put in the same jar. Small fish may be preserved whole. All
jars shall be labeled on the inside and outside (jar cap) to indicate
ship, cruise number, station number, species, number of fish, and
length category. Samples will be preserved in 10% Formalin.
STOMACH CONTENTS EXAMINED
AT SEA
Stomach contents of spiny dogfish, pollock, white hake,
red hake, and goosefish are to be examined at sea. Details
are given in the “Procedure for Examining Stomach Contents at Sea
[1981-84]” section. If time permits, the examination of nonpriority
species such as large sharks, skates, rays, summer flounder, bluefish
(i.e., fish eating species) or predominant species in the
catch, other than those listed in Table
A7, will be useful to personnel in the Feeding Ecology
Project. Any additional samples received will be appreciated.
NUMBER OF SAMPLES
If sampling time becomes critical at any particular
station, only five randomly chosen individual fish of the three top
priority species (i.e., silver hake, Atlantic cod, and spiny
dogfish) need be sampled. No more than 40 individual fish,
in total, need be sampled at any one station under any circumstances.
PROCEDURE FOR EXAMINING STOMACH
CONTENTS AT SEA [1981-84]
1. Record pertinent information such as vessel, cruise
number, station number, and predator species in the space provided
at the top lefthand corner of the log.
2. Inspect the buccal cavity (inside of mouth) for signs
of regurgitated food and the esophageal area (via the body cavity)
for inversion. Also check the gills to be sure the fish was
freshly caught (white or pink gills indicate the fish was probably
from a previous station). If any of the above conditions exist,
discard the fish.
3. Determine the length (cm), sex, and maturity stage
of the fish, and record this information on the appropriate line
of the log (simply circle the sex and maturity stage; i.e.,
F = female, M = male, I = immature, D = developing, R = ripe [or
spawning], S = spent, and Rt = resting).
4. Excise the stomach and empty the contents onto a
tray or sieve. The fullness measurement is recorded on the “Fullness” line
of the log as the estimated volume (in cubic centimeters) made up
by the stomach contents. If the stomach is empty, record the
fullness measurement as “0” and proceed to the next fish. The
volume may be estimated to the nearest 0.5 cm3 by comparing the appropriately
sized, premeasured piece of wood to the volume made up by the stomach
contents.
5. It will be helpful if the relative state of digestion
of the prey items is recorded. If the stomach contents (as
a whole) appear relatively fresh (“FR”), partly digested (“PD”),
or well digested (“WD”), they should be recorded as such in the “State
of Digestion” column of the log.
6. The stomach contents may now be spread apart and
separated into prey groups. An estimate of the percentage that
each prey group makes up of the total stomach content volume should
be written on the line of the log corresponding to the particular
prey. If prey can readily be identified to species, the name
may be written into one of the open spaces, and the percentage recorded
as indicated earlier. The number and size (or mean size) of
organisms may also be recorded, either along with the percentage,
or in the “Comments” section of the log. A small number of
prey species (or groups) usually account for a large percentage of
a fish’s food. Some organisms are common food for many species
of fish. Organisms repeatedly noted in the stomachs of a particular
species (or in the stomachs of several species), and not identified
to species, should be preserved in 10% Formalin and brought back
to the laboratory for identification. The stomach contents
of all fish recorded on a particular log should be saved in the same
jar when returning material to the laboratory for identification. The “Pisces” (i.e.,
fish) column of the log should never be used if the fish found in
the stomach can be identified to any lower classification. The
length(s) and number of fish prey should always be recorded (the
total length at time of ingestion may be estimated if fish prey are
in pieces or partially digested).
7. Space for pathological information is provided at
the bottom of the log. Simply circle the abbreviation for the
affected organ and/or record additional information in the “Comments” section
of the log.
STOMACH SAMPLING PROTOCOL DURING
1985-91
The Northeast Fisheries Center seeks stomach content
data for the priority species listed in Table
A8 whenever they occur in trawl catches. It is especially
important to document the size and species of fish prey being consumed
by piscivorous predators in the Georges Bank - Southern New England
area (Strata 1-25). The stomach contents of all fish are to
be examined at sea (see “Procedure for Examining Stomach Contents
at Sea [1985-91].”). If time permits, the sampling of species
not listed in Table A8 such as large sharks and rays (i.e.,
fish-eating species) or any other predominant species in the catch
will also be desirable. Table
A9 lists 18 secondary species for which a random sample
of 10 fish should be taken for each watch.
If sampling time is inadequate to achieve target numbers
of priority species, then try to obtain at least five individual
fish (randomly chosen) of each of the top three priority species
in the catch. However, if possible, it is desirable to sample
about 50 fish in total at each station to provide large enough samples
to detect spatial and temporal shifts in diet.
PROCEDURE FOR EXAMINING STOMACH
CONTENTS AT SEA [1985-91]
1. Record pertinent information such as vessel, cruise
number, station number, and predator species in the spaces provided
at the top of the log. Please use entire common name of predator
(and prey) fishes when completing the log.
2. Inspect the buccal cavity (inside of mouth) for regurgitated
food and the esophageal area (via the body cavity if necessary) for
signs of stomach inversion. Also inspect the gills to be sure
the fish was freshly caught (white or pink gills indicate the fish
is probably from a previous station). If any of the above conditions
exist, discard the fish.
3. Determine the length (cm) (as per standard survey
measurements), sex, and maturity stage of the fish, and record that
information in the appropriate spaces of the log (see codes for sex
and maturity stage at the top of log).
4. Excise the stomach and empty the contents onto a
clean measuring board. Determine and record the stomach fullness
(volume to the nearest 0.5 cm3) by comparing the appropriately sized
volumetric gauge to the entire volume of the bolus. If the
stomach is empty, record stomach fullness as “0.0” and proceed to
the next fish.
5. Separate the stomach contents into prey groups and
record each group to the lowest taxon practical (see Watch Chief
if necessary). Estimate the percentage that each prey group
makes up of the total stomach content volume and write it on the
corresponding line of the log. For each prey category, record
the number (you may estimate for small organisms), individual sizes
as per survey standards (or minimum, maximum, and average size in
millimeters if more than 10 organisms are present), and state of
digestion (see codes at top of log). It is only necessary to
measure organisms >15 mm in length (e.g., fish, squid, crabs,
and decapod shrimp). The length(s) and number of fish prey
should be estimated to reflect the number and size at time of ingestion
if only pieces or partly digested fish are present. Larvae
and juvenile fish which can’t be identified should be preserved (with
pertinent station information) for microscopic examination at the
Woods Hole Laboratory. Organisms repeatedly observed as prey,
but not identified to species, may occasionally be preserved (10%
Formalin) and brought to the laboratory for positive identification.
STOMACH SAMPLING PROTOCOL DURING
1992-99
Information on predator-prey interactions among fishes
is critical for recruitment and multispecies models. Routine
stomach sampling on groundfish surveys provides an extremely valuable
time series of gut-content data which is needed for evaluating major
changes in the diets of fishes in relation to composition and abundance
of their prey. The primary objective is to estimate predation
on fish (particularly larval and juvenile stages). The current
focus is on piscivorous species representing major components of
the finfish biomass along the continental shelf off the northeastern
United States.
First priority is assigned to silver hake, Atlantic
cod, spiny dogfish, and skates. Sampling guidelines for these
and other species are outlined in the accompanying Table A10, Table A11,
and Table A12. Any
unusual or interesting species not included in Tables A10-A12 can
be sampled as time permits.
PROCEDURE FOR VOLUMETRIC EXAMINATION
OF FISH STOMACHS AT SEA
1. Select fish according to priorities in Tables A10-A12.
2. Inspect gills to be sure fish was freshly caught
(pale pink or white gills means fish is probably from an earlier
station).
3. For each species sampled, use a separate log sheet. Print
complete name of species. Abbreviations can be confused (e.g., “s.
hake” could be silver hake or spotted hake).
4. Record each predator length to nearest centimeter
following standard NEFSC survey methods. Record individual
fish weight to the nearest gram. Record sex and maturity stage. Fill
in “F” block with an “E” to show that the stomach was examined at
sea. Excise stomach and empty stomach contents onto a clean
measuring board or into a sorting tray.
5. If stomach is everted (blown) or shows signs of regurgitation
(food in mouth), leave the “F” block and the “Fullness” section of
the log blank. Record “BLOWN” as the prey name. Continue
on to next fish.
6. If stomach is empty, record “0.0” in the “Fullness” section
and record “EMPTY” as the prey name. Continue on to next fish.
7. If stomach contains food, estimate total volume of
bolus using the volumetric gauges (i.e., “wind chimes”). Record
volume to nearest 0.1 cm3. Trace amounts <0.1cm3 in volume
should be recorded as 0.1cm3. ALWAYS USE GAUGE.
8. Sort stomach contents into separate piles of prey
groups whenever possible. Prey should be identified to the
lowest taxon practical. Fish prey should be identified to species
as much as possible. Unidentified larval fish should be measured,
recorded as “larval fish,” and preserved in the provided vials. Label
vial lid with cruise number, station number, predator, and identification
number. Invertebrate prey should at least be recorded to major
taxonomic group (e.g., crab, polychaete, gammarid, bivalve)
Use the illustrated prey sheet to aid identification and spelling,
or ask the food habits representative on your watch. Unidentifiable
remains should be recorded as “AR.”
9. Estimate and record percentage of total volume represented
by each prey group. Record the average digestion for
each group.
10. All fish, crabs, and squid should be measured using
standard NEFSC survey methods. ALL species should be counted
or have the count estimated by volume if possible. It is of
critical importance to record the number and individual lengths of
all fish prey. A MAXIMUM of 10 individuals should be measured
to the nearest millimeter. If over 10 animals are present select
a random subsample of 10 individuals and record their lengths to
the nearest mm. For partly digested prey remains or large fragments
of prey that cannot be accurately measured, estimate the length of
the animal to nearest 10 mm at time of ingestion. Record these
estimated lengths, followed by an “E” (e.g., 250E, 110E). Be
sure to record the total number of prey present, not the number of
prey measured. RECORD ALL PREY LENGTHS IN MILLIMETERS.
11. Use the “Remarks” section to note larval fish preserved
for further identification, comments on prey, or general comments
on feeding (i.e., net feeding or scallop draggers in vicinity). Remember
to check “R” block if “Remarks” section is used.
IF YOU HAVE ANY QUESTIONS, ASK THE FOOD HABITS REPRESENTATIVE
ON YOUR WATCH.
CRITERIA FOR ASSESSING DIGESTION
STAGE
FRESH = No obvious sign of digestion. No skin
discoloration. Crustacean carapace is hard. Prey are
easily identifiable to family or species, but do not have to be identified
to this level to fit category.
PARTIAL = Some recognizable external characteristics
remain. Crustacean carapace is intact but soft. Prey
can frequently be identified to family or species, but do not have
to be identified to this level to fit category.
WELL = No external characteristics remaining. Prey
cannot be identified to family or species using external features. Prey
may sometimes be identified using otoliths or other remaining hard
internal structures.
STOMACH SAMPLE PROCESSING
During 1992-93 surveys, sampling ranges (i.e.,
number of fish per range of length) and maximum stomach sampling
levels were set by species and station. Refer to Table
A10. Maxima were removed in 1994, and priority species
were designated for each survey beginning in 1995. Refer to Table
A11. In 1999, the composition of species to be sampled,
sampling ranges, and sampling priorities changed. Refer to Table
A12.
An effort should be made to complete all food habits
sampling requested. If catches become overwhelming, complete
stomach analyses in order of priority. At the Watch Chief’s
discretion, a cap of five randomly selected fish may be placed on
the number of stomachs examined, excluding empty or blown stomachs.
Acronyms |
BEP |
= |
Benthic Ecology Program (predecessor to the
FEP) |
EMI |
= |
Ecosystem Modeling Investigation |
FCDI |
= |
Food Chain Dynamics Investigation (predecessor
to the FWDP) |
FEP |
= |
Feeding Ecology Project (predecessor to the
FCDI) |
FWDP |
= |
Food Web Dynamics Program |
GLOBEC |
= |
(U.S.) Global Ocean Ecosystems Dynamics Program |
NEFC |
= |
Northeast Fisheries Center (predecessor to
the NEFSC) |
NEFSC |
= |
Northeast Fisheries Science Center |
NMFS |
= |
National Marine Fisheries Service |
YOY |
= |
young of the year |