CONTENTS Abstract Introduction Methods Results Discussion Acknowledgements References Cited Appendix
NOAA Technical Memorandum NMFS-NE-216
The Trophic Dynamics of 50 Finfish and 2 Squid Species on the Northeast US Continental ShelfBrian E. Smith and Jason S. Link
National Marine Fisheries Service, Northeast Fisheries Science Center 166 Water St, Woods Hole, MA, 02543
Web version posted November 18, 2010Citation: Smith B, Link J. 2010. The Trophic Dynamics of 50 Finfish and 2 Squid Species on the Northeast US Continental Shelf . NOAA Technical Memorandum NMFS NE 216 640 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026, or online at http://www.nefsc.noaa.gov/nefsc/publications/
Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.ABSTRACT
This document describes the feeding habits of 50 fish and 2 squid species inhabiting the Northeast US (NEUS) continental shelf ecosystem and provides a current context for the Northeast Fisheries Science Center ’s Food Web Dynamics Program (FWDP). These descriptions are based on the examination of over 510,000 stomachs from over 150 predators since 1973. Trophic dynamics were examined with respect to decadal, spatial, seasonal, and ontogenetic variations in feeding habits. Most species are opportunistic, generalist feeders exhibiting broad diets, but feeding patterns were identified over broad temporal and spatial scales and in relation to ontogenetic stages. Dietary overlap among numerous fish species within this ecosystem was moderate, although for the entire shelf community, diet overlap was generally low among all species, suggesting relatively minimal competition. Given the wide range of feeding habits of most species in this ecosystem, changes in prey or predator abundance are less likely to impact populations and the community compared to ecosystems with a high number of specialists. The recognition of patterns and processes in the NEUS continental shelf fish community over large temporal and spatial scales has remained a key objective for the FWDP given ongoing efforts with food habits sampling, particularly during periods of intense fishing pressure.INTRODUCTION
The examination of fish feeding habits across the Northeast US (NEUS) continental shelf has remained an interest to fisheries science for over a century. Since the decline of fish populations was formally acknowledged in the late 1800s, ecological interactions (e.g., fish trophic dynamics) were considered a potential cause for those declines (Baird 1873). This interest and those considerations have remained in many of the present issues facing the National Marine Fisheries Service (NMFS; Fogarty and Murawski 1998; Link et al. 2002b) such that trophic ecology has continued to be an important consideration.
Prior to the 1960s, fish stomach sampling in conjunction with surveys (to monitor trends in NEUS shelf fish populations) by the US Fish and Wildlife Service Woods Hole Laboratory (currently NMFS, Northeast Fisheries Science Center ) explored the mechanisms behind individual species decline and their relationships to prey availability (e.g., gadids and benthic macrofauna). Ad hoc diet studies were initiated as part of a standardized bottom trawl survey to track trends in fish populations beginning in 1963, with a general emphasis on sampling commercially important groundfish. However, it was not until the late 1960s and early 1970s when multispecies considerations for fish stomach sampling were first applied with a systematic sampling of food habits initiated in 1973. The inception of the Feeding Ecology Project (FEP), followed by the formation of the Food Chain Dynamics Investigation (FCDI; predecessor to the Food Web Dynamics Program [FWDP]) occurred at that time.
A programmatic history of the FWDP, including descriptions of the many precursory programs leading up to its current development, was provided in Link and Almeida (2000). Here we aim to extend the documentation of fish trophic ecology for the NEUS in general and the FWDP in particular.
The major objectives of this work were to describe the diets of 50 major fish and 2 squid species occurring on the NEUS continental shelf and to examine feeding trends over broad temporal (i.e., decadal and seasonal) and spatial sampling scales and ontogeny (i.e., size class). Diet overlaps among the 52 major predator species for the entire NEUS shelf were also evaluated with the Bray-Curtis similarity index. A current description of FWDP stomach sampling requests, priorities, and methodology from 2000-2008 have also been provided to update previous documentation.Uses of the data
There have been numerous summaries of the diets of these species which use these and associated data such as for haddock Melanogrammus aeglefinus (Wigley 1956), yellowtail flounder Limanda ferruginea (Langton 1983), and silver hake Merluccius bilinearis (Bowman 1984). More comprehensive diet summaries for various fishes and squids of this region have also been provided as single documents (e.g., Bowman 1981; Langton 1982; Bowman and Michaels 1984; Bowman et al. 2000; Link and Almeida 2000). Here we build upon these previous works and focus on a presentation of the basic diet descriptions by using the sampling factors: decade, geographic area, season, and size class. For more detailed diet analyses of many of these species, the reader is directed to the following literature: the common hake species (Order Gadiformes; Garrison and Link 2000b), flatfishes (Order Pleuronectiformes; Link et al. 2002a), Atlantic cod (Gadus morhua; Link and Garrison 2002b; Smith et al. 2007), and the “lesser-appreciated” fishes (e.g., goosefish Lophius americanus; Link 2007). Additionally, the consumptive demands for many species of this shelf region have been reported, for example, six major piscivores of Georges Bank (Link and Garrison 2002a), pollock (Pollachius virens; Tyrrell et al. 2007), and skates (Family Rajidae; Link and Sosebee 2008). Furthermore, fish trophic guild analysis (Garrison and Link 2000a), major feeding-reproductive patterns (Link and Burnett 2001), and the use of these fishes as “samplers” of invertebrates which are otherwise difficult to sample (Link 2006; Link and Ford 2006) have been examined.
More recently the FWDP has emphasized evaluation of and explored the feasibility of incorporating ecological interactions (namely predation) directly into models to support fisheries science and management. The integration of ecological considerations into standard stock assessments and associated multispecies models is one approach to implementing ecosystem-based fisheries management (EBFM). Additions such as the predatory removal of commercially valuable forage species via a large predator complex (i.e., combined skate biomass for the NEUS continental shelf; 10-15% of total finfish biomass surveyed [Link 2007]) have shown the combined consumptive demands of seven skate species (Link and Sosebee 2008) and separately, various demersals (Overholtz and Link 2007) to be comparable or higher than the magnitude of commercial fisheries. Accordingly, predation mortality has been shown to exceed fishing mortality rates for various commercial fishes and invertebrates within this continental shelf region (e.g., age-1 and age-2 Atlantic mackerel (Scomber scombrus; Moustahfid et al. 2009), Atlantic herring (Clupea harengus; Overholtz et al. 2008), and northern shrimp (Pandalus borealis; NEFSC 2007)). Furthermore, these studies highlight the critical need to incorporate fish food habits data into fisheries models such that the miscalculation of magnitude and model uncertainty for various biological reference points and indices has been shown in a fisheries-only model (Hollowed et al. 2000; Tjelmeland and Lindstrøm 2005; Overholtz et al. 2008; Moustahfid et al. 2009).
The FWDP data have also been used to initialize, parameterize, and calibrate a wide range of multispecies and ecosystem models. The expansion of traditional multispecies virtual population analysis (MSVPA; Garrison and Link 2004) has been applied to forage species population dynamics within the NEUS continental shelf community, revealing the importance of predation mortality rates which exceed 0.2 for juvenile Atlantic herring, and Atlantic mackerel (Tyrrell et al. 2008). In general, the inclusion of predation into fisheries science, albeit well-accepted conceptually for over a century, has not become operationally routine in fisheries management despite focused efforts and evidence of its appropriateness (e.g., Christensen 1996; NMFS 1999; NRC 1999; Hollowed et al. 2000; Link 2002; Tjelmeland and Lindstrøm 2005; Overholtz and Link 2007; Overholtz et al. 2008; Moustahfid et al. 2009).
Further examples demonstrating the use of FWDP data in a modeling context include the development of multispecies production models (e.g., MS-PROD; Gamble and Link 2009) as extensions of the Graham-Schaefer production model (Quinn and Deriso 1999) which simulate the relative importance of predation, competition within and between functional feeding groups (see also Garrison and Link 2000a), and fisheries removals. The Energy Modeling and Analysis eXercise (EMAX) created an ecological network model (i.e., energy budget) for the entire NEUS food web (Link et al. 2006, 2008). Other ecosystem models have involved specific regions of the NEUS shelf (e.g., Gulf of Maine ; Ecosystem Gulf of Maine Aggregate (ECOGOMAGG; Overholtz and Link 2009) and have included numerous ecological processes spanning multiple trophic levels (i.e., primary production to seabirds and marine mammals). In addition, models have also incorporated suites of ecological and bio-physical processes for the entire NEUS shelf though this approach can be quite exhausting given the complexity and parameterization requirements of these factors (e.g., Atlantis; Gamble et al. in prep.). These examples represent a wide range of uses of the food habits data that will continue to be implemented as we move towards EBFM and the specific application of integrated ecosystem assessments (IEAs; Levin et al. 2009).
Ultimately, the underlying goals of the FWDP are to examine trophic interactions within the NEUS continental shelf ecosystem with an emphasis on demersal and pelagic finfish including various elasmobranchs and commercially important invertebrates. The FWDP research objectives are to quantify predation mortality relative to fishing mortality for commercially important species; model species interactions that influence the status of these stocks; relate diet variability to changes in population level processes; and advance our understanding of the NEUS continental shelf ecosystem.
The food habits data maintained by the FWDP are generated from multiple sources that provide stomach content information in the form of: total and individual prey weights (0.01 g) or volumes (0.1 cm3), diet composition, prey abundance, and prey length (1.0 mm). A major source of this information is the Northeast Fisheries Science Center (NEFSC) standardized bottom trawl survey and these food habits data are what we will focus upon in this document. These seasonal surveys were implemented to monitor the distribution and abundance of the fishes and invertebrates inhabiting the NEUS continental shelf ecosystem as well as to investigate biological and ecological interests (e.g., fish maturity, competition). Stomach sampling is currently a standard protocol for more than 60 species during these surveys (Appendix).
Additional data sources include process-oriented cruises and cooperative projects with industry partners that address specific questions pertaining to the feeding ecology of the fishes on the continental shelf. Recent projects have focused on such topics as spatial variations in benthivorous fish diet as a function of benthic disturbance (i.e., bottom fishing and invasive benthos; Link et al. 2005; Smith 2009), predation on larval fishes (Garrison et al. 2000, 2002; Almeida et al. 1999), and localized (~800 km2) fish feeding effects for selected predators (e.g., Atlantic cod Gadus morhua; Smith et al. 2007). The data from these studies, while an important research element in terms of fish trophic dynamics, were not included in this document.
Food habits data have been collected from the NEFSC Bottom Trawl Survey from Nova Scotia to Cape Hatteras , NC (~293,000 km2 or 85,300 nm2; Figure 1). Seasonal surveys have been conducted regularly in the fall since 1963, in the spring since 1968, and less frequently in the winter and summer. Sampling has occurred south of Cape Hatteras , NC , (i.e., South Atlantic Bight) although minimally in those southern locales with regard to fish feeding ecology.
Sampling locations were selected by using a stratified random design with strata defined by depth and latitude. Approximately 350-400 stations per fall and spring season were sampled in depths ranging between 8-400 m across the NEUS continental shelf. One station per approximately 690 km2 or 200 nm2 was employed such that the number of stations randomly assigned was proportional to the stratum area. A minimum of two stations were sampled per stratum to permit statistical inference. The catch was sorted by species and weighed (0.001 kg); individuals were measured (1.0 cm) and classified by sex and maturity stage, and a subset of species were sampled for food habits and age data. A detailed description of the survey design and protocols are available in Azarovitz (1981), NEFC (1988), and Reid et al. (1999).
Quantitative food habits sampling by the NEFSC has been conducted since 1973 to the present, and the data for the current study were restricted to this time series. From 1973-1981, stomachs were preserved and brought back to the laboratory for prey identification. Total stomach content and individual prey mass were measured to the nearest 0.01g. After 1981, food habits data were primarily collected at sea. The total volume (0.1 cm3) of stomach contents (i.e., an entire bolus) was measured and the proportion of each prey item estimated. A complete description of the history of NEFSC stomach content sampling through 2000 has been provided by Link and Almeida (2000), including conversion methods for stomach content volume (X, cm3) to mass (Y, g) using the formula: Y = a + bX with a = 0 and b = 1.1 (N = 10,806, r2 > 0.90, p = 0.0001). Although the species sampling requests for food habits have fluctuated over the 35+ year time series (Table 1), the general at-sea procedures for examining stomach content since 1981 have remained effectively the same for all sampled species. Spiny dogfish (Squalus acanthias) and silver hake were consistently the most highly sampled species for each decade (Figures 2-5). Since 1999 the emphasis has been placed not only on historically prioritized species but also has expanded to ecologically important species that appeared to be undersampled. In more recent years, the FWDP has also directed efforts to collect fewer stomachs per species to allow for an increased number of species sampled within the NEUS continental shelf ecosystem. Tables describing species, length ranges, and species priorities for collecting food habits data throughout the time series have been provided in the Appendix. Since 2004 through the present, at approximately every 25th station, stomach contents that regularly would be processed at sea were preserved and then processed in the laboratory. This was done as an additional form of data quality control.
Prey Taxonomic Resolution
The taxonomic resolution of invertebrate prey species prior to 1981 was greater than that of more recent decades. To correct for possible differences in prey taxonomic resolution between laboratory and at-sea processed stomach samples, four major prey categories were established. These categories span the lowest taxonomic levels feasible (i.e., occasionally genus and species) to a more broad phylum- or class-level category (Table 2). For the diet summaries discussed in this report, the lowest appropriate taxonomic grouping category (i.e., collection category) was used to describe the diets. It was not thought that the differences in sampling protocols over time would interfere, given a broad range of taxonomic resolution.
Data Analyses: Diet Summaries and Overlap
The 52 predator species selected for diet description across decadal, spatial, and seasonal scales and ontogeny were based on a minimum of 200 stomachs (Table 3). The predator species and their respective diet summaries were grouped by taxonomic order according to Nelson et al. (2004); Order Teuthida (i.e., two squid species) was placed at the end. To minimize redundancy, predators with similar feeding habits per taxonomic order were grouped when appropriate. To begin, the general feeding habits across all factors have been provided for each predator. The factors used to describe diet variability included decade: 1970s, 1980s, 1990s, and 2000s; geographic area: Mid-Atlantic Bight, Southern New England, Georges Bank, Gulf of Maine , and Scotian Shelf; season: fall, spring, winter, and summer; and size class: extra-small, small, medium, large, and extra-large. Size class definitions by species are listed in Table 3. For each factor considered per species, only those treatments (e.g. 1970s and 1980s) with a minimum of 200 stomachs were reported to facilitate comparisons. The prey categories shown for each species-group represented approximately 85% or more of the diet by mass. The trophic guild classifications (e.g., planktivorous or benthivorous) when specified were adopted from Garrison and Link (2000a) which examined an earlier version of the Food Habits Database (FHDBS). Specific trophic guilds reported by the current study (e.g., echinoderm specialist) followed the criteria of having more than 30 – 50% of the diet by mass composed of the indicated taxon across all sampling factors.
To assess dietary overlap, the Bray-Curtis index of similarity was used as a diet similarity measure whereby values ranged from 0-100% (i.e., no similarity to identical diets). Prey taxonomic resolution was limited to the analytical category (Table 2; e.g., invertebrates grouped by taxonomic order and fishes grouped by taxonomic family) with the understanding that dissimilarities among these broader taxonomic groupings were sufficient.
Food Habits Database Metadata
Currently there are over 510,000 stomach records in the FHDBS. Predator sizes range from 1 cm to over 2.4 m (Table 4). More than 150 species have been sampled, with 39 species having more than 1,000 stomachs sampled, and 52 species having more than 200 stomachs sampled. Approximately 30-40% of the stomachs examined by species were empty.
The elasmobranchs were generally the largest fishes sampled and were highly piscivorous thus, they had the largest mean total stomach contents. Some of the skates and rays were notable exceptions, feeding primarily on benthic macroinvertebrates. Other large mean total stomach contents were observed with goosefish, white hake (Urophycis tenuis), Atlantic cod, striped bass (Morone saxatilis), and Atlantic halibut (Hippoglossus hippoglossus) which were also highly piscivorous and had large mean lengths. Planktivorous species (e.g., herrings, mackerels, and northern sand lance [Ammodytes dubious]) and to a lesser degree small benthivores (e.g., fawn cusk-eel [Lepophidium profundorum]) had the smallest total mean stomach masses, reflecting a smaller mean fish size, and small crustacean (e.g., various zooplankton and gammarids respectively) diet.
There are over 630,000 individual prey records in FHDBS for the 510,000 stomachs previously described. Prey sizes range from 0.1 mm to 1 m. There are 1,376 unique prey items composing 10 major taxa: arthropods, fishes, molluscs, polychaetes, ctenophores, echinoderms, cnidarians, urochordates, chaetognaths, and bryozoans. The top 10 prey items by percent frequency of occurrence for all predators include: unidentified and miscellaneous fishes, gammarids and other amphipods, various crustacean shrimps (i.e., euphausiids, Crangon, and pandalids), Cancer crabs and other decapod crabs, polychaetes, ctenophores, bivalves, and copepods. Other major prey items include sand lance (Ammodytes sp.), cephalopods (primarily squids), mysids, and ophiuroids (Figure 6). There are a large number of empty stomachs (N = 169,774) in the database, and unidentified fishes and well-digested prey (i.e., unidentifiable animal remains) were observed most frequently, suggesting most individual prey items are difficult to identify macroscopically when highly digested.
Diets of Major Species Grouped by Taxonomic Classification
The diet of the squalid shark spiny dogfish had a large proportion of fishes (clupeids (e.g.,Atlantic herring), scombrids (e.g., Atlantic mackerel), and various other fishes including unidentified fish; Figure 7). Ctenophores, Loligo squid, and bivalves were additional prey items to note by mass.
The prey composition of spiny dogfish has varied over the past 40 years and in general parallels the population dynamics of commercially important forage species (e.g., herring, Overholtz 2002; Overholtz and Friedland 2002) (Figures 8A-D). In the 1970s, squids and unidentified cephalopods (i.e., Loligo sp. and Illex sp.) composed a substantial percentage of the diet (combined by mass; ~20%) although decreased to less than 10% throughout the remaining three decades. In contrast, clupeids, including Atlantic herring, increased in the diet composition of spiny dogfish from the 1970s and 1980s (~4%) to the 1990s and 2000s (~18%). Accordingly, the unidentified fish component has remained remarkably consistent over the entire time span (~20-25%).
Unidentified fishes were a large dietary component for spiny dogfish across geographic area (Figures 9A-E). Spiny dogfish diet on Georges Bank was dominated by ctenophores as well as unidentified fish with Atlantic herring and various clupeids occupying the largest percentages of identified fish. Within the Gulf of Maine , ctenophores and unidentified fish were also major prey items, but larger percentages of clupeids in comparison to Georges Bank were observed. The Southern New England and Mid-Atlantic Bight regions revealed lesser ctenophore and clupeid diet components respectively, although greater percentages of squid and unidentified cephalopods (i.e., Loligo sp. or Illex sp.), scombrids (i.e., Atlantic mackerel), and bivalves were present.
Seasonal differences in diet for spiny dogfish were minor (Figures 10A-D); however, slight ontogenetic shifts in diet were suggested over the three size classes (Figures 11A-C). In general, medium and to a lesser extent small spiny dogfish ate larger proportions of ctenophores (~10-18%) in comparison with the large size class (~5%). This prey item may also contribute to increased amounts of well-digested prey for appropriate size classes given the relatively immediate gastric evacuation of ctenophores (Arai et al. 2003). Large spiny dogfish were predominantly piscivorous (e.g., clupeids, scombrids, and unidentified fishes).
The two ground sharks (smooth dogfish [Mustelus canis] and Atlantic sharpnose shark [Rhizoprionodon terraenovae]) exhibited distinct diets across broad temporal and spatial scales and ontogeny. Smooth dogfish fed predominantly on benthic macroinvertebrates, with Cancer crabs (Cancer borealis and Cancer irroratus) and other decapod crabs dominating the diet throughout the four decades of sampling (Figures 12 and 13A-D). Similar diet preferences were observed over spatial area, season, and size category (large and medium categories only; Figures 14A-C, 15A-D, and 16A-B). Conversely, the Atlantic sharpnose shark was a bentho-pelagic feeder, consuming various fishes distributed throughout the bentho-pelagic environment (e.g., pleuronectids, sciaenids, and engraulids), Loligo squid, and decapod crabs (Figure 17).
The skates within the NEUS shelf system are primarily benthic invertebrate feeders, yet barndoor (Dipturus laevis) and winter skate (Leucoraja ocellata) were also piscivorous. Barndoor skates consume various decapods, including Cancer crabs, pandalid and Crangon shrimps, and fishes such as Atlantic herring, silver hake, and other unidentified fish species (Figure 18). Barndoor diet also remained relatively constant across spatial and seasonal scales (Figures 19A-B and 20A-B). Additionally, winter skate fed on Ammodytes sp., their presence dominating the diets of the 1970s and 1980s (Figures 21 and 22A-B). Notable increases in diet composition of polychaetes, gammarids, and bivalves were observed in the 1990s and 2000s, but the presence of Ammodytes sp. remained, albeit in lesser amounts (Figures 22C-D). The diet variability of winter skate across the four geographic regions and seasons was generally minor (Figures 23A-D, 24A-D). Nonetheless, increases in percent diet composition of gammarids for Southern New England and bivalves in the Mid-Atlantic Bight were apparent. In general, no major ontogenetic shifts in diet for winter skate were observed with most size classes consuming gammarids, polychaetes, other benthos, and small fishes (i.e., Ammodytes sp.) (Figures 25A-D).
Clearnose (Raja eglanteria) and thorny skate (Amblyraja radiate) generally followed a bentho-piscivorous diet, consuming approximately equal proportions of benthic invertebrates and fish. Clearnose fed on decapod and Cancer crabs, Loligo squid, and unidentified fish (Figure 26). Although few diet differences were detected across seasons (Figure 27A-C), a slight diet shift from predominantly benthic macroinvertebrates to approximately equal proportions of invertebrate benthos, various benthic fishes, and Loligo squid occurred between medium and large size classes (Figures 28A-B). Thorny skate diet primarily consisted of polychaetes and unidentified fish including Atlantic herring (Figure 29). These major components persisted throughout the four decades of sampling with the exception of the 1970s when a substantial proportion of squid (Loligo sp., Illex sp., and unidentified cephalopods) was present (Figures 30A-D). Spatial, seasonal, and ontogenetic diet variations of thorny skate were generally minor with piscivory and invertebrate benthivory continuous throughout all factors (Figures 31A-C, 32A-C, and 33A-C).
The remaining three skates—rosette (Leucoraja garmani), little (Leucoraja erinacea), and smooth (Malacoraja senta)-- are principally benthivorous. Their feeding habits consist of decapods, including Cancer crabs, Crangon and pandalid shrimps, along with polychaetes and gammarids (Figures 34, 35, and 40). Smooth skate will also feed on pelagic organisms with a diet that includes euphausiids and a small proportion of various fishes. Because of the benthivory throughout the life histories of little and smooth skates, decadal, spatial, seasonal and ontogenetic diet trends were essentially absent (Figures 36A-D, 37A-E, 38A-D, 39A-B, 41A-B, 42A-B, 43A-B).
The clupeids (i.e., Atlantic herring, alewife [Alosa pseudoharengus], blueback herring [Alosa aestivalis], and American shad [Alosa sapidissima]) were planktivorous, feeding mostly on pelagic organisms such as copepods, euphausiids, amphipods (i.e., hyperiids and gammarids), and various shrimp-like organisms (e.g., mysids) (Figures 44, 49, 50, and 53). The common occurrence of well-digested prey (i.e., unidentifiable animal material) was also seen with these species because of difficulties in identifying small prey at sea and the rapid digestion of small individuals. The general diet of Atlantic herring remained consistent over the decadal time series, although a large proportion of amphipods and lesser amounts of mysids, various other crustaceans, Ammodytes sp., and well-digested prey were observed in the 1980s (Figure 45A). In the later decades, copepods, euphausiids, and well-digested prey were the predominant food items (Figures 45B-C). Euphausiids composed approximately 6% of the diet by mass for Atlantic herring collected in the Mid-Atlantic Bight and Southern New England (Figures 46A-B). In contrast, individuals sampled from Georges Bank to the Scotian Shelf had approximately 18-60% euphausiids in their diet (Figures 46C-E). Furthermore, within these two southern regions, copepods were proportionally large diet components, although the presence of well-digested prey was noteworthy as well. A seasonal trend in Atlantic herring diet was observed across the NEUS continental shelf with a large proportion of euphausiids consumed in the fall, whereas the spring revealed greater amounts of copepods and well-digested prey (Figures 47A-B). Similar feeding patterns by Atlantic herring on copepods and euphausiids were seen in the winter and summer seasons respectively (Figures 47C-D). These results parallel the diets of the northern and southern regions previously described, and were believed to be an artifact of Atlantic herring’s seasonal migration patterns as demonstrated by Overholtz (2002) and Overholtz and Friedland (2002). In general, no major ontogenetic shifts were observed for Atlantic herring diet (Figures 48A-C).
The differences in blueback herring and American shad diets across the geographic areas and size categories sampled were minor (Figures 51A-C, 52A-C, 54A-B, 55A-C). For both species, well-digested prey and copepods composed large proportions of the diet categories and additionally, various crustacean shrimps (e.g., euphausiids, mysids, and pandalids) in the feeding habits of American shad.
The benthic macroinvertebrate feeder, fawn cusk-eel, ate gammarids, polychaetes, and other small benthos, including a substantial amount of well-digested prey (Figure 56). The stomach sampling of fawn cusk-eel did not begin until after 2000; thus the ability to detect change was limited, and only minor variations in diet were observed for the geographic areas, seasons, and size categories adequately sampled (Figures 57A-B, 58A-C, and 59A-B).
The larger gadoid species-- Atlantic cod, haddock, and pollock-- have broad, extensive diets comprising benthic and pelagic prey. In general, these predators occupy three relatively distinct feeding niches with haddock’s principal prey being benthic invertebrates (i.e., ophiuroids, gammarids, and polychaetes), cod with its generalist feeding habits in between, and pollock having a more pelagic diet consisting of various fish and crustacean shrimps (i.e., silver hake, Ammodytes sp., clupeids, and euphausiids) (Figures 60, 65, and 70). The diet of haddock showed no major variations across decadal and seasonal scales, or ontogeny (Figures 61A-D, 63A-D, and 64A-C). However, diet shifts were apparent across geographic area in which haddock primarily ate ophiuroids or fish eggs in the Gulf of Maine and Scotian Shelf, a more general benthic invertebrate diet of gammarids, polychaetes, and ophiuroids on Georges Bank, and similarly amphipods and polychaetes in Southern New England (Figures 62A-D). In comparison, cod are more of a mixture of bentho-pelagic feeders, with the diet including large proportions of fish (i.e., clupeids, Ammodytes sp., silver hake, and unidentified individuals), and benthic macroinvertebrates (i.e., Cancer crabs, various crustacean shrimps, bivalves, gastropods, ophiuroids, and other benthos) in their diet (Figure 65). A general increase in clupeids (primarily Atlantic herring) was observed over the decadal time series with the percent diet composition equal to approximately 12%, 6%, 24%, and 20% from the 1970s through the 2000s (Figures 66A-D). The broad diet of cod remained relatively constant across the geographic and seasonal scales sampled although an ontogenetic shift in diet from benthivory (i.e., macroinvertebrates) towards piscivory was identified throughout the size classes (Figures 67A-D, 68A-D, and 69A-D). Smaller cod appear to prefer small benthic macroinvertebrates (i.e., gammarids, shrimps, ophiuroids, and polychaetes) whereas an increase in fish particularly clupeids and silver hake made up greater proportions of medium to extra-large cod diet; a confirmed occurrence across multiple sampling scales in the northwest Atlantic Ocean (Link and Garrison 2002b; Smith et al. 2007). Similarly, the diet of pollock remained fairly constant and mainly focused on pelagic prey over the decadal, spatial, and seasonal sampling scales (Figures 71A-D, 72A-C, and 73A-C). Furthermore, a dietary shift from euphausiids and other shrimp-like crustaceans to silver hake, Ammodytes sp., and other fishes occurred throughout the general life history of pollock (i.e., medium through extra-large size classes; Figures 74A-D).
The hakes within this ecosystem (offshore hake [Merluccius albidus], silver hake, white hake, red hake [Urophycis chuss], and spotted hake [Urophycis regia]) are generally piscivorous (i.e., feeding on silver hake, Atlantic herring, and unidentified fish) but also feed on pelagic invertebrates such as euphausiids and various other crustacean shrimps and squid (Figures 75, 77, 82, 87, and 92). The general diet composition of these predators experienced only minor variations over decadal, spatial, and seasonal sampling scales (Figures 76A-B, 79A-E, 80A-D, 83A-D, 84A-D, 85A-D, 88A-D, 89A-E, 90A-D, 93A-D, 94A-C, and 95A-C). Noteworthy exceptions to this include the increase in Atlantic herring and unidentified clupeids in the diet of silver hake over the decades sampled (Figures 78A-D) as well as increased piscivory across size class for those hakes with adequate sample sizes (i.e., silver hake, white hake, red hake, and spotted hake; Figures 81A-C, 86A-C, 91A-C, and 96A-B).
Goosefish was a piscivorous specialist with various demersal fishes (e.g., pleuronectids, skates, and gadiformes), clupeids (e.g., Atlantic herring), scombrids (e.g., Atlantic mackerel), and a large proportion of unidentified individuals in the diet (Figure 97). The percent diet composition of clupeids increased over the time series (i.e., ~4% clupeid taxa combined for each decade: 1970s and 1980s; ~10-15% for each decade 1990s and 2000s; Figures 98A-D). For the other factors examined, no major shifts in feeding habits were observed as the prey categories previously described remained relatively constant (Figures 99A-D, 100A-D, and 101A-C).
The two scorpaenids-- Acadian redfish (Sebastes fasciatus) and blackbelly rosefish (Helicolenus dactylopterus)-- can be classified as shrimp-fish feeders with euphausiids, pandalids, silver hake, and various other fishes composing their diets (Figures 102 and 107). The proportions of these major prey items were variable across the broad sampling scales and ontogeny although the general diet remained consistent (Figures 103A-D, 104A-B, 108A-B, 109A-B, and 110A-B). In contrast to the fall diet of Acadian redfish dominated by various shrimps and few fishes (~80% combined shrimp taxa; ~8% combined fish taxa), larger proportions of fishes (e.g., silver hake and unidentified fish) were observed in the spring diet (~30% combined fish taxa) (Figures 105A-B). Likewise for Acadian redfish diet, a slight increase in the amount of fish was apparent from small to medium size classes (Figures 106A-B).
Longhorn sculpin (Myoxocephalus octodecemspinosus) and sea raven (Hemitripterus americanus) were predominantly benthic predators. Longhorn sculpin feed on decapods, including Cancer crabs, shrimps (i.e., Crangon and pandalids), gammarids, as well as some fishes (Figure 111). In comparison, sea raven was a benthic piscivore, eating ocean pout (Zoarces americanus), pleuronectiformes, silver hake, longhorn sculpin, and other gadiformes, along with some Cancer crabs (Figure 116). The general feeding patterns of these species did not vary drastically over time, space, or life history (Figures 112A-D, 113A-D, 114A-D, 115A-B, 117A-C, 118A-D, 120A-C). Nonetheless, a seasonal variation in benthic invertebrates was observed for sea raven (i.e., a greater proportion of Cancer crabs in the fall diet, and the spring, summer, and winter diets with greater proportions of various fishes as previously described; Figures 119A-D).
The two Prionotus searobins (i.e., northern [Prionotus carolinus] and striped [Prionotus evolans]) were primarily benthivorous, eating decapod crabs (e.g., Cancer crabs), Crangon shrimp, polychaetes, and gammarids (Figures 121 and 125). The food habits sampling of these species was sporadic in the 1970s through the 1990s and did not become routine until the early 2000s; thus, limited feeding inferences are reported (i.e., minor feeding variations for northern searobin over geographic area, season, and size class; Figures 122A-B, 123A-C, and 124A-B). Nonetheless, the general diet of striped searobin can be distinguished from the principally macroinvertebrate diet of northern searobin by the presence of various fish species in its diet (e.g., engraulids, scup [Stenotomus chrysops], and unidentified individuals; Figure 125).
Planktivorous feeding habits were predominant for Atlantic mackerel, butterfish (Peprilus triacanthus), and northern sand lance. The diet comprised copepods, euphausiids, various crustacean shrimps, and ctenophores as primary prey items for these fishes (Figures 126, 131, and 136). Similar to the diets of the clupeids sampled, well-digested prey accounted for a substantial proportion of the diet of butterfish and Atlantic mackerel because of the sampling limitations previously described and was probably one of several zooplankton or crustacean shrimp species. The diets of these predators did not vary markedly across decadal, spatial, and seasonal scales, or ontogeny (Figures 127A-C, 129A-C, 130A-C, 132A-D, 133A-C, 134A-D, 135A-B, 137A-B, 138A-C, 139A-C). In the case of Atlantic mackerel, a larger proportion of euphausiids were seen in Gulf of Maine diets in contrast to the southern regions (e.g., Mid-Atlantic Bight) which had greater amounts of copepods (Figures 128A-D).
Bluefish (Pomatomus saltatrix), weakfish (Cynoscion regalis), and to a lesser degree striped bass were piscivorous specialists with their diets composed of Atlantic herring, other clupeids, engraulids, silver hake, various other fishes, and squids (Figures 140, 145, and 150). Variations in these predators’ diets reflected prey availability and distribution across temporal and geographic scales, including fluctuations in the diet composition of Atlantic herring over the time series, spatial regions, and seasons similar to the diets of other piscivores previously described (Figures 141A-D, 142A-C, 143A-B, 146A-D, 147A-B, 148A-B, 151A-B, 152A-B, and 153A-B). In general, no major shifts in diet were observed with ontogeny for these three species except for the medium size class of striped bass which consumed benthic macroinvertebrates (i.e., bivalves, Crangon shrimp, gammarids, isopods, and polychaetes) in addition to the piscivorous diet already discussed (Figures 144A-B, 149A-C, and 154A-B). Weakfish, along with being highly piscivorous, had a relatively unique diet targeting engraulids (all engraulids combined were greater than 35% of diet by mass; Figure 145).
The two sciaenids regularly sampled (i.e., Atlantic croaker [Micropogonias undulates] and spot [Leiostomus xanthurus]) were mainly benthivorous with polychaetes, bivalves, gammarids, other small benthic crustaceans, well-digested prey, and small proportions of fishes occupying their diets (Figures 155 and 158). Although recent efforts to characterize Atlantic croaker and spot diet within the past decade have been attempted, generally no major trends were observed across decade and size class for Atlantic croaker (Figures 156A-B and 157A-B). Similarly, only minor dietary variations were seen for spot across decade and season (Figures 159A-B and 160A-B).
The broad benthic diets of scup and black sea bass (Centropristis striata) included gammarids, polychaetes, Cancer and unidentified decapod crabs, and small fishes; the later three taxa (particularly Cancer and decapod crabs) were primarily eaten by black sea bass (Figures 161 and 166). Diet variations across decadal, spatial, and seasonal scales, and ontogeny were relatively minor given the consistently benthivorous feeding habits of these predators (Figures 162A-C, 165A-B, 167A-C, 168A-B, 169A-C, and 170A-B). However, scup diet in the fall showed approximately equal proportions of polychaetes and gammarids (~18% and ~20% respectively), whereas in the spring a minimal amount of gammarids (<2%) was found in the diet, possibly because of subtle differences in regional growth of the benthos (Figures 163A-B and 164A-C; Theroux and Wigley 1998).
Ocean pout can be considered an echinoderm specialist much the same as American plaice (Hippoglossoides platessoides; discussed below) and to a lesser degree haddock with echinoids (e.g., sand dollars and sea urchins), ophiuroids, and asteroids being major diet components (Figure 171). Other prey taxa included Cancer crabs, gammarids, and polychaetes. The marked presence of these prey items in the diet of ocean pout remained relatively constant over the time series (Figures 172A-C). However, the amounts of these prey did vary spatially with a larger proportion of ophiuroids and lesser proportion of echinoids in ocean pout stomachs collected in the Gulf of Maine (Figures 173A-D). Few diet differences were observed across seasons; nonetheless, the percent compositions by mass of ophiuroids and echinoids were variable as fall diets had ~20% ophiuroids and ~5% echinoids, and the spring and winter diets each had less than 5% ophiuroids and greater than 40% echinoids (Figures 174A-C). Diet variability with size class was present as smaller individuals tended to consume greater amounts of smaller benthos (i.e., gammarids and other amphipods) while larger ocean pout fed primarily on echinoids and asteroids (Figures 175A-C).
Flatfish diet can be categorized into one of four general feeding groups: piscivores (i.e., Atlantic halibut, summer flounder [Paralichthys dentatus], and fourspot flounder [Hippoglossina oblonga] that eat mainly fish and squids; Figures 176, 178, and 183), polychaete-gammarid predators (i.e., yellowtail flounder [Limanda ferruginea], winter flounder [Pseudopleuronectes americanus], witch flounder [Glyptocephalus cynoglossus], and Gulf Stream flounder [Citharichthys arctifrons]; Figures 188, 193, 198, and 203), shrimp-fish predator (i.e., windowpane flounder [Scophthalmus aquosus]; Figure 206), or echinoderm specialist (i.e., ophiuroids and echinoids; American plaice; Figure 211). In general, no major diet variations were exhibited across decade, spatial area, season, and size class as most flatfish diets per sampling factor did not deviate from the feeding classifications previously described (Figures 177A-B, 179A-D, 181A-C, 182A-C, 184A-D, 186A-D, 189A-D, 190A-C, 191A-D, 192A-C, 194A-D, 195A-E, 196A-D, 197A-C, 199A-C, 200A-E, 201A-D, 202A-C, 204A-B, 205A-C, 207A-D, 208A-D, 209A-D, 210A-B, 212A-D, 214A-C, and 215A-C). Noteworthy exceptions included increased percent diet compositions of cephalopods and Loligo squid within the Southern New England and Mid-Atlantic Bight regions for summer and fourspot flounders (Figures 180A-C and 185A-D respectively); a similar shift was seen between small and medium fourspot flounder size classes (Figures 187A-B). A slight increase in the percent diet composition of ophiuroids (by mass) for American plaice in the Gulf of Maine was evident as shown for the other echinoderm specialists (i.e., haddock and ocean pout) (Figures 213A-C).
The two squids (northern shortfin [Illex illecebrosus] and longfin inshore squid [Loligo pealeii]) had large amounts of well-digested prey in their diets which can be attributed to the high degree of prey mastication associated with these predators (Figures 216 and 221). Cannibalism, as seen by unidentified cephalopods (i.e., well-digested squid material) in the diet, along with unidentified fish were the largest dietary components. Only minor variations in feeding were observed across the broad sampling scales of decade, region, and season, and ontogenetic stages (Figures 217A-B, 218A-E, 219A-C, 220A-B, 222A-B, 223A-C, 224A-C, and 225A-B).
The average diet similarity for the 52 predators examined was generally low (Bray-Curtis Index (BCI) average = 31.5%), suggesting relatively minimal potential for competition within this NEUS shelf community (Figure 226). However, greater dietary overlap (BCI greater than 40%, occasionally BCI greater than 60%) was observed among the seven skate species (barndoor, winter, clearnose, rosette, little, smooth, and thorny skate), and additionally between skates and separate pairings with searobins, longhorn sculpin, Acadian redfish, blackbelly rosefish, scup, black sea bass, some gadiformes, and flatfish (primarily fourspot and windowpane flounders). High overlap was also seen for some but not all of the planktivorous feeders (i.e., Atlantic herring, blueback herring, and Atlantic mackerel), among the gadiformes, and between various gadiformes and longhorn sculpin, sea raven, blackbelly rosefish, and the searobins. Nonetheless, a moderate similarity in the feeding habits of benthivorous flatfish (i.e., yellowtail, winter, witch, and Gulf Stream flounders; BCI = 40-60%), and to a higher degree, the pairing of longhorn sculpin and northern searobin (BCI greater than 60%) was observed.
Food Habits Summary
The summary of food habits for 52 species provided here expands and updates previous diet descriptions for the major fish and squid species of the NEUS continental shelf (e.g., Sherman et al. 1978; Bowman 1981, 1984; Langton 1982, 1983; Durbin et al. 1983; Bowman and Michaels 1984; Bowman et al. 1984; Bowman et al. 2000; Link and Almeida 2000). Diet variability over decade, geographic area, season, and ontogeny for greater than 60% of the species reported here was, generally speaking, relatively minor. Major patterns in feeding habits were observed for approximately 20 predator species. For instance, the increase in diet proportion of principal pelagic species (e.g., Atlantic herring, mackerel, etc.) over the decadal time series was observed in the diets of spiny dogfish, Atlantic cod, many of the hakes (e.g., silver, white, and red hake), and other major piscivores (i.e., bluefish and goosefish). This observation most likely reflected the availability of major pelagic prey in response to variations in fishing intensity over the time series (Fogarty and Murawski 1998; Overholtz 2002; Overholtz and Friedland 2002). Spatial variations in prey availability across the broad geographic sampling scale were apparent for some fishes feeding on benthic macroinvertebrates (e.g., ophiuroids for haddock, American plaice, and ocean pout) as well as pelagic crustaceans (e.g., euphausiids and copepods for Atlantic herring and Atlantic mackerel). An increased proportion of ophiuroids was observed in the diets of haddock, ocean pout, and to a lesser extent American plaice in the Gulf of Maine than in the more southern regions of the Mid-Atlantic Bight, Southern New England, and Georges Bank ; a similar result was shown by Link (2006). Atlantic herring diet revealed an equivalent latitudinal shift in major prey taxa such that euphausiid percent diet composition was markedly greater for the northern regions: Scotian Shelf, Gulf of Maine, and Georges Bank, whereas copepods and well-digested prey (primarily digested zooplankton) were the dominant prey taxa in the southern regions (i.e., Mid-Atlantic Bight and Southern New England). Similar dietary trends for these prey, albeit less dramatic, were seen with Atlantic mackerel. In general, few seasonal diet variations were identified particularly for those predators consuming pelagic taxa (namely euphausiids and various fishes) such as with Atlantic herring and Acadian redfish. Additionally, seasonal differences in feeding were suggested for ocean pout with variable proportions of echinoderm taxa (i.e., echinoids and ophiuroids), further highlighting the interplay among the broad-scale factors examined by this work.
Increased piscivory with increased size was the most common ontogenetic diet shift observed. This was evident for both demersal and pelagic fishes: spiny dogfish, Atlantic cod, pollock, most hakes (i.e., silver, white, red, and spotted hake), striped bass, and Acadian redfish. Whether these fishes ate benthic macroinvertebrates or pelagic crustaceans during small or medium size classes, there was a shift to piscivory with larger size classes.
The fish community of the NEUS shelf is primarily composed of generalist feeders. Given the wide range of feeding habits and generally low dietary overlap for the entire shelf community, changes in prey or predator abundance are less likely to impact populations and the community compared to ecosystems with a high number of specialists. In some limited instances there was evidence of dietary specialization (e.g., echinoderm feeders: haddock, American plaice, and ocean pout; or decapod crab feeders: smooth dogfish and black sea bass). Garrison and Link (2000a) noted the generalized feeding preferences for many major fish and squid predators of the NEUS continental shelf, grouping species into trophic guilds that accounted for ontogenetic diet shifts (i.e., crab eaters, planktivores, amphipod/shrimp eaters, shrimp/small fish eaters, benthivores, and piscivores). The diet summaries presented here support the feeding guilds proposed by Garrison and Link (2000a).
Some species still remain relatively undersampled, particularly over the broad temporal, spatial, and seasonal scales examined. Sampling requests and species priorities are regularly modified as appropriate every two or three years to address modeling needs and research interests. It is difficult to predict which species of low commercial value will gain importance, yet the multispecies food habits sampling currently used has provided reasonable coverage over such variability. Nonetheless, sampling the feeding habits of the entire NEUS continental shelf fish community remains a major challenge.
An important component of understanding fish community structure and function is knowledge of fish feeding ecology through a continuous diet monitoring program such as that described in this report. We assert that these data constitute the preliminary information necessary for implementing EBFM. Thus, the requisite monitoring and modeling of such ecological interactions for the NEUS continental shelf ecosystem will continue to remain a priority for the FWDP and the NEFSC in the near future.
We acknowledge the many individuals who have collected the food habits data used in this report over the 35+ years of sampling on NEFSC Bottom Trawl Surveys. We thank the previous staff of the FWDP for their efforts on the development and management of the FHDBS. We thank H. Blossom for her assistance with the production of numerous graphics throughout this document. We also thank two anonymous reviewers for their comments.
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