Fish consumption by harbor seals (Phoca vitulina) in the San Juan Islands, Washington

The harbor seal (Phoca vitulina) is a large-bodied and abundant predator in the Salish Sea ecosystem, and its population has recovered since the 1970s after passage of the Marine Mammal Protection Act and the cessation of bounties. Little is known about how this large predator population may affect the recovery of fish stocks in the Salish Sea, where candidate marine protected areas are being proposed. We used a bioenergetics model to calculate baseline consumption rates in the San Juan Islands, Washington. Salmonids (Oncorhynchus spp.) and herring (Clupeidae) were the 2 most energetically important prey groups for biomass consumed by harbor seals. Estimated consumption of salmonids was 783 (±380 standard deviation [SD]) metric tons (t) in the breeding season and 675 (±388 SD t in the nonbreeding season. Estimated consumption of herring was 646 (±303 SD) t in the breeding season and 2151 (±706 SD) t in the nonbreeding season. Rockfish, a depressed fish stock currently in need of population recovery, composed one of the minor prey groups consumed by harbor seals (84 [±26 SD] t in the nonbreeding season). The variables of seal body mass and proportion of prey in seal diet explained >80% of the total variation in model outputs. Prey groups, such as rockfish, that are targeted for recovery may still be affected by even low levels of predation. This study highlights the importance of salmonids and herring for the seal population and provides a framework for refining consumption estimates and their confidence intervals with future data.

Habitat and diet overlap of 4 piscivorous fishes: variation on the inner continental shelf off New Jersey

Piscivorous fishes, many of which are economically valuable, play an important role in marine ecosystems and have the potential to affect fish and invertebrate populations at lower trophic levels. Therefore, a quantitative understanding of the foraging ecology of piscivores is needed for ecosystem-based fishery management plans to be successful. Abundance and stomach contents of seasonally co-occurring piscivores were examined to determine overlap in resource use for Summer Flounder (Paralichthys dentatus; 206–670 mm total length [TL]), Weakfish (Cynoscion regalis; 80–565 mm TL), Bluefish (Pomatomus saltatrix; 55–732 mm fork length [FL]), and Striped Bass (Morone saxatilis; 422–920 mm FL). We collected samples from monthly, fishery-independent trawl surveys conducted on the inner continental shelf (5–27 m) off New Jersey from June to October 2005. Fish abundances and overlaps in diet and habitat varied over this study period. A wide range of fish and invertebrate prey was consumed by each species. Diet composition (determined from 1997 stomachs with identifiable contents) varied with ontogeny (size) and indicated limited overlap between most of the species size classes examined. Although many prey categories were shared by the piscivores examined, different temporal and spatial patterns in habitat use seemed to alleviate potential competition for prey. Nevertheless, the degree of overlap in both fish distributions and diets increased severalfold in the fall as species left estuaries and migrated across and along the study area. Therefore, the transitional period of fall migration, when fish densities are higher than at other times of the year, may be critical for unraveling resource overlap for these seasonally migrant predators.

Spatial and temporal variation in the diet of the California sea lion (Zalophus californianus) in the Gulf of California, Mexico

Between June 1995 and May 1996 seven rookeries in the Gulf of California were visited four times in order to collect scat samples for studying spatial and seasonal variability California sea lion prey. The rookeries studied were San Pedro Mártir, San Esteban, El Rasito, Los Machos, Los Cantiles, Isla Granito, and Isla Lobos. The 1273 scat samples collected yielded 4995 otoliths (95.3%) and 247 (4.7%) cephalopod beaks. Fish were found in 97.4% of scat samples collected, cephalopods in 11.2%, and crustaceans in 12.7%. We identified 92 prey taxa to the species level, 11 to genus level, and 10 to family level, of which the most important were Pacific cutlassfish (Trichiurus lepturus), Pacific sardine (Sardinops caeruleus), plainfin midshipman (Porichthys spp.), myctophid no. 1, northern anchovy (Engraulis mordax), Pacific mackerel (Scomber japonicus), anchoveta (Cetengraulis mysticetus), and jack mackerel (Trachurus symmetricus). Significant differences were found among rookeries in the occurrence of all main prey (P≤0.04), except for myctophid no. 1 (P>0.05). Temporally, significant differences were found in the occurrence of Pacific cutlassfish, Pacific sardine, plainfin midshipman, northern anchovy, and Pacific mackerel (P<0.05), but not in jack mackerel (χ 2=2.94, df=3, P=0.40), myctophid no. 1 (χ 2=1.67, df= 3, P=0.64), or lanternfishes (χ 2=2.08, df=3, P=0.56). Differences were observed in the diet and in trophic diversity among seasons and rookeries. More evident was the variation in diet in relation to availability of Pacific sardine.

Diet shifts of juvenile red snapper (Lutjanus campechanus) with changes in habitat and fish size

We examined the diets and habitat shift of juvenile red snapper (Lutjanus campechanus) in the northeast Gulf of Mexico. Fish were collected from open sand-mud habitat (little to no relief), and artificial reef habitat (1-m3 concrete or PVC blocks), from June 1993 through December 1994. In 1994, fish settled over open habitat from June to September, as shown by trawl collections, then began shifting to reef habitat — a shift that was almost completed by December as observed by SCUBA visual surveys. Stomachs were examined from 1639 red snapper that ranged in size from 18.0 to 280.0 mm SL. Of these, 850 fish had empty stomachs, and 346 fish from open habitat and 443 fish from reef habitat contained prey. Prey were identified to the lowest possible taxon and quantified by volumetric measurement. Specific volume of particular prey taxa were calculated by dividing prey volume by individual fish weight. Red snapper shifted diets with increasing size. Small red snapper (<60 mm SL) fed mostly on chaetognaths, copepods, shrimp, and squid. Large red snapper (60–280 mm SL) shifted feeding to fish prey, greater amounts of squid and crabs, and continued feeding on shrimp. We compared red snapper diets for overlapping size classes (70–160 mm SL) of fish that were collected from both habitats (Bray-Curtis dissimilarity index and multidimensional scaling analysis). Red snapper diets separated by habitat type rather than fish size for the size ranges that overlapped habitats. These diet shifts were attributed to feeding more on reef prey than on open-water prey. Thus, the shift in habitat shown by juvenile red snapper was reflected in their diet and suggested differential habitat values based not just on predation refuge but food resources as well.

Diet changes of Pacific cod (Gadus macrocephalus) in Alaska between 1980 and 1995

The diet of Pacific cod (Gadus macrocephalus) in the area of Pavlof Bay, Alaska, was studied in the early 1980s by Albers and Anderson (1985). They found that the dominant prey species were forage species like pandalid shrimp, capelin (Mallotus villosus), and walleye pollock (Theragra chalcogramma). The shrimp fishery in Pavlof Bay began in 1968 and closed in 1980 because of low shrimp abundance (Ruccio and Worton1). Survey data indicate that, during the period between 1972 and 1997, the abundance of forage species such as pandalid shrimp and capelin declined and higher trophic-level groundfish such as Pacific cod increased. There is a general recognition that a long-term ocean climate shift in the Gulf of Alaska has been partially responsible for the observed reorganization of the community structure (Anderson and Piatt, 1999).

Diet of the minimal armhook squid (Berryteuthis anonychus) (Cephalopoda: Gonatidae) in the northeast Pacific during spring

The stomach contents of the minimal armhook squid (Berryteuthis anonychus) were examined for 338 specimens captured in the northeast Pacific during May 1999. The specimens were collected at seven stations between 145−165°W and 39−49°N and ranged in mantle length from 10.3 to 102.2 mm. Their diet comprised seven major prey groups (copepods, chaetognaths, amphipods, euphausiids, ostracods, unidentified fish, and unidentified gelatinous prey) and was dominated by copepods and chaetognaths. Copepod prey comprised four genera, and 86% by number of the copepods were from the genus Neocalanus. Neocalanus cristatus was the most abundant prey taxa, composing 50% by mass and 35% by number of the total diet. Parasagitta elegans (Chaetognatha) occurred in more stomachs (47%) than any other prey taxon. Amphipods occurred in 19% of the stomachs but composed only 5% by number and 3% by mass of the total prey consumed. The four remaining prey groups (euphausiids, ostracods, unidentified fish, and unidentified gelatinous prey) together composed <2% by mass and <1% by number of the diet. There was no major change in the diet through the size range of squid examined and no evidence of cannibalism or predation on other cephalopod species.

Prey consumption of Steller sea lions (Eumetopias jubatus) off Alaska: How much prey do they require?

The effects of seasonal and regional differences in diet composition on the food requirements of Steller sea lions (Eumetopias jubatus) were estimated by using a bioenergetic model. The model considered differences in the energy density of the prey, and differences in digestive efficiency and the heat increment of feeding of different diets. The model predicted that Steller sea lions in southeast Alaska required 45–60% more food per day in early spring (March) than after the breeding season in late summer (August) because of seasonal changes in the energy density of the diets (along with seasonal changes in energy requirements). The southeast Alaska population, at 23,000 (±1660 SD) animals (all ages), consumed an estimated 140,000 (±27,800) t of prey in 1998. In contrast, we estimated that the 51,000 (±3680) animals making up the western Alaska population in the Gulf of Alaska and Aleutian Islands consumed just over twice this amount (303,000 [±57,500] t). In terms of biomass removed in 1998 from Alaskan waters, we estimated that Steller sea lions accounted for about 5% of the natural mortality of gadids (pollock and cod) and up to 75% of the natural mortality of hexagrammids (adult Atka mackerel). These two groups of species were consumed in higher amounts than any other. The predicted average daily food requirement per individual ranged from 16 (±2.8) to 20 (±3.6) kg (all ages combined). Per capita food requirements differed by as much as 24% between regions of Alaska depending on the relative amounts of low–energy-density prey (e.g. gadids) versus high–energy-density prey (e.g. forage fish and salmon) consumed. Estimated requirements were highest in regions where Steller sea lions consumed higher proportions of low–energy-density prey and experienced the highest rates of population decline

Diet composition of large striped bass (Morone saxatilis) in Chesapeake Bay

Large (>458 mm) striped bass (Morone saxatilis) are dominant predators in Chesapeake Bay. In recent years, the Chesapeake Bay stock of striped bass has increased dramatically, raising concerns about their predatory impact and their forage requirements. In response to these concerns and the need for more recent ecological studies, this investigation was conducted to characterize feeding habits of large striped bass in Chesapeake Bay. Stomach contents from 1225 striped bass from 458 to 1151 mm TL were examined in the spring and fall of 1997 and 1998. Striped bass consumed 52 different species of vertebrates and invertebrates; however, only a few species of clupeoid and sciaenid fishes dominated diets across both the seasons and size ranges of striped bass examined. Of finfish species, menhaden (Brevoortia tyrannus) was the dominant prey in most areas and gizzard shad (Dorosoma cepedianum) replaced menhaden in importance in lower salinity waters. Spot (Leiostomus xanthurus) and other sciaenid fishes and anadromous herrings (Alosa spp.) also contibuted large percentages of striped bass diet. Although pelagic schooling fishes formed the majority of the diet, benthic fishes contributed a higher percentage to the diet than in previous studies of striped bass diet composition.

Ontogenetic shifts in natural diet during benthic stages of American lobster (Homarus americanus), off the Magdalen Islands

The natural diet of 506 American lobsters (Homarus americanus) ranging from instar V (4 mm cephalothorax length, CL) to the adult stage (112 mm CL) was determined by stomach content analysis for a site in the Magdalen Islands, Gulf of St. Lawrence, eastern Canada. Cluster and factor analyses determined four size groupings of lobsters based on their diet: <7.5 mm, 7.5 to <22.5 mm, 22.5 to <62.5 mm, and ≥62.5 mm CL. The ontogenetic shift in diet with increasing size of lobsters was especially apparent for the three dominant food items: the contribution of bivalves and animal tissue (flesh) to volume of stomach contents decreased from the smallest lobsters (28% and 39%, respectively) to the largest lobsters (2% and 11%, respectively), whereas the reverse trend was seen for rock crab Cancer irroratus (7% in smallest lobsters to 53% in largest lobsters). Large lobsters also ate larger rock crabs than did small lobsters.

Differences in diet of Atlantic bluefin tuna (Thunnus thynnus) at five seasonal feeding grounds on the New England continental shelf

The stomachs of 819 Atlantic bluefin tuna (Thunnus thynnus) sampled from 1988 to 1992 were analyzed to compare dietary differences among five feeding grounds on the New England continental shelf (Jeffreys Ledge, Stellwagen Bank, Cape Cod Bay, Great South Channel, and South of Martha’s Vineyard) where a majority of the U.S. Atlantic commercial catch occurs. Spatial variation in prey was expected to be a primary influence on bluefin tuna distribution during seasonal feeding migrations. Sand lance (Ammodytes spp.), Atlantic herring (Clupea harengus), Atlantic mackerel (Scomber scombrus), squid (Cephalopoda), and bluefish (Pomatomus saltatrix) were the top prey in terms of frequency of occurrence and percent prey weight for all areas combined. Prey composition was uncorrelated between study areas, with the exception of a significant association between Stellwagen Bank and Great South Channel, where sand lance and Atlantic herring occurred most frequently. Mean stomach-contents biomass varied significantly for all study areas, except for Great South Channel and Cape Cod Bay. Jeffreys Ledge had the highest mean stomach-contents biomass (2.0 kg) among the four Gulf of Maine areas and Cape Cod Bay had the lowest (0.4 kg). Diet at four of the five areas was dominated by one or two small pelagic prey and several other pelagic prey made minor contributions. In contrast, half of the prey species found in the Cape Cod Bay diet were demersal species, including the frequent occurrence of the sessile fig sponge (Suberites ficus). Prey size selection was consistent over a wide range of bluefin length. Age 2–4 sand lance and Atlantic herring and age 0–1 squid and Atlantic mackerel were common prey for all sizes of bluefin tuna. This is the first study to compare diet composition of western Atlantic bluefin tuna among discrete feeding grounds during their seasonal migration to the New England continental shelf and to evaluate predator-prey size relationships. Previous studies have not found a common occurrence of demersal species or a pre-dominance of Atlantic herring in the diet of bluefin tuna.

Improving pinniped diet analyses through identification of multiple skeletal structures in fecal samples

Marine mammal diet is typically characterized by identifying fish otoliths and cephalopod beaks retrieved from stomachs and fecal material (scats). The use and applicability of these techniques has been the matter of some debate given inherent biases associated with the method. Recent attempts to identify prey using skeletal remains in addition to beaks and otoliths are an improvement; however, difficulties incorporating these data into quantitative analyses have limited results for descriptive analyses such as frequency of occurrence. We attempted to characterize harbor seal (Phoca vitulina) diet in an area where seals co-occur with several salmon species, some endangered and all managed by state or federal agencies, or both. Although diet was extremely variable within sampling date, season, year, and between years, the frequency and number of individual prey were at least two times greater for most taxa when prey structures in addition to otoliths were identified. Estimating prey mass in addition to frequency and number resulted in an extremely different relative importance of prey in harbor seal diet. These data analyses are a necessary step in generating estimates of the size, total number, and annual biomass of a prey species eaten by pinnipeds for inclusion in fisheries management plans.

Pinniped diet composition: a comparison of estimation models

Along the west coast of the United States, the potential impact of increasing pinniped populations on declining salmonid (Oncorhynchus spp.) stocks has become an issue of concern. Fisheries managers need species-specific estimates of consumption by pinnipeds to evaluate their impact on salmonid stocks. To estimate consumption, we developed a model that estimates diet composition by reconstructing prey biomass from fecal samples. We applied the model to data collected from harbor seals (Phoca vitulina) that are present year-round in the lower Columbia River where endangered stocks of salmonids pass as returning adults and as seaward-migrating smolts. Using the same data, we applied the split-sample frequency of occurrence model, which avoids reconstructing biomass by assuming that each fecal sample represents an equal volume of consumption and that within each sample each prey item represents an equal proportion of the volume. The two models for estimating diet composition yielded size-specific differences in consumption estimates that were as large as tenfold for the smallest and largest prey. Conclusions about the impact of harbor seal predation on adult salmonids, some of their largest prey species, remain uncertain without some appropriate rationale or further information (e.g. empirical captive studies) to discriminate between these models.

A note on the use of ground water for successful breeding of common carps in extremely cold climate

Common carps are known for prolific breeding habits but they generally do not breed in water with temperature value less than 20 degree C. During winter months of 1985 when the temperature ranged from 15.5-20.5 degree C, the common carps were successfully bred by using ground water having temperature of 25-26 degree C and the results are discussed.