Dominant Prey Types Research Articles

Foraging by larval fish: a full stomach is indicative of high performance but random encounters with prey are also important

Abstract This study contrasts diet composition patterns of larval fish categorized as strong and weak foragers, identified from quadratic relationships between larval length and the number of prey eaten, for 11 fish species. Two sets of alternative hypotheses test whether strong foragers (1) exhibit precocious behaviour by eating later developmental stages of copepods, and (2) take advantage of random encounters with zooplankton, based on the contrast between the two categories in each 1 mm length-class. Results indicate that strong foragers shift their feeding toward earlier copepod developmental stages, which was most apparent in four flatfish species, and demonstrate stronger overall prey selectivity than weak foragers. Inverse modeling revealed the latter is achieved through increases in apparent prey perception and/or responsiveness to dominant prey types (i.e. nauplii and copepodites) and declines for less frequent prey (e.g. veliger and Cladocera). Foraging strength increased modestly with larger eye diameter and mouth gape. Two possible explanations for prey selection patterns are that strong foragers have inherently different capacity to perceive and attack prey, or that after initially eating sufficient large prey to meet metabolic requirements fuller stomachs depend on the ability of larval fish to take advantage of random encounters.

Capturing shell-crushing by large mobile predators using passive acoustics technology

Durophagy (shell-crushing) is a predatory mode that has evolved across numerous marine taxa from relatively sessile crustaceans to large and highly mobile fishes and mammals. Despite its preponderance in the marine environment, the ecology of durophagy (i.e., dynamics and spatiotemporal distribution) remains poorly understood especially for highly mobile species, limiting our ability to quantify their predatory effects on benthic communities. Here, we used passive acoustics (i.e., remote monitoring of underwater sounds with acoustic recorders) to characterize consumption of hard-shelled mollusk prey by a model predator, the whitespotted eagle ray (Aetobatus narinari). Acoustic recordings were made in captivity for 434 total prey items, spanning eight species of hard-shelled mollusks (1 bivalve, 7 gastropods). For all prey types, consumption sequences were generally characterized by an initial high-energy signal (Sound Pressure Level > 160 dB re 1μPa), presumably associated with shell failure, followed by numerous additional signals of lesser energy as prey was further fractured and winnowed by the predator. Fracture events were short-lived (<0.1 s) with peak frequencies ranging from 3.1 to 5.0 kHz, depending on prey type. Statistical analyses showed capacity to distinguish between the two dominant prey types offered (hard clam, Mercenaria mercenaria and banded tulip, Cinctura lilium) based on processing time, the number of fractures, as well as using a suite of energy and spectral features associated with these acoustic signals. Importantly, we noted that the directionality of these relationships (i.e., relative differences in signals between prey types) can change depending on the chronological location within a consumption sequence and the amount of data analyzed (e.g., first fracture event vs. all fracture event), which may present analytical challenges. Additionally, in situ simulation of fracture events in the target environment suggested events could be detected above ambient noise out to 100 s of m. To our knowledge, this is the first attempt to both quantify and classify durophagy using passive acoustics. We recommend that future studies conduct extensive testing in controlled and target environments to build robust data sets capable of supporting feature extraction as well as detection-classification schemes via machine-learning. Lastly, collaborations with biomechanical scientists are suggested to facilitate a better understanding of the mechanisms driving acoustic variation of shell fracture across prey taxa.

Spatial variation in feeding ecology of age-0 lake whitefish Coregonus clupeaformis in Lake Huron

Age-0 lake whitefish Coregonus clupeaformis stomachs from western Lake Huron were examined in 2018 to evaluate spatial variation in feeding ecology. Overall, age-0 lake whitefish had narrow diet niches and similar feeding strategies across sites in western Lake Huron, with all sizes of age-0 lake whitefish specializing on one prey at each site, with some other prey eaten occasionally by most individuals as fish grew at some sites. Although prey selectivity and the dominant prey types varied among sites, feeding success was generally similar among sites for most sizes of fish. For fish collected in neuston nets (13–21 mm), food biomass per fish did not differ between Saginaw Bay and Thunder Bay for fish that had lost their yolk sac even though cyclopoid copepods were selected in Saginaw Bay and calanoid copepods were selected in Thunder Bay, suggesting that the type of copepod available to early stage age-0 lake whitefish might not be as important as previously thought. Fish from beach seines (20–51 mm) also had a narrow diet niche with a fair degree of diet specialization on calanoids, cyclopoids, or Bosmina, depending on site, along with generalized feeding on chironomid pupae at some sites. Selectivity patterns on zooplankton prey by larger age-0 fish followed patterns in prey availability, indicating the ability to adapt to prevailing prey conditions. Despite these spatial differences in prey importance, most fish had food in their stomachs and food biomass per fish did not differ among sites for most size groups, suggesting that variable diet patterns might not result in differential conditions for growth or survival of age age-0 lake whitefish. Adaptability in prey use might not entirely buffer age-0 lake whitefish from environmental variability, as we noted strong differences in both diet composition and feeding success among years (2014–2018) at the Monaghan Point site.

Dietary analysis reveals the vulnerability of the endangered Maugean skate (Zearaja maugeana) to benthic changes in Macquarie Harbour

Species occupying high trophic positions with a small population size, limited diet and restricted range are vulnerable to extinction. The endangered Maugean skate (Zearaja maugeana) in Macquarie Harbour is potentially threatened by changing benthic environmental conditions (declining dissolved oxygen concentrations and decreased macrofaunal diversity). To inform investigations on potential indirect impacts of benthic environmental changes on the Maugean skate and its prey, non-lethal techniques were used to identify dietary composition and trophic position of the species. The Maugean skate occupies a high trophic position (3.70) in the food web, with both stable-isotope (median 86% contribution to isotopic signatures, credible interval (CI)=44–100%) and stomach-content analysis showing crustaceans, particularly brachyuran crabs (58.5% index of relative importance), as the dominant prey type. Aquaculture feed from salmonid fish farms operating in the harbour was not found to be important in skate diet. This study showed that the Maugean skate consumes a small number of benthic prey species, highlighting its vulnerability to changing environmental conditions in the harbour.

Energy dynamics of subyearling Chinook salmon reveal the importance of piscivory to short‐term growth during early marine residence

AbstractVariation in prey quantity and quality can influence growth and survival of marine predators, including anadromous fish that migrate from freshwater systems. The objective of this study was to examine the energy dynamics of subyearling Chinook salmon (Oncorhynchus tshawytscha) following freshwater emigration. To address this objective, a population of Chinook salmon and their marine prey were repeatedly sampled from June to September over 2 years in coastal waters off Oregon and Washington. Subyearlings from the same population were also reared under laboratory conditions. Using a bioenergetics model evaluated in the laboratory, we found that growth rate variability in the field was associated more with differences in northern anchovy (Engraulis mordax) consumption and less with variation in diet energy density or ocean temperature. Highest growth rates (2.43–3.22% body weight/day) occurred in months when anchovy biomass peaked, and the timing of peak anchovy biomass varied by year. Our results support a general pattern among subyearling Chinook salmon occurring from Alaska to California that feeding rates contribute most to growth rate variability during early marine residence, although dominant prey types can differ seasonally, annually, or by ecosystem. In the northern California Current, faster growth appears to be associated with the availability of age‐0 anchovy. Identifying factors that influence the seasonal development of the prey field and regulate prey quantity and quality will improve understanding of salmon growth and survival during early marine residence.

Variation in the diet of Arctic Cod (Boreogadus saida) in the Pacific Arctic and Bering Sea

Arctic cod, Boreogadus saida (Lepechin, 1774), is a nodal species in Arctic marine foodwebs as an important prey of many birds, marine mammals, and other fishes, as well as an abundant predator of zooplankton and epibenthic fauna. We examined the summer diet of Arctic cod across a latitudinal gradient extending from the southern limit of their distribution in the eastern Bering Sea to the northern margins of the eastern Chukchi Sea (ECS) continental shelf. Specimens were collected from demersal and pelagic trawls conducted between 1999 and 2012, and across a range of predator sizes (3–26 cm). Arctic cod diets vary with body size and between regions within the study area, and appear to vary between years in the eastern Bering Sea, indicating opportunistic feeding habits. Constrained Analysis of Principal Coordinates was conducted on ECS demersal samples and revealed consumption of fish and decapod crustacea were positively correlated with Arctic cod length while consumption of euphausiids and copepods had the opposite relationship. The demersal Arctic cod diet in the northern latitudes of the ECS was dominated by copepod consumption (47% by weight, %W), but copepods were less important (12–26%W) in the central and southern latitudes of the ECS and in the northern and eastern Bering Sea—areas where diets were more varied in their composition. High levels of variation in the diet of Arctic cod highlights the need to monitor Arctic cod diets to identify consistently dominant prey types and potential future changes to trophic relationships related to climate change or increasing anthropogenic activity.

Diet of the Torrent Sculpin, Cottus rhotheus, in the Cedar River, Washington: Effect of Season, Habitat Type, and Predator Size with Emphasis on Piscivory

Abstract Torrent Sculpin (Cottus rhotheus) can play an important role in riverine ecosystems in the Pacific Northwest, but factors that influence their feeding ecology are not well understood. To better understand how environmental conditions may influence the diet of Torrent Sculpin in the Cedar River, Washington, we collected diet information from fish across seasons and habitats. Numerically, aquatic insects were usually the dominant prey type for each size class in each sampling effort. Overall, Torrent Sculpin prey on a wide variety of aquatic invertebrates but can also take advantage of other prey types, such as fry of Sockeye Salmon (Oncorhynchus nerka), juvenile Chinook Salmon (O. tshawytscha), and eggs of Largescale Suckers (Catostomus macrocheilus), when these prey become abundant. In February and March, predation on Sockeye Salmon fry was substantially higher in large, deep pools (>2.5-m depth) than in either riffles or slow-water secondary habitats. Besides fry of Sockeye Salmon, the only salm...

Diet compositions and trophic guild structure of the eastern Chukchi Sea demersal fish community

Fishes are an important link in Arctic marine food webs, connecting production of lower trophic levels to apex predators. We analyzed 1773 stomach samples from 39 fish species collected during a bottom trawl survey of the eastern Chukchi Sea in the summer of 2012. We used hierarchical cluster analysis of diet dissimilarities on 21 of the most well sampled species to identify four distinct trophic guilds: gammarid amphipod consumers, benthic invertebrate generalists, fish and shrimp consumers, and zooplankton consumers. The trophic guilds reflect dominant prey types in predator diets. We used constrained analysis of principal coordinates (CAP) to determine if variation within the composite guild diets could be explained by a suite of non-diet variables. All CAP models explained a significant proportion of the variance in the diet matrices, ranging from 7% to 25% of the total variation. Explanatory variables tested included latitude, longitude, predator length, depth, and water mass. These results indicate a trophic guild structure is present amongst the demersal fish community during summer in the eastern Chukchi Sea. Regular monitoring of the food habits of the demersal fish community will be required to improve our understanding of the spatial, temporal, and interannual variation in diet composition, and to improve our ability to identify and predict the impacts of climate change and commercial development on the structure and functioning of the Chukchi Sea ecosystem.

Diet of the Exotic American Bullfrog,Lithobates catesbeianus, in a Stream of Northwestern Baja California, Mexico

We studied the diet of the exotic American bullfrog (Lithobates catesbeianus) in Arroyo San Carlos, located in northwestern Baja California, Mexico, during spring and summer 2009. Analysis of 64 stomach contents revealed 15 prey types, of which the exotic red crayfish (Procambarus clarkii) and terrestrial isopods (Armadillidiidae) were the most significant items (according to an index of their relative importance), making up 64% and 31% of the bullfrog's diet, respectively. Diet differed between the sexes, with red crayfish eaten more frequently by males (84%) than females (69%). Bullfrog diet changed with body size, with smaller individuals (≤86 mm) containing woodlice (61%) as the dominant prey type and larger individuals (≥129 mm) containing red crayfish (97%) as the dominant prey type. The average prey size consumed was significantly correlated with body length.

Importance of Crayfish Prey to Nesting White Ibis (Eudocimus albus)

Abstract. Wading bird prey populations, prey availability and wading bird nesting success are all thought to be associated with hydrologic conditions in wetlands, but the relationship between successful nesting and essential prey types fed to chicks is poorly understood in many cases. Prey fed to White Ibis (Eudocimus albus) chicks were quantified and compared for three colonies in the northern Everglades within and between the 2008 and 2009 nesting seasons. Crayfish were the dominant prey type in both years, being present in 58–88% of chick boluses across all collections. Prey composition was not temporally variable within 2008, but chick diet shifted slightly toward more fish at the end of 2009 when the wetland wras at its driest. Crayfish were the dominant (55–65%) energetic component of White Ibis chick diets in the northern Everglades during both a good nesting year (6,000 nests) with higher water levels (2008) and during an excellent nesting year (9,300 nests) when water levels were lower (2009). Th...

Prey Selection by an Apex Predator: The Importance of Sampling Uncertainty

The impact of predation on prey populations has long been a focus of ecologists, but a firm understanding of the factors influencing prey selection, a key predictor of that impact, remains elusive. High levels of variability observed in prey selection may reflect true differences in the ecology of different communities but might also reflect a failure to deal adequately with uncertainties in the underlying data. Indeed, our review showed that less than 10% of studies of European wolf predation accounted for sampling uncertainty. Here, we relate annual variability in wolf diet to prey availability and examine temporal patterns in prey selection; in particular, we identify how considering uncertainty alters conclusions regarding prey selection.Over nine years, we collected 1,974 wolf scats and conducted drive censuses of ungulates in Alpe di Catenaia, Italy. We bootstrapped scat and census data within years to construct confidence intervals around estimates of prey use, availability and selection. Wolf diet was dominated by boar (61.5±3.90 [SE] % of biomass eaten) and roe deer (33.7±3.61%). Temporal patterns of prey densities revealed that the proportion of roe deer in wolf diet peaked when boar densities were low, not when roe deer densities were highest. Considering only the two dominant prey types, Manly's standardized selection index using all data across years indicated selection for boar (mean = 0.73±0.023). However, sampling error resulted in wide confidence intervals around estimates of prey selection. Thus, despite considerable variation in yearly estimates, confidence intervals for all years overlapped. Failing to consider such uncertainty could lead erroneously to the assumption of differences in prey selection among years. This study highlights the importance of considering temporal variation in relative prey availability and accounting for sampling uncertainty when interpreting the results of dietary studies.

Latitudinal and taxonomic patterns in the feeding ecologies of fish larvae: A literature synthesis

The longtime focus on factors that influence the survival of marine fish larvae has yielded an extensive number of studies on larval fish diets and feeding success. In light of a recent increase in such studies within lower latitudes, results from the peer-reviewed literature were synthesized to examine both latitudinal and taxonomic differences in several trophic-related categories, including feeding incidence, trophic niche breadth, ontogenetic diet shifts, dominant prey types, diet broadness, and larval piscivory. A total of 204 investigations (taxon–article combinations) contained suitable results for at least one of these categories. Feeding incidences (proportions of larvae containing food) were significantly higher in lower latitudes with all taxa combined, as well as only within the order Perciformes. Feeding incidences also differed among orders, with Perciformes and Scorpaeniformes having the highest values. The number of larval taxa exhibiting a significantly increasing niche breadth (SD of the log of prey sizes) with larval size decreased toward lower latitudes, with some taxa in lower latitudes exhibiting a decrease in niche breadth with size. The frequency of exhibiting ontogenetic diets shifts decreased with decreasing latitude, as did relative diet broadness (a function of prey types). The most common dominant prey types in the diets of higher latitude larvae were nauplii and calanoid copepods, with cyclopoids being rare in higher latitudes. Dominant prey types in lower latitudes were more diverse, with nauplii, calanoids, and cyclopoids being equally important. Appendicularians increased in importance with decreasing latitude, and one of the clearest latitudinal distinctions was the display of larval piscivory (almost exclusively by scombroid taxa), which was highly concentrated in lower latitudes. Overall, the latitudinal differences observed for multiple trophic-related factors highlight inherent distinctions in larval fish feeding ecologies, likely reflecting differences in the overall structure of planktonic food webs over large latitudinal gradients.

Prey selection in a nocturnal web-building spider, Eriophora edax (Araneae Araneidae)

A field study was conducted to compare hourly captures by nocturnal adult female orb-web spiders (Eriophora edax) and the insect fauna collected at the same time, using a UV light trap. Predation of the spiders is related to the activity patterns of Lepidoptera, since this order of insects was the dominant prey type of E. edax. Also, E. edax showed a positive selection (Ivlev's index of electivity) for Lepidoptera, which means that the spiders showed a selective over-predation of this prey taxon from the pool of insect prey available in the habitat. Based on our results, we suggest that this spider's behaviour might represent an adaptation to the first step in the evolution of a spider-moth specialisation.

Feeding behaviour of Black Sea bottom fishes: Did it change over time?

This study was designed to improve knowledge in feeding behaviour of the round goby ( Apollonia melanostomus (Pallas, 1814)), the red mullet ( Mullus barbatus ponticus Essipov, 1927), the whiting ( Merlangius merlangus (Linnaeus, 1758)), the flounder ( Platichthys flesus (Linnaeus, 1758)), the sole ( Solea solea (Linnaeus, 1758)), the turbot ( Psetta maeotica (Pallas, 1814)) and the starry sturgeon ( Acipenser stellatus Pallas, 1771) from the north-western Black Sea. Gut content coupled with stable isotope analysis allowed describing food web variations according to species, in two seasons and at two areas located seawards the Danube River. Present results showed that most fishes have likely changed their feeding behaviour compared to past studies from the same area. Trophic niches were reduced and dietary overlap was common, as different fish species consumed the same dominant prey types. Fishes probably adapted their feeding behaviour to the increasingly low biodiversity of the Black Sea communities.

Feeding Habits of Young-of-the-year Black and Copper Rockfish in Eelgrass Habitats of Humboldt Bay, California

Abstract Temperate eelgrass beds are temporally and spatially complex habitats that often provide a transition habitat for many species of juvenile fishes. The feeding habits of young-of-the-year Black Rockfish (Sebastes melanops) and Copper Rockfish (S. caurinus) inhabiting eelgrass (Zostera marina) beds in Humboldt Bay, California, were assessed in 2003 and 2004. A total of 16 prey types were noted for both species. Gammarid amphipods and copepods were the dominant prey types. Shifts in prey type were observed for Copper Rockfish as prey preference shifted from smaller planktonic prey to larger epiphytic prey types over time. No shifts in prey type were noted for Black Rockfish. Prey types for both species were consistent with previous studies conducted in temperate eelgrass habitats, which noted the importance of small crustaceans as prey.

Variation in the trophic basis of production and energy flow associated with emergence of larval salamander assemblages from forest ponds

Summary1. Larval amphibians are a dominant consumer in many freshwater systems, yet limited data on energy transfers between aquatic food resources and larvae and between metamorphosed larvae and adjacent habitats preclude an accurate assessment of their roles as links between aquatic and terrestrial food webs.2. During 2003–04, we derived prey‐specific assimilation efficiencies, analysed stomach contents, and intensively sampled ambystomatid salamander assemblages in four ponds to quantify the trophic basis of larval production. Using estimates of the contribution of each prey taxon to larval production, we constructed quantitative food webs and assessed variation in pathways of energy flow associated with emergences.3. Overall, metamorphosed salamanders exported 3–8% of total prey production, required to account for total salamander production, to adjacent forest. Aquatic insects, zooplankton and amphibian prey were most important to energy flow associated with emergence; amounts of larval production attributed to each of these prey types shifted during development and varied among salamander taxa.4. The majority of variation in the trophic basis of production among species was attributed to copepods (Cyclopidae) and three families of aquatic insects (Chironomidae, Chaoboridae and Culicidae). Dominant prey types contributing to the production of metamorphosed salamanders varied among ponds, representing different pathways for energy transfers between aquatic resources and forest habitats. These findings further our understanding of the ecological roles of amphibians and thus the consequences of amphibian declines and extinctions.

Diet of large-eye dentex Dentex macrophthalmus (Pisces: Sparidae) off Angola and Namibia

The large-eye dentex Dentex macrophthalmus is a major target species of the demersal fishery off Angola, and is taken as bycatch off northern Namibia. Its prey spectrum off Angola and Namibia is described from the stomach contents of 593 individuals collected between northern Namibia (21°S) and northern Angola (6°S) during a bottom trawl survey carried out in 1997. In all, 20 prey types were recorded in the diet, demonstrating that it is an opportunist feeder. The main food items were fish (80.9%), euphausiids (12.0%), polychaetes (4.8%), crustaceans (2.5%) and cephalopods (0.1%). The variety of prey increases with fish size, but there is no systematic effect of depth on the diet. The dominant prey types varied as a function of fish size as well as spatially, demonstrating the different ecological niches provided by the Angola Current off northern Angola, the cold Benguela Current off northern Namibia, and the Angola-Benguela Front off southern Angola.

Convergent evolution of chemical defense in poison frogs and arthropod prey between Madagascar and the Neotropics.

With few exceptions, aposematically colored poison frogs sequester defensive alkaloids, unchanged, from dietary arthropods. In the Neotropics, myrmicine and formicine ants and the siphonotid millipede Rhinotus purpureus are dietary sources for alkaloids in dendrobatid poison frogs, yet the arthropod sources for Mantella poison frogs in Madagascar remained unknown. We report GC-MS analyses of extracts of arthropods and microsympatric Malagasy poison frogs (Mantella) collected from Ranomafana, Madagascar. Arthropod sources for 11 "poison frog" alkaloids were discovered, 7 of which were also detected in microsympatric Mantella. These arthropod sources include three endemic Malagasy ants, Tetramorium electrum, Anochetus grandidieri, and Paratrechina amblyops (subfamilies Myrmicinae, Ponerinae, and Formicinae, respectively), and the pantropical tramp millipede R. purpureus. Two of these ant species, A. grandidieri and T. electrum, were also found in Mantella stomachs, and ants represented the dominant prey type (67.3% of 609 identified stomach arthropods). To our knowledge, detection of 5,8-disubstituted (ds) indolizidine iso-217B in T. electrum represents the first izidine having a branch point in its carbon skeleton to be identified from ants, and detection of 3,5-ds pyrrolizidine 251O in A. grandidieri represents the first ponerine ant proposed as a dietary source of poison frog alkaloids. Endemic Malagasy ants with defensive alkaloids (with the exception of Paratrechina) are not closely related to any Neotropical species sharing similar chemical defenses. Our results suggest convergent evolution for the acquisition of defensive alkaloids in these dietary ants, which may have been the critical prerequisite for subsequent convergence in poison frogs between Madagascar and the Neotropics.

DIETS OF FIN, SEI, AND SPERM WHALES IN BRITISH COLUMBIA: AN ANALYSIS OF COMMERCIAL WHALING RECORDS, 1963–1967

AbstractDiets of fin (Balaenoptera physalus), sei (Balaenoptera borealis), and sperm whales (Physeter macrocephalus) were estimated from the stomach contents of individuals killed along the British Columbia coast from 1963 to 1967. The dominant prey types of fin whales were euphausiids, with minor contributions from copepods and fish. Sei whale stomachs contained primarily copepods in three years, whereas euphausiids or a variety of fish dominated the diet in the other two years. Sperm whales consumed primarily North Pacific giant squid (Moroteuthis robusta), but secondary prey differed between males and females. Female sperm whales frequently consumed ragfish (Icosteus spp.) and other fish, whereas the male diet also contained rockfish (Sebastes spp.). The high abundance of euphausiids along the British Columbia coast likely contributed to the presence of a summer resident population of fin whales. The high abundance of large copepods farther north probably influenced the migration of sei whales through the offshore waters of British Columbia. Sperm whale stomach contents differed by sex reflecting location and possibly breeding behaviors.

DIET OF RINGED SEALS (PHOCA HISPIDA) ON THE EAST AND WEST SIDES OF THE NORTH WATER POLYNYA, NORTHERN BAFFIN BAY

AbstractIn conjunction with the International North Water Polynya Study (NOW) in northern Baffin Bay, we examined the diets of ringed seals (Phoca hispida)1 on the west (Grise Fiord, Nunavut) and east (Qaanaaq, Greenland) sides of the polynya, using conventional stomach content analysis, as well as inferences from stable isotope ratios in seal muscle. Between May and July 1998, stomach and muscle tissue samples were collected from 99 ringed seals taken near Grise Fiord and 100 taken near Qaanaaq. The amphipod Themisto libellula was the dominant prey type in the diet of immature ringed seals from Grise Fiord, whereas arctic cod (Boreogadus saida) and polar cod (Arctogadus glacialis) predominated in the diet of adults. Both immature and adult seals collected near Qaanaaq fed predominantly on arctic cod. Overall, seals collected near Grise Fiord had significantly higher δ13C values than those collected near Qaanaaq (P &lt; 0.001), but there was no statistical separation in δ15N values between the two samples (P= 0.06). Differences in diets of ringed seals from the east and west sides of the North Water Polynya may be due to differences in prey distribution and/or differences in biological productivity and fish biomass within the polynya.