Changes in diets of individual Baltic ringed seals (Phoca hispida botnica) during their breeding season inferred from stable isotope analysis of multiple tissues

The stable isotope ratios (δ¹³C and δ¹⁵N) of three tissues with different metabolic rates (plasma, liver, and muscle) were used to investigate temporal variation in diet among nine individual Baltic ringed seals (Phoca hispida botnica Gmelin) from the Bothnian Bay, northeast Baltic Sea. The isotope values from plasma should reflect the most recent diet, values from liver the diet of the past weeks prior to sampling, and values from muscle should integrate diet over almost the entire breeding season of the ringed seals. In general, δ¹³C values of liver were more enriched in ¹³C than were those of either muscle or plasma, suggesting that the diet of the seals may have included a higher proportion of ¹³C‐enriched benthic prey in April. Females showed more variable δ¹³C values than males, suggesting possible gender differences in diet or in foraging locations. The differences that were apparent between females possibly reflect individual variation in the onset and duration of parturition and lactation, both of which likely restrict female foraging. Previous data from parasite infections and from alimentary tract contents of the same seals were linked to the isotope data to assist in drawing inferences about changes in the diets of individual seals.     

Fast carnivores and slow herbivores: differential foraging strategies among grizzly bears in the Canadian Arctic

Categorizing animal populations by diet can mask important intrapopulation variation, which is crucial to understanding a species’ trophic niche width. To test hypotheses related to intrapopulation variation in foraging or the presence of diet specialization, we conducted stable isotope analysis (δ¹³C, δ¹⁵N) on hair and claw samples from 51 grizzly bears (Ursus arctos) collected from 2003 to 2006 in the Mackenzie Delta region of the Canadian Arctic. We examined within-population differences in the foraging patterns of males and females and the relationship between trophic position (derived from δ¹⁵N measurements) and individual movement. The range of δ¹⁵N values in hair and claw (2.0–11.0‰) suggested a wide niche width and cluster analyses indicated the presence of three foraging groups within the population, ranging from near-complete herbivory to near-complete carnivory. We found no linear relationship between home range size and trophic position when the data were continuous or when grouped by foraging behavior. However, the movement rate of females increased linearly with trophic position. We used multisource dual-isotope mixing models to determine the relative contributions of seven prey sources within each foraging group for both males and females. The mean bear dietary endpoint across all foraging groups for each sex fell toward the center of the mixing polygon, which suggested relatively well-mixed diets. The primary dietary difference across foraging groups was the proportional contribution of herbaceous foods, which decreased for both males and females from 42–76 to 0–27% and 62–81 to 0–44%, respectively. Grizzlies of the Mackenzie Delta live in extremely harsh conditions and identifying within-population diet specialization has improved our understanding of varying habitat requirements within the population.     

Penguins, fur seals, and fishing: prey requirements and potential competition in the South Shetland Islands, Antarctica

Antarctic and sub-Antarctic seabirds, marine mammals, and human fisheries concentrate their foraging efforts on a single species, Antarctic krill (Euphausiasuperba). Because these predators may have a significant effect on krill abundance, we estimated the energy and prey requirements of Adelie (Pygoscelisadeliae), chinstrap (Pygoscelisantarctica), and gentoo (Pygoscelispapua) penguins and female Antarctic fur seals (Arctocephalusgazella) breeding on the South Shetland Islands, Antarctica and compared these estimates with catch statistics from the Antarctic krill fishery. Published data on field metabolic rate, population size, diet, prey energy content, and metabolic efficiency were used to estimate prey requirements of these breeding, adult, land-based predators and their dependent offspring. Due to their large population size, chinstrap penguins were the most significant krill predators during the period examined, consuming an estimated 7.8 × 10⁸ kg krill, followed by Adelie penguins (3.1 × 10⁷ kg), gentoo penguins (1.2 × 10⁷ kg), and Antarctic fur seals (3.6 × 10⁶ kg). Total consumption of all land-based predators on the South Shetland Islands was estimated at 8.3 × 10⁸ kg krill. The commercial krill fishery harvest in the South Shetland Island region (1.0 × 10⁸ kg) was approximately 12% of this. Commercial harvest coincides seasonally and spatially with peak penguin and fur seal prey demands, and may affect prey availability to penguins and fur seals. This differs from the conclusions of Ichii et al. who asserted that the potential for competition between South Shetland predators and the commercial krill fishery is low. Received: 26 August 1997 / Accepted: 16 December 1997     

Ecological implications of unprecedented warm water anomalies on interannual prey preferences and foraging areas of Guadalupe fur seals

The Guadalupe fur seal (GFS) currently is recovering from near extinction and prey availability is an important factor. Scat and stable isotope analyses (SIA) were used to assess GFS foraging on Guadalupe Island during 2013–2016. This period was characterized by normal (2013) ocean temperatures followed by warm conditions (2014–2016). Scat samples (~40/year) were collected for prey identification. Additionally, lanugo samples from 1-month-old pups (50/year) were processed for SIA (δ¹³C/δ¹⁵N). Interannual prey preferences were identified (ANOSIM, p < .05), including significant differences between 2013 and the anomalous years. The most important prey species among these years was the jumbo squid; followed by the neon flying squid (2014–2016). The GFS diet was more specialized in 2013–2015 and more generalized in 2016. Prey from higher trophic levels (determined by scat analysis) were consumed during 2014–2016, but with significantly lower δ¹³C/δ¹⁵N values than in 2013 (ANOVA, p < .05 for both isotope ratios). This pattern may indicate more northerly or offshore foraging areas, with the presence of oceanic prey (neon flying squid). The widest isotopic niche was observed in 2015 (2.2‰²), reflecting a broader foraging area. Our findings are an important step toward better understanding the impacts of climate change on the recovery of GFS.     

Using Satellite Tracking and Isotopic Information to Characterize the Impact of South American Sea Lions on Salmonid Aquaculture in Southern Chile

Apex marine predators alter their foraging behavior in response to spatial and/or seasonal changes in natural prey distribution and abundance. However, few studies have identified the impacts of aquaculture that represents a spatially and temporally predictable and abundant resource on their foraging behavior. Using satellite telemetry and stable isotope analysis we examined the degree of spatial overlap between the South American sea lion (SASL) and salmon farms, and quantify the amount of native prey versus farmed salmonids in SASL diets. We instrumented eight SASL individuals with SRDL-GPS tags. Vibrissae, hair and skin samples were collected for δ¹³C and δ¹⁵N analyses from five of the tagged individuals and from four males captured in a haul-out located adjacent to salmon farms. Tracking results showed that almost all the foraging areas of SASL are within close proximity to salmon farms. The most important prey for the individuals analyzed was farmed salmonids, with an estimated median (±SD) contribution of 19.7 ± 13.5‰ and 15.3 ± 9.6‰ for hair and skin, respectively. Using vibrissae as a temporal record of diet for each individual, we observed a remarkable switch in diet composition in two SASL, from farmed salmonids to pelagic fishes, which coincided with the decrease of salmon production due to the infectious salmon anemia virus that affected salmon farms in Chile at the end of 2008. Our study demonstrates the usefulness of integrating stable isotope derived dietary data with movement patterns to characterize the impacts of a non-native prey on the foraging ecology of an apex marine predator, providing important applied implications in situations where interactions between aquaculture and wildlife are common.     

Isotopic tracking of foraging and long-distance migration in northeastern Pacific pinnipeds

We investigated the impact of foraging location (nearshore vs offshore) and foraging latitude (high vs middle) on the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope compositions of bone collagen of northern fur seals (Callorhinus ursinus), harbor seals (Phoca vitulina), California sea lions (Zalophus californianus), and northern elephant seals (Mirounga angustirostris). Nearshore-foraging harbor seals from California had δ¹³C values 2.0‰ higher than female northern elephant seals foraging offshore at similar latitudes. Likewise, nearshore-foraging harbor seals from Alaska had values 1.7‰ higher than male northern fur seals, which forage offshore at high latitudes. Middle-latitude pinnipeds foraging in either the nearshore or offshore were ¹³C enriched by ∼1.0‰ over similar populations from high latitudes. Male northern elephant seals migrate between middle and high latitudes, but they had δ¹³C values similar to high-latitude, nearshore foragers. Female northern fur seal δ¹³C values were intermediate between those of high- and middle-latitude offshore foragers, reflecting their migration between high- and middle-latitude waters. The δ¹³C values of California sea lions were intermediate between nearshore- and offshore-foraging pinnipeds at middle latitudes, yet there was no observational support for the suggestion that they use offshore food webs. We suggest that their “intermediate” values reflect migration between highly productive and less-productive, nearshore ecosystems on the Pacific coasts of California and Mexico. The relative uniformity among all of these pinnipeds in δ¹⁵N values, which are strongly sensitive to trophic level, reveals that the carbon isotope patterns result from differences in the δ¹³C of organic carbon at the base of the food web, rather than differences in trophic structure, among these regions. Finally, the magnitude and direction of the observed nearshore-offshore and high-to middle-latitude differences in δ¹³C values suggest that these gradients may chiefly reflect differences in rates and magnitudes of phytoplankton production as well as the δ¹³C value of inorganic carbon available for photosynthesis, rather than the input of ¹³C-enriched macroalgal carbon to nearshore food webs. Received: 8 September 1998 / Accepted: 24 February 1999     

Spatially Explicit Estimates of Prey Consumption Reveal a New Krill Predator in the Southern Ocean

Development in foraging behaviour and dietary intake of many vertebrates are age-structured. Differences in feeding ecology may correlate with ontogenetic shifts in dispersal patterns, and therefore affect foraging habitat and resource utilization. Such life-history traits have important implications in interpreting tropho-dynamic linkages. Stable isotope ratios in the whiskers of sub-yearling southern elephant seals (Mirounga leonina; n = 12) were used, in conjunction with satellite telemetry and environmental data, to examine their foraging habitat and diet during their first foraging migration. The trophic position of seals from Macquarie Island (54°30′S, 158°57′E) was estimated using stable carbon (δ^{1 3}C) and nitrogen (δ¹⁵N) ratios along the length of the whisker, which provided a temporal record of prey intake. Satellite-relayed data loggers provided details on seal movement patterns, which were related to isotopic concentrations along the whisker. Animals fed in waters south of the Polar Front (>60°S) or within Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Statistical Subareas 88.1 and 88.2, as indicated by both their depleted δ^{1 3}C (<−20‰) values, and tracking data. They predominantly exploited varying proportions of mesopelagic fish and squid, and crustaceans, such as euphausiids, which have not been reported as a prey item for this species. Comparison of isotopic data between sub-yearlings, and 1, 2 and 3 yr olds indicated that sub-yearlings, limited by their size, dive capabilities and prey capture skills to feeding higher in the water column, fed at a lower trophic level than older seals. This is consistent with the consumption of euphausiids and most probably, Antarctic krill (Euphausia superba), which constitute an abundant, easily accessible source of prey in water masses used by this age class of seals. Isotopic assessment and concurrent tracking of seals are successfully used here to identify ontogenetic shifts in broad-scale foraging habitat use and diet preferences in a highly migratory predator.     

Feeding ecology of phocid seals and some walrus in the Alaskan and Canadian Arctic as determined by stomach contents and stable isotope analysis

Feeding habits of ringed (Phoca hispida), bearded (Erignathus barbatus), spotted (Phoca largha) and ribbon (Phoca fasciata) seals and walrus (Odobenus rosmarus) were studied using stomach contents and stable carbon and nitrogen isotopes. Bearded seals fed benthically, primarily crustaceans and mollusks. Both zooplankton and fish were significant prey for ringed seals, while fish was principal spotted seal prey. Few gastric contents were available from ribbon seals. δ¹⁵N was positively correlated with age in ribbon seals and δ¹³C was positively correlated with age in ringed and ribbon seals. δ¹⁵N was highest in spotted seals, in agreement with their fish-dominated diet. δ¹⁵N was not different between Alaskan-harvested ringed and bearded seals, while δ¹⁵N was lowest in ribbon seals and walrus. Carbon-13 was most enriched in bearded seals and walrus reflecting benthic ecosystem use. Canadian ringed seals were depleted in ¹³C compared to Alaskan pinnipeds, likely because of Beaufort Sea versus Chukchi and Bering seas influence.     

Sexual differences in the foraging ecology of 19th century beluga whales (Delphinapterus leucas) from the Canadian High Arctic

Marine mammals often exhibit significant sexual segregation in their diet and habitat use but these differences have not been studied systematically in historic or ancient populations due to the difficulties associated with determining the sex of skeletal elements based on gross morphology. Using a combined ancient DNA and stable isotope approach, we document a sexual difference in the foraging ecology of late 19th century beluga whales (Delphinapterus leucas) from the Canadian High Arctic. Using two PCR assays that coamplify fragments of the Y-linked SRY and X-linked ZFX genes, we assigned reproducible sex identities to 35 beluga specimens. This provided a basis for investigating sex-specific differences in foraging ecology using stable carbon and nitrogen isotope analyses of bone collagen. These isotopic data demonstrate that although both males and females primarily consumed Arctic cod, males utilized a wider range of prey than females, feeding on high trophic level benthic prey (sculpins) to a greater extent. Because bone collagen integrates prey isotopic compositions over the course of several years these sex-based differences in beluga bone collagen isotopic compositions reflect long-term and sustained sexual differences in foraging.     

The diet of the Antarctic fur seal Arctocephalus gazella at Harmony Point, Nelson Island, South Shetland Islands

The diet of non-breeding male Antarctic fur seals (Arctocephalus gazella) was investigated at Harmony Point, Nelson Island, South Shetland Islands, by the analysis of 18 and 33 scats collected during February 1996 and 1997 respectively. Overall, fish were the most frequent prey (74.5%) and predominated by mass (54.4%), whereas krill predominated by number (94.2%). This coincides well with the pattern observed in 1997, but in 1996 krill was the most important prey by number and mass (50.2%). The importance of the remaining taxa represented in the samples (octopods, hyperiids and bivalves) was negligible. Among fish, myctophids represented 85.2% of the fish mass, with Gymnoscopelus nicholsi and Electrona antarctica being the main prey. These two species predominated in 1997, whereas the channichthyid Cryodraco antarcticus and the nototheniid Gobionotothen gibberifrons were dominant in 1996. The importance of the myctophids as prey of the Antarctic fur seal is discussed. Received: 7 October 1997 / Accepted: 24 May 1998     

Diet differences among age classes of Arctic seals: evidence from stable isotope and mercury biomarkers

A basic understanding of current food web dynamics and baseline data from which to measure future change is necessary to understand species re-distribution and altered competition for food with climate change. We use mercury (Hg) and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios as biomarkers to understand species diet differences and age class differences among ringed (Phoca hispida), bearded (Erignathus barbatus), and harbour (P. vitulina) seals in a subarctic marine ecosystem. Adult bearded seals had significantly lower δ¹⁵N and muscle Hg than bearded seal pups, whereas the opposite was observed in ringed seals where pups had lower δ¹⁵N than adults, suggesting age specific foraging differences in trophic food level for both species. For harbour seals, δ¹⁵N did not differ significantly among ages while Hg and δ¹³C did. The δ¹³C in muscle supports that bearded seals in this study are benthic feeders and are part of a separate food web from ringed seals and harbour seals. Harbour seals had the highest levels of mercury and δ¹⁵N, indicating they feed at a higher trophic level relative to the other two seal species. Carbon and nitrogen isotopic ratios and Hg levels illustrate how resources are partitioned among three seal species and offer evidence for separation based on life stages within species.     

Inferring prey size variation from mandible acceleration in northern elephant seals

Prey size is an important factor for predators as it affects prey quality (energy content) and hence total energy gain. However, it remains challenging to obtain information about prey size from free‐ranging marine predators. Here, we developed a method that estimates prey size using mandible acceleration in captive northern elephant seals and then applied it to 34 free‐ranging seals. In captive seals, the number of feeding‐related acceleration signals were positively related to prey size category (<15 cm). In free‐ranging seals, smaller number of acceleration signals occurred frequently in both mesopelagic (200–1,000 m) and bathypelagic layers (>1,000 m), suggesting that seals foraged mainly on smaller prey (possibly <15 cm). However, the quantity of larger acceleration signals increased in the bathypelagic layers, suggesting that seals were more likely to forage on larger prey (>15 cm) at deeper depths. These results suggest that seals might compensate for higher energetic costs of deeper‐diving by targeting larger prey. Although our study has practical limitations (e.g., calibrating prey size in captive conditions), our method allows concurrent inference of prey size and foraging behavior, being potentially useful to investigate how predators adjust their behavior in response to the changes in the foraging environment.     

Seasonal diving behaviour in lactating subantarctic fur seals on Amsterdam Island

Diving behaviour was investigated in female subantarctic fur seals (Arctocephalus tropicalis) breeding on Amsterdam Island, Indian Ocean. Data were collected using electronic Time Depth Recorders on 19 seals during their first foraging trip after parturition in December, foraging trips later in summer, and during winter. Subantarctic fur seals at Amsterdam Island are nocturnal, shallow divers. Ninety-nine percent of recorded dives occurred at night. The diel dive pattern and changes in dive parameters throughout the night suggest that fur seals follow the nycthemeral migrations of their main prey. Seasonal changes in diving behaviour amounted to the fur seals performing progressively deeper and longer dives from their first foraging trip through winter. Dive depth and dive duration increased from the first trip after parturition (16.6 ± 0.5 m and 62.1 ± 1.6 s respectively, n=1000) to summer (19.0 ± 0.4 m and 65 ± 1 s, respectively, n=2000) through winter (29.0 ± 1.0 m and 91.2 ± 2.2 s, respectively, n=800). In summer, subantarctic fur seals increased the proportion of time spent at the bottom during dives of between 10 and 20 m, apparently searching for prey when descending to these depths, which corresponded to the oceanic mixed layer. In winter, fur seals behaved similarly when diving between 20 and 50 m, suggesting that the most profitable depths for feeding moved down during the study period. Most of the dives did not exceed the physiological limits of individuals. Although dive frequency did not vary (10 dives/h of night), the vertical travel distance and the time spent diving increased throughout the study period, while the post-dive interval decreased, indicating that subantarctic fur seals showed a greater diving effort in winter, compared to earlier seasons. Accepted: 1 August 1999     

Stable isotope composition and parasitic infections of harbor seal young‐of‐the‐year used as prey‐based diet indicators

The stable isotope composition (δ¹³C and δ¹⁵N values) of harbor seals (Phoca vitulina) is influenced by their diet. Young‐of‐the‐year during lactation and postweaning fast are expected be enriched in ¹⁵N compared to foraging seals. We studied the temporal variation of stable isotope composition of young‐of‐the‐year and adults to determine from which point in time the young‐of‐the‐year tissues (i.e., muscles and vibrissae) are influenced by independent foraging only. These results were compared with the development of trophically transmitted parasitic infections. The δ¹⁵N values in young‐of‐the‐year muscles decreased from June (20.3‰ ± 0.5‰) to October (18.5‰ ± 0.4‰), while those of foraging seals were all year long below 19.2‰. This decrease coincides with the increase of parasitic infections in young‐of‐the‐year, reflecting a shift to fish diet. Together these results suggest that the muscles of the young‐of‐the‐year older than 5–6 mo reflect independent foraging and that they can therefore be used in community diet studies. The nursing signal in vibrissae was unclear, as the δ¹⁵N values of young‐of‐the‐year were stable over time, whereas those of adults varied seasonally. However, δ¹⁵N values of nursing pups were significantly higher than those of adults in May and June, maybe due to their reliance on milk.     

Large‐scale molecular barcoding of prey DNA reveals predictors of intrapopulation feeding diversity in a marine predator

Predator–prey interactions are critical in understanding how communities function. However, we need to describe intraspecific variation in diet to accurately depict those interactions. Harbor seals (Phoca vitulina) are an abundant marine predator that prey on species of conservation concern. We estimated intrapopulation feeding diversity (variation in feeding habits between individuals of the same species) of harbor seals in the Salish Sea. Estimates of feeding diversity were examined relative to sex, month, and location using a novel approach that combined molecular techniques, repeated cross‐sectional sampling of scat, and a specialization metric (within‐individual consistency in diet measured by the Proportional Similarity Index ()). Based on 1,083 scat samples collected from five haul‐out sites during four nonsequential years, we quantified diet using metabarcoding techniques and determined the sex of the scat depositor using a molecular assay. Results suggest that intrapopulation feeding diversity was present. Specialization was high over short periods (24–48 hr,  = 0.392, 95% CI = 0.013, R = 100,000) and variable in time and space. Females showed more specialization than males, particularly during summer and fall. Additionally, demersal and benthic prey species were correlated with more specialized diets. The latter finding suggests that this type of prey likely requires specific foraging strategies and that there are trade‐offs between pelagic and benthic foraging styles for harbor seals. This differential feeding on prey species, as well as between sexes of harbor seals, indicates that predator–prey interactions in harbor seals are complex and that each sex may have a different impact on species of conservation concern. As such, describing intrapopulation feeding diversity may unravel hitherto unknown complex predator–prey interactions in the community.     

The take of fish species by seabirds and marine mammals in the Australian Fisheries Zone around Heard Island: the potential for competition with a commercial fishery

. The number of predators from Heard Island foraging in shelf waters, their prey requirements, and the proportion of their diet that was commercial and non-commercial fish were estimated. The calculated annual consumption of commercial fish species varied between 36,360 and 84,166 tonnes. The non-commercial Krefftichthys anderssoni was the preferred prey for most predators, and when its occurrence in diets was low it was replaced by crustaceans and commercial fish species. The estimated annual consumption of Champsocephalus gunnari was approximately 2 and 6 times the highest and lowest estimates respectively of the biomass of this species, obtained from three fisheries research cruises. For Dissostichus eleginoides, the maximum estimate was 28% of the highest estimate of biomass. The current fishery for D. eleginoides will most likely impact on southern elephant seals, whose population decreased by 50% between the 1950s and the 1980s, possibly as a result of overfishing around Iles Kerguelen. Received: 26 January 1998 / Accepted: 20 April 1998     

Diet of ringed seals (Pusa hispida) from Northeast Greenland

The diet of ringed seals (Pusa hispida) from coastal and offshore areas of Northeast Greenland was determined by identifying, to the lowest taxonomic limit possible, all hard-part contents from the gastrointestinal tract of 51 seals sampled (2002–2004) in spring (April to June, N = 35) and autumn (September to October, N = 16). The autumn diet was characterized by high numbers of Parathemisto libellula, and the spring diet was comprised primarily of polar cod (Boreogadus saida), with few invertebrates consumed. The coastal seal diet samples had a diverse fish prey composition (during both the spring and autumn), whereas the open water seals had eaten mostly crustaceans with P. libellula being most abundant. The sample sizes from the various locations and seasons were not large enough to explore age-class effects on diet in addition. Similar to earlier studies, this study suggests that the ringed seal is a generalist that exploits prey based on availability, with a few key species dominating the diet in an area at least on a seasonal basis.     

Annual and seasonal changes in diets of martens: evidence from stable isotope analysis

Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available. Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents (Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly, from mid-February to mid-December 1992–1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992, from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991 and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis. Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition during years when small rodents were not readily available. Received: 1 May 1996 / Accepted: 24 February 1997     

The contrasting diet of Harpagifer antarcticus (Notothenioidei, Harpagiferidae) at two localities of the South Shetland Islands, Antarctica

The diet of Harpagifer antarcticus was studied at two localities of the South Shetland Islands, Antarctica. The analysis of the stomach contents of specimens collected in tide pools at Potter Cove, King George Island, indicated that gammarid amphipods (mainly Gondogeneia sp.) were the main prey of this fish, followed by polychaetes, gastropods and isopods. By contrast, the specimens from Harmony Point, Nelson Island, which were recovered from stomach contents of Antarctic shags Phalacrocorax bransfieldensis feeding at depths of 46–110 m, preyed almost exclusively on the Antarctic krill Euphausia superba. These results are discussed and compared with previous studies. Received: 27 August 1997 / Accepted: 17 November 1997     

Feeding ecology of two planktonic sculpins, Comephorus baicalensis and Comephorus dybowskii (Comephoridae), in Lake Baikal

Planktonic sculpins (Comephorus baicalensis and Comephorus dybowskii) are endemic to Lake Baikal, and their migration behavior is characterized by their strange movements, which occur only vertically. We investigated their detailed food sources at each growth stage by stable isotope analyses in combination with stomach content analysis. At all growth stages, both fishes and amphipods were the main diet for C. baicalensis, whereas amphipods alone were the main diet for C. dybowskii. Our results indicate that the relationship between the two species shifts from a competitive interaction to a predator–prey interaction with the growth stages of C. baicalensis.