AERIAL CENSUS OF PACIFIC WALRUSES IN THE CHUKCHI SEA, 1985

The U.S. Fish and Wildlife Service conducted a survey of the walruses in the pack ice of the Chukchi Sea between 16 September and 2 October 1985, as part of a joint effort with the Soviet Union to estimate the size of the Pacific walrus population. American observers conducted censuses from two aircraft along randomly selected north–south lines over the pack ice. The observers counted walruses within a constant viewing angle that corresponded to a total strip width of 1.38 km at an altitude of 152 m.
In nine days of flying, 15,312 walruses were observed, of which 10,140 were on 5,764 km² of census strips. Few walruses were observed east of 161°W longitude or west of 170°W longitude, hence the census effort was stratified. Walrus concentrations between 161° and 170° shifted slightly westward during the 2-wk duration of the censuses. The differences among days in observed walrus density were due to changes in the numbers of walruses on the ice within 37 km of the ice edge. The number of observable walruses in pack ice of the eastern Chukchi Sea was estimated to be 62,177 (SD = 19,480), based on censuses conducted on 29 and 30 September and 1 October. At that time there were also at least 15,238 in Bristol Bay, Bering Sea. The Soviets counted 39,572 on the shores of the western Chukchi and Bering seas and estimated 115,531 in pack ice of the western Chukchi Sea. Summing U.S. and Soviet estimates, the total population of Pacific walruses in 1985 was 232,518. This number was comparable with earlier estimates from censuses conducted jointly by the U.S. and the Soviets. However, information on fraction hauled out, segregation, and movements is needed for more precise estimates.     

Pacific Walrus (Odobenus rosmarus divergens) Resource Selection in the Northern Bering Sea

The Pacific walrus is a large benthivore with an annual range extending across the continental shelves of the Bering and Chukchi Seas. We used a discrete choice model to estimate site selection by adult radio-tagged walruses relative to the availability of the caloric biomass of benthic infauna and sea ice concentration in a prominent walrus wintering area in the northern Bering Sea (St. Lawrence Island polynya) in 2006, 2008, and 2009. At least 60% of the total caloric biomass of dominant macroinfauna in the study area was composed of members of the bivalve families Nuculidae, Tellinidae, and Nuculanidae. Model estimates indicated walrus site selection was related most strongly to tellinid bivalve caloric biomass distribution and that walruses selected lower ice concentrations from the mostly high ice concentrations that were available to them (quartiles: 76%, 93%, and 99%). Areas with high average predicted walrus site selection generally coincided with areas of high organic carbon input identified in other studies. Projected decreases in sea ice in the St. Lawrence Island polynya and the potential for a concomitant decline of bivalves in the region could result in a northward shift in the wintering grounds of walruses in the northern Bering Sea.     

DNA metabarcoding of feces to infer summer diet of Pacific walruses

Environmental conditions in the Chukchi Sea are changing rapidly and may alter the abundance and distribution of marine species and their benthic prey. We used a metabarcoding approach to identify potentially important prey taxa from Pacific walrus (Odobenus rosmarus divergens) fecal samples (n = 87). Bivalvia was the most dominant class of prey (66% of all normalized counts) and occurred in 98% of the samples. Polychaeta and Gastropoda occurred in 70% and 62% of the samples, respectively. The remaining nine invertebrate classes comprised <21% of all normalized counts. The common occurrence of these three prey classes is consistent with examinations of walrus stomach contents. Despite these consistencies, biases in the metabarcoding approach to determine diet from feces have been highlighted in other studies and require further study, in addition to biases that may have arisen from our opportunistic sampling. However, this noninvasive approach provides accurate identification of prey taxa from degraded samples and could yield much-needed information on shifts in walrus diet in a rapidly changing Arctic.     

Faecal DNA amplification in Pacific walruses (Odobenus rosmarus divergens)

Dietary information is critical for assessing the population status of seals, sea lions and walruses—and is determined for most species of pinnipeds using non-invasive methods. However, diets of walruses continue to be described from the stomach contents of dead individuals. Our goal was to assess whether DNA could be extracted from the faeces of Pacific walruses (Odobenus rosmarus divergens) collected at haulout sites, and whether potential prey species or taxa could be amplified from that DNA. We extracted DNA from 70 faecal samples collected from ice pans in the Bering Sea during the spring of 2008 and 2009 (with between 4.6 and 308.9 ng/μl of DNA in every sample). We also extracted DNA from 12 potential prey species or taxa collected by bottom-grabs in 2009 to identify positive controls for primers and to test the ability of previously published taxon-specific and species-specific primers to correctly identify the prey using conventional PCR. We tested primers that successfully amplified DNA from the tissue of at least one potential prey species or taxon on all 70 walrus faecal samples. We found that two sets of primers successfully amplified many of the potential prey species or taxa using DNA from their tissue, and that one of these primer sets produced positive amplification in 4 of the 70 faecal samples. The band size that was produced for prey organisms and in the faecal samples was consistent with expectations, although prey identities were not verified with sequencing. Our pilot study demonstrates that DNA can be successfully extracted and amplified from walrus faeces, providing a stepping stone towards describing the diets of walruses from faecal DNA.     

Regional walrus abundance estimate in the United States Chukchi Sea in autumn

Human activities (e.g., shipping, tourism, oil, gas development) have increased in the Chukchi Sea because of declining sea ice. The declining sea ice itself and these activities may affect Pacific walrus (Odobenus rosmarus divergens) abundance; however, previous walrus abundance estimates have been notably imprecise. When sea ice is absent from the eastern Chukchi Sea, walruses in waters of the United States usually rest together onshore at a single Alaska coastal haulout, where they can be surveyed more easily than when they rest on dispersed offshore ice floes. We estimated the number of walruses on land (herd size) at this haulout from 13 unoccupied aircraft system (UAS) surveys flown within a 10-day period in each of 2018 and 2019. We estimated population size of walruses using the haulout over the course of the surveys by combining herd size data with data from satellite-linked transmitters that indicated whether tagged walruses were in or out of water during each survey. Our estimates of the population size of walruses using the haulout during each year's survey period were similar to each other and more precise than historical walrus abundance estimates: posterior means (95% credibility intervals) were 166,000 (133,000–201,000) for 2018 and 189,000 (135,000–251,000) for 2019. Auxiliary observations support using these estimates to represent the size of the population using the eastern Chukchi Sea in autumn during the surveyed years. Our study site was the only substantial Chukchi Sea coastal haulout in the United States during the survey periods and study-specific tracking data (consistent with known distribution and movement patterns) indicated tagged walruses remained in eastern Chukchi waters during the survey periods. In addition, the imagery, telemetry, and analytical methods developed for this study advance the prospect for precise range-wide walrus population size estimates.     

Inter-annual variability in the proportional contribution of higher trophic levels to the diet of Pacific walruses

Pacific walruses (Odobenus rosmarus divergens) depend on Arctic sea ice as a resting and foraging platform; however, recent years have seen unprecedented seasonal reductions in ice extent. Previous researchers proposed that during unfavorable ice conditions, walruses might prey on other pinnipeds. To examine this hypothesis, we analyzed carbon and nitrogen stable isotope ratios of muscle from walruses (n = 155) sampled from the Bering and Chukchi seas during 2001–2010. We used a Bayesian stable isotope mixing model to examine the proportional contribution of higher trophic level prey (HTLP) (e.g., seals, seabirds) to walrus diets and extrapolated a tissue-specific turnover rate to compare diet of individuals over time. Mode HTLP across years was 19 % ± 8. Results indicate a significant decrease (P < 0.05) in the reliance on HTLP during 2008–2009 (mode HTLP 13 %), one of two sampling periods that experienced great seasonal loss of pan-Arctic sea ice (the other being 2007–2008 with mode HTLP of 23 %). We also reveal intra-annual fluctuations in the contribution of HTLP to the diet of a walrus sampled in 2011 with seal remains in its stomach through high-resolution sectioning along a whisker length. Our findings suggest that walruses forage opportunistically as a result of multiple environmental factors and that sea ice extent alone does not drive consumption of HTLP.     

Seal eating by walruses in the Bering and Chukchi Seas

Summary Walruses (Odobenus rosmarus) feed primarily on benthic invertebrates, but they are known to eat seals (Phocidae) occasionally, ostensibly when the benthic foods are unavailable. We investigated reports of a marked increase in occurrence of seal-eating walruses in the Bering Strait region in the late 1970's by examining stomach contents of animals taken in the spring harvests by Eskimos. We also obtained relevant information during visual surveys of marine mammals in the region. Our findings from the stomachs indicated that seal eating was 10 to 100 times more common during the 1970's and early 1980's (0.6–3.0%, N=645) than it had been in the previous three decades (0.07–0.20%, N=4015). In addition, we observed walruses in possession of seal remains in 1978 and 1979, where we had not seen such a phenomenon before in the previous 25 years. We attribute the increased predatory interaction between seals and walruses partly to a larger walrus population and, especially in 1979, to unusually restrictive spring ice conditions, which tended to cause greater than usual overlap of their distributions. Stomach contents of walruses taken in the Chukchi Sea in summer, where the ranges of walruses and seals overlap broadly in all years, have indicated a similarly high rate of occurrence of seal eaters (8.6% in the 1960's, N=35; 11.4% in 1983, N=44). As a whole, our findings indicate that most of the seal eating is predation, rather than scavenging of carrion. They also indicate that it is not rare or aberrant but common behavior, and that it could exert a significant impact on seal populations in some areas.     

Antibodies to marine caliciviruses in the Pacific walrus (Odobenus rosmarus divergens Illiger)

Sera from 155 Pacific walruses (Odobenus rosmarus divergens Illiger), sampled in the Chukchi Sea during the summer of 1983, were tested for serum neutralizing (SN) antibodies to six marine calicivirus serotypes. Serotypes tested included San Miguel sea lion virus (SMSV) types 1, 5, 8, and 10, previously isolated from northern fur seals (Callorhinus ursinus Linné) in the Bering Sea; walrus calicivirus (WCV), previously isolated from walrus feces collected off sea ice in the Chukchi Sea; and Tillamook calicivirus (TCV), a bovine isolate from Oregon of suspected marine origin. No antibodies were found to SMSV-1, SMSV-10, or TCV. Antibodies to SMSV-5 were found in two animals (titers 1:20 and 1:160); antibodies to SMSV-8 were found in four animals (all 1:20); and antibodies to WCV were found in one animal (titer 1:40). Antibodies to WCV have been found in the Pacific walrus previously; however, this represents the first report of antibodies to any of the SMSV serotypes in this marine mammal.     

Fish feeding guilds in shallow rocky and soft bottom areas on the Swedish west coast

Multivariate analyses of the diet relationships within fish assemblages resulted in the recognition of five major feeding guilds in shallow (0-9 m) rocky- and soft-bottom habitats on the Swedish west coast. The two habitats had similar guilds that could be broadly described as piscivores, decapod feeders, copepod feeders and amphipod and mollusc feeders. The last guild on the rocky bottom was more strictly one of amphipod feeders, whereas the last guild on soft bottoms consisted of infauna feeders (mainly polychaetes and bivalves). A comparison of the diet of the entire fish assemblages showed that amphipods and gastropods were more prevalent as prey taxa on rocky bottoms, whereas bivalves and teleosts were more common on soft bottoms. A large proportion (12 out of 27 species) of the fish species investigated was found in both habitats. These fish species generally seemed to rely on the same major prey groups in the two habitats, but also included prey taxa characteristic for the habitat in which they resided. Sea trout Salmo trutta , mackerel Scombrus scombrus , saithe Pollachius virens , whiting Merlangius merlangus and cod Gadus morhua were found to be the major piscivores in both the investigated habitats. Herring Clupea harengus and members of the family gobiidae were the most prevalent prey species for piscivores within the rocky- as well as the soft-bottom fish assemblage. A comparison of feeding modes suggested that the fish assemblage on rocky bottoms predominantly (60% of the food items) relied on food found in vegetation. On soft bottoms, the average distribution of food categories among fish species was 44% benthic, 35% pelagic and 21% vegetation-associated food items.     

Caloric content of dominant benthic species from the northern Bering and Chukchi Seas: historical comparisons and the effects of preservation

Energy density of a prey item provides information about the nutrition of a predator. Here, we provide data on energy content of 24 common benthic taxa (18 epifauna and six infauna) from the northern Bering and Chukchi Seas. Mollusks (both bivalves and gastropods) and amphipods had the highest caloric content of the taxa investigated. In light of changing environmental conditions and benthic communities in the Arctic, we compared present-day caloric content of several taxa with measurements conducted for the same taxa in the 1970s. In most cases, caloric content did not differ between historic and present-day values but was higher in two out of seven comparisons for both formalin-preserved samples and frozen materials. We also examined the effect of preservation methods (frozen vs. formalin-preserved) on the caloric content for seven epifaunal taxa. In three of the seven taxa, formalin-preserved specimens had significantly higher energy densities than frozen specimens. These data should make investigators aware of the opportunities and also possible limitations of including energy density values derived in different decades and from different preservation types into their calculations, although more comprehensive and time-controlled studies are needed to generate possible corrections factors.     

Diet and feeding strategy of thornback ray Raja clavata

The diet and feeding strategy of thornback ray Raja clavata, from the eastern-central Adriatic Sea, were investigated. Stomach contents of 428 specimens, total length (L(T) ) of 14·0-75·1 cm, were collected from commercial bottom trawls. The prey items identified in the stomachs belong to eight major groups: Cephalopoda, Polychaeta, Stomatopoda, Decapoda (Natantia and Reptantia), Mysidacea, Isopoda, Amphipoda and Teleostei. Decapods were the most important prey (index of relative important, %I(RI) ,= 72·8) followed by teleosts (%I(RI) = 20·4), whereas other prey groups were only occasionally ingested. Small-sized individuals (<25 cm L(T) ) fed primarily on small crustaceans (mysids and amphipods), whereas large-sized specimens consumed larger prey, such as decapods, cephalopods and teleosts. Diet composition showed little seasonal variation; decapods were the most important prey in all seasons. There was high dietary similarity between sampling locations. The percentage of empty stomachs did not differ significantly among size classes and seasons. In terms of composition by species, the diet of R. clavata was characterized by a variety of rare or unimportant prey. As a result, R. clavata could be considered a generalist predator.     

A comparative study of the diet of auks killed during an oil incident in the Skagerrak in January 1981

Stomach contents were examined from guillemots Uria aalge (Pont.), Razorbills Alca torda Linnaeus and Little auks Alle alle (Linnaeus) killed by oil in the Skagerrak in January 1981. The range of prey species was small; the commonest fish found in the stomachs were Gobiusculus flavescens (Fabricius), Crystallogobius linearis (Düben), Sprattus sprattus (L.), Trisopterus minutus (L.) and Ammodytes sp. The relative importance of these fish depended on bird species and locality. Guillemots contained gadoids more frequently and sandeels less frequently than razorbills, and the composition of guillemot samples from Oslo Fjord differed from that of samples from Sweden. Invertebrate remains were uncommon in all bird species; the commonest items recorded were polychaete jaws. The frequency of occurrence of items in the diet of guillemots from Oslo Fjord was apparently influenced by the extent of oiling suffered by the birds.     

楚科奇海和白令海网采浮游植物群聚的多元分析

根据1999年夏季(7~8月)我国首次北极科学考察期间“雪龙”号考察船在楚科奇海(66°0.3'~75°18.6' N, 153°36.5'~174°59.5' W)和白令海(55°59.8'~66°0.3' N,173°21.1' E~175°53.9' W)49个站采集的样品研究了该区网采浮游植物的分布特征。结果显示,两个调查区的浮游植物多样性指数和均匀度平均值几乎相等且都较低,分别变动在1.88~1.90和0.43~0.45之间。然而,两个区存在明显的空间异质性。就丰富度和种类组成而言,楚科奇海的细胞密度和物种总数量远高于白令海,前者为9 122.56×10⁴ cells·m^-3 和103种,而后者为161.78×10⁴ cells·m^-3和71种。上述生态学指标不同站点间的变异也大。用多元分析方法对浮游植物群聚的分布异质性加以讨论。主成分分析(PCA)结果表明,解析调查区浮游植物分布方差的33.0%的两个最主要成分(即主要物种变量)分别是北极-北方浅海群落(泛北极群落)和北方大洋群落的主要种群。前者包括某些圆心硅藻,如旋链角毛藻 (Chaetoceros curvisetus)、柔弱角毛藻(C. debilis)、聚生角毛藻(C. socialis)、叉尖角毛藻(C.furcellatus)等多种寡色体类角毛藻以及冷淡劳德藻 (Lauderia glacialis)、丹麦细柱藻 (Leptocylindrus danicus)、诺登海链藻(Thalassiosira nordenskioldii)和脆杆链藻(Bacteriosira fragilis)等;后者主要有小环毛藻 (Corethron criophilum)、平截鼻状藻 (Proboscia truncata)、无刺鼻状藻 (P. inermis)、西氏细齿状藻(Dentecula seminae)和一些多色暗角毛类角毛藻,如大西洋角毛藻(Chaetoceros atlanticus)等。泛北极群落主要分布于楚科奇海南部和中部,而北方大洋群落的主要分布区在白令海调查区的西南部。通过聚类分析还辨认出7个包含不同群落的主要网采浮游植物分布区,但是由于不同水系的不同程度影响,以及极具不确定性的浮冰的位置及其物理状态(聚集、开裂和消融等)也会在短时间内给调查区生物过程导入变异性,从而使得这些分布区的归类复杂化,尤其在楚科奇海。     

Results and evaluation of a survey to estimate Pacific walrus population size, 20061

In spring 2006, we conducted a collaborative U.S.–Russia survey to estimate abundance of the Pacific walrus (Odobenus rosmarus divergens). The Bering Sea was partitioned into survey blocks, and a systematic random sample of transects within a subset of the blocks was surveyed with airborne thermal scanners using standard strip‐transect methodology. Counts of walruses in photographed groups were used to model the relation between thermal signatures and the number of walruses in groups, which was used to estimate the number of walruses in groups that were detected by the scanner but not photographed. We also modeled the probability of thermally detecting various‐sized walrus groups to estimate the number of walruses in groups undetected by the scanner. We used data from radio‐tagged walruses to adjust on‐ice estimates to account for walruses in the water during the survey. The estimated area of available habitat averaged 668,000 km² and the area of surveyed blocks was 318,204 km². The number of Pacific walruses within the surveyed area was estimated at 129,000 with 95% confidence limits of 55,000–507,000 individuals. Poor weather conditions precluded surveying in other areas; therefore, this value represents the number of Pacific walruses within about half of potential walrus habitat.     

Diet composition and food consumption rate of harbor porpoises (Phocoena phocoena) in the western Baltic Sea

Stomach content composition and prey‐specific consumption rates of juvenile and adult harbor porpoises (Phocoena phocoena) were estimated from a data set including 339 stomachs collected over a 32 yr period (1980–2011) in the western Baltic Sea. The stomach contents were mainly hard parts of fish prey and in particular otoliths. The bias originating from differential residence time of otoliths in the stomachs was addressed by use of a recently developed approach. Atlantic cod and herring were the main prey of adults, constituting on average 70% of the diet mass. Juvenile porpoises also frequently consumed gobies. Here, the mass contribution by gobies was on average 25%, which was as much as cod. Other species such as whiting, sprat, eelpout, and sandeels were of minor importance for both juveniles and adults. The diet composition differed between years, quarters, and porpoise acquisition method. Yearly consumption rates for porpoises in the western Baltic Sea were obtained in three scenarios on the daily energy requirements of a porpoise in combination with an estimate including the 95% CLs of the porpoise population size. Cod of age groups 1 and 2 and intermediate‐sized herring suffered the highest predation from porpoises.     

Proportion of higher trophic-level prey in the diet of Pacific walruses (Odobenus rosmarus divergens)

During nutritionally stressful situations, Pacific walruses (Odobenus rosmarus divergens) may switch from preying on benthic invertebrates to higher trophic-level prey (HTLP) (e.g., pinnipeds and/or seabirds). We applied a Bayesian mixing model to stable isotope (C and N) data from analyses of various tissues (tongue and lumbar muscle, skin, and liver) to quantify the proportional contribution of HTLP to walruses (n = 293 individuals). The mode contribution of HTLP to walrus diet was ~22 % (±10 %) based on muscle mixing models, which is consistent with results from contaminant studies of Atlantic walruses (Odobenus rosmarus rosmarus), but higher than estimates based on historical stomach content analyses of Pacific walruses. A broader range in the proportion of HTLP (0–60 %) shown by mixing models using stable isotope data from liver and skin of walruses indicated they pursue an opportunistic foraging strategy. Data from the HTLP-consuming walruses were comparable with our stable isotope data of a known “seal-eating” walrus. No significant difference was evident between the estimated contributions of HTLP to the diet of male versus female walruses (P > 0.01). This finding suggests that changes in diet base for walruses are not influenced by the sex of the predator.     

The influence of ice conditions on terrestrial haulouts of the pacific walrus Odobenus rosmarus divergens Illiger, 1815 in the Gulf of Anadyr,Bering Sea

The boundaries of the area of appropriate depths for feeding by the Pacific walrus were determined. Based on the ice maps for the period of 1997–2011, the long-term dynamics of ice breakup and formation in the Gulf of Anadyr (Bering Sea) were analyzed. There was a tendency towards a reduction in the ice-cover period in the Gulf of Anadyr, especially during 2007–2011. In 2008, the number and duration of stay of walruses at terrestrial haulout sites on Meeskyn Spit Island and on Retkyn Spit continued to decrease. Walruses also used other haulouts located in the Gulf of Anadyr at Cape Gek and Cape Retkyn. The age and sex structure of the walrus herd changed: the proportions of adult males and young animals decreased, while the proportion of females with calves increased.     

The stomach contents of Patagonian toothfish around South Georgia (South Atlantic)

The proportion of Patagonian toothfish Dissostichus eleginoides , caught around South Georgia in the south-west Atlantic, with empty stomachs was much lower in fish caught in pots compared to longlines (28 and 91%, respectively, of examined individuals). It is hypothesized that pots caused reduced levels of stress on capture. Stomach content data examined from pot-caught fish will probably therefore be more comprehensive than that from fish caught using longlines. A wide range of prey items was identified in the stomachs of D. eleginoides and stomach contents of individuals caught using the two fishing methods were significantly different. The most common prey item for D. eleginoides caught using pots was the decapod prawn Nauticaris sp., which was restricted in location and depth. However, prawns were not common in the stomachs of D. eleginoides caught from the same location using longlines. Stomach contents from the two fishing methods remained significantly different when Nauticaris sp. were eliminated from the assessment, although fishes then dominated the diet of D. eleginoides caught using either fishing gear. The study confirms that D. eleginoides is an opportunistic carnivore, and indicates that feeding habits depend on the local availability of food items, as well as factors such as depth and predator size. The potential ecological impacts of fishing for D. eleginoides on the South Atlantic ecosystem are discussed.     

Long‐term changes in the feeding of Pollachius virens on the Scotian Shelf: responses to a dynamic ecosystem

The diet and feeding ecology of pollock Pollachius virens from the Scotian Shelf and Bay of Fundy in the north‐west Atlantic changed over the last few decades, which was associated with decreases in euphausiid abundance. Stomach contents data for 2078 pollock collected during the 1958–1967 period and for 1230 pollock collected during the 1996–2002 period indicated that pollock diet contained fewer euphausiids and feeding activity decreased. During the early period, euphausiids were present in 65% of the pollock stomachs that contained food and only 9% of these stomachs in the recent period. The decrease of euphausiids was not wholly offset by an increase in piscivory, since there was little increase in the frequency of fish prey in the diet or in the fullness index for this prey type. Empty stomachs were significantly more common in the recent period during both the winter and summer. The decreased occurrence of euphausiids in stomach samples coincided with a significantly decreased abundance of this prey, suggesting that the near‐absence of euphausiids in recently collected pollock stomachs reflected prey abundance. Concurrent with changes in diet and feeding intensity, the condition or 'plumpness' of pollock significantly declined from the early to the late sampling periods.     

Indication of two Pacific walrus stocks from whole tooth elemental analysis

The Pacific walrus (Odobenus rosmarus divergens) is considered to be a single panmictic population for management purposes. However, studies on population structuring in this species are limited; in part, because portions of the population’s range are often inaccessible. Therefore, alternative and complementary methods for investigating stock structure in the Pacific walrus are of particular interest. We used measures of elemental concentrations in whole tooth sections from ICP-MS in a discriminant analysis to investigate evidence of stock separation between walruses from two of three known breeding areas (S.E. Bering, St Lawrence, and Anadyr Gulf). Elemental compositions of teeth from female and male walruses from the S.E. Bering and St Lawrence breeding areas were significantly different, providing evidence of separate stocks. We also obtained insights into the potential relation of walruses from non-breeding areas to walruses from these breeding groups based on similarities in their dental elemental profiles.