Pollock Biomass Research Articles

Variability of stock status walleye pollock fishery

The analysis of the variability of walleye pollock stocks assessment and fisheries showed that in most areas the degree of exploitation is at a low level (4,7–13,5%), with the exception of the Navarinsky region of the Bering Sea, the coast of eastern Kamchatka, the northern Kuril Islands and western Sakhalin (16,7–18,9%). Climate change has a significant impact on the of walleye pollock stocks assessment. Positive anomalies of the PDO climate index (Pacific Decadal Oscillation) create favorable conditions for northern populations, whereas for southern populations living on the southern border of the area, on the contrary, those periods when negative anomalies are noted are more comfortable. Subject to the implementation of the cooling scenario in the North Pacific Ocean, walleye pollock biomass centers may shift to the southern regions (the economic zone of the DPRK), which will lead to a significant reduction in catches in the economic zones of the Russian Federation and the United States, which are currently the main players in the walleye pollock market.

The influence of prey availability on behavioral decisions and reproductive success of a central-place forager during lactation

Marine central-place foragers are increasingly faced with altered prey landscapes, necessitating predictions of the impact of such changes on behavior, reproductive success, and population dynamics. We used state-dependent behavioral life history theory implemented via Stochastic Dynamic Programming (SDP) to explore the influence of changes in prey distribution and energy gain from foraging on the behavior and reproductive success of a central place forager during lactation. Our work is motivated by northern fur seals (Callorhinus ursinus) because of the ongoing population decline of the Eastern Pacific stock and projected declines in biomass of walleye pollock (Gadus chalcogrammus), a key fur seal prey species in the eastern Bering Sea. We also explored how changes in female and pup metabolic rates, body size, and lactation duration affected model output to provide insight into traits that might experience selective pressure in response to reductions in prey availability. Simulated females adopted a central-place foraging strategy after an initial extended period spent on land (4.7–8.3 days). Trip durations increased as the high energy prey patch moved farther from land or when the energy gain from foraging decreased. Increases in trip duration adversely affected pup growth rates and wean mass despite attempts to compensate by increasing land durations. Metabolic rate changes had the largest impacts on pup wean mass, with reductions in a pup’s metabolic rate allowing females to successfully forage at distances of 600+ km from land for up to 15+ days. Our results indicate that without physiological adaptations, a rookery is unlikely to be viable if the primary foraging grounds are 400 km or farther from the rookery. To achieve pup growth rates characteristic of a population experiencing rapid growth, model results indicate the primary foraging grounds need to be <150 km from the rookery.

Reproduction, stock status and fishery for walleye pollock in Peter the Great Bay in the modern period

The aim of the study is to analyze the current state of reproduction, resources and pollock fishery in Peter the Great Bay.Result: It is shown that the increase in pollock abundance is associated with the appearance of a strong yearclass of 2014 in Peter the Great Bay. Pollock biomass has increased significantly, mass spawning has resumed in traditional spawning areas, commercial catch of pollock increased to record levels. However, during the three spawning seasons of the year-class 2014 there was no significant recruitment, so the fish stock is expected to return to a low level.Novelty: The results of scientific research conducted in the Peter the Great Bay in 2019–2022 made it possible to obtain new data on the reproduction, current state and structure of pollock stock.Practical significance: The obtained estimates of the current state and structure of pollock stock will be used for short-term and long-term forecasts.Research methods: The data of research bottom trawl and ichthyoplankton surveys and pollock catch statistics from the Industry Monitoring System (OSM «Fishing») were used.

Modern status, formation and exploitation of walleye pollock stocks off the southern Kuril Islands

The aim of the study is analyze the state and clarify modern status of walleye pollock stocks in the southern Kuril Islands waters.Result: Strong year-classes are revealed, which determined pollock stock dynamics in the South Kuril region in the 2010s — e arly 2020s. It is shown, that the maximum pollock biomass since the early 1990s was observed in the early 2010s, and in the early 2020s decreased to the average level. The organization of fishing and the composition of pollock commercial catches off the southern Kuril Islands are differ at low and high levels of stocks.Novelty: New data on structure and pollock stocks assessment in the South Kuril region since 2011 based on the integrated surveys results are presented. Long-term data on the effectiveness of fishing and the structure of pollock commercial catches in the South Kuril zone are considered.Practical significance: The obtained estimates of the current state and structure of pollock stock will be used for short-term and long-term forecasts.Research methods: walleye pollock stocks assessment in the South Kuril region is based on the annual integrated research expeditions in accordance with the standard methods used in TINRO to monitor pollock stocks.

Time-varying predation as a modifier of constant natural mortality for Gulf of Alaska walleye pollock

Although highly variable in time and space, predation remains the greatest source of mortality for juvenile and lower trophic-level fishes. As such, predation can have substantial and long-term effects on the dynamics of these prey. Gulf of Alaska walleye pollock (Gadus chalcogrammus) has shown considerable variability in biomass over the past four decades. During this same time, the demersal fish community transitioned from being dominated by pollock to a system comprised primarily of upper trophic-level predators. We estimated time-varying predation mortality to better understand its effects on the population dynamics of pollock in this currently “top heavy” system. Our index of predation accounted for spatiotemporal variation in predator biomass, bioenergetics-based rations, and age-specific proportions of pollock consumed (1990–2019). To evaluate population-level impacts of predation, we included an index of removals as part of the stock assessment model. This formulation allowed for non-annual data inputs and included a proportionality constant with which to scale predation. Age-specific natural mortality was allowed to vary according to a penalized random walk. We found that natural mortality ranged from 37% higher to 17% lower than the long-term mean. Resulting estimates of total pollock biomass differed by as much as 37% relative to a model without time-varying natural mortality, though the maximum difference for exploitable biomass was only 14%. Using an empirically-derived predation index to modify constant natural mortality allows stock assessment scientists to evaluate impacts of time-varying predation on assessed species. This approach provides a relatively simple way of incorporating ecological information into single-species stock assessments and may reduce bias compared to conventional models that do not account for changes in predation mortality. Notably, including predation mortality in single-species assessments may help identify inconsistencies in biomass estimates that warrant further consideration.

Estimating spatiotemporal availability of transboundary fishes to fishery‐independent surveys

Abstract Taxa can expand beyond historical scientific survey footprints and into new areas with different survey protocols as they move to track their preferred climate. In global groundfish fisheries, for example, scientists estimate population dynamics within the spatial extent of a fishery‐independent survey using an index known as a design‐based estimator. Observed changes in species distribution in recent years suggest that some groundfish are moving beyond the spatial extent of single surveys. We must intercalibrate disparate data that cover a larger spatial extent to maintain our ability to accurately index populations as their availability to historical surveys changes. We combine US and Russian data from the northern, eastern and western Bering Sea to understand the proportion of fish biomass within the extent of the eastern survey (‘availability’). Surveys are within close proximity to each other, but with different sampling protocols (hence catch a different proportion of local densities, termed ‘sampling efficiency ratio’). We use Alaska pollock Gadus chalcogrammus, Pacific cod Gadus macrocephalus and Alaska plaice Pleuronectes quadrituberculatus as case studies to calculate survey efficiency ratios and two area‐swept estimators, termed local and conventional, to summarize groundfish biomass over various spatial scales across the Bering Sea. We estimated variation in spatial availability of transboundary stocks to the eastern Bering Sea (EBS) survey. In 2017, the most recent available year of survey coverage that included all three Bering Sea regions, estimated availability in the EBS of pollock biomass was ~33%, cod biomass was ~27% and plaice biomass was ~26%, down from ~58%, ~71% and ~30%, respectively, in 2010. Synthesis and applications. This is the first study to provide an empirical way to combine Russian and US data in the Bering Sea to assess changes in the availability of groundfish biomass, which, in turn, will alter the interpretations and values of population indices used in regional management. We recommend leveraging this approach using existing global fishery‐independent datasets that span different spatiotemporal footprints to monitor transboundary stocks, and as a template to initiate international cooperation on the assessment of spatial availability of stocks common to multiple countries.

Practical Application of a Bioenergetic Model to Inform Management of a Declining Fur Seal Population and Their Commercially Important Prey

Food availability is a key concern for the conservation of marine top predators, particularly during a time when they face a rapidly changing environment and continued pressure from commercial fishing activities. Northern fur seals (Callorhinus ursinus) breeding on the Pribilof Islands in the eastern Bering Sea have experienced an unexplained population decline since the late-1990s. Dietary overlap with a large U.S. fishery for walleye pollock (Gadus chalcogrammus) in combination with changes in maternal foraging behavior and pup growth has led to the hypothesis that food limitation may be contributing to the population decline. We developed age- and sex-specific bioenergetic models to estimate fur seal energy intake from May–December in six target years, which were combined with diet data to quantify prey consumption. There was considerable sex- and age-specific variation in energy intake because of differences in body size, energetic costs, and behavior; net energy intake was lowest for juveniles (18.9 MJ sea-day–1, 1,409.4 MJ season–1) and highest for adult males (66.0 MJ sea-day–1, 7,651.7 MJ season–1). Population-level prey consumption ranged from 255,232 t (222,159 – 350,755 t, 95% CI) in 2006 to 500,039 t (453,720 – 555,205 t) in 1996, with pollock comprising between 41.4 and 76.5% of this biomass. Interannual variation in size-specific pollock consumption appeared largely driven by the availability of juvenile fish, with up to 81.6% of pollock biomass coming from mature pollock in years of poor age-1 recruitment. Relationships among metabolic rates, trip durations, pup growth rates, and energy intake of lactating females suggest the most feasible mechanism to increase pup growth rates is by increasing foraging efficiency through reductions in maternal foraging effort, which is unlikely to occur without increases in localized prey density. By quantifying year-specific fur seal consumption of pollock, our study provides a pathway to incorporate fur seals into multispecies pollock stock assessment models, which is critical for fur seal and fishery management given they were a significant source of mortality for both juvenile and mature pollock.

Adapting to climate‐driven distribution shifts using model‐based indices and age composition from multiple surveys in the walleye pollock (Gadus chalcogrammus) stock assessment

AbstractThe northern Bering Sea is transitioning from an Arctic to subarctic fish community as climate warms. Scientists and managers aim to understand how these changing conditions are influencing fish biomass and spatial distribution in this region, as both are used to inform stock assessments and fisheries management advice. Here, we use a spatio‐temporal model for walleye pollock (Gadus chalcogrammus) to provide two inputs to its stock assessment model: (a) an alternative model‐based biomass index and (b) alternative model‐based age compositions. Both inputs were derived from multiple fishery‐independent data that span different regions of space and time. We developed an assessment model that utilizes both the standard and model‐based inputs from multiple surveys despite inconsistencies in spatial and temporal coverage, and we found that using these data provide an improved spatial and temporal scope of total pollock biomass. Age composition information indicated that pollock density is increasing and moving farther north, particularly for older pollock. We found that including an index of cold pool extent could be used to extrapolate pollock densities in the northern Bering Sea in unsampled years. Stock assessment parameter estimates were similar for standard and model‐based input. This study demonstrates that spatio‐temporal model‐based estimates of a biomass index and age composition can facilitate rapid changes in stock assessment structure in response to climate‐driven shifts in spatial distribution. We conclude that assimilating data from regions neighboring standard survey areas, such as the Chukchi Sea and western Bering Sea, would improve understanding and management efforts as fish distributions change under a warming climate.

Биологические исследования в российских дальневосточных и арктических морях в трансарктической экспедиции ВНИРО

The results of biological research on the RV “Professor Levanidov” in the Bering, Chukchi, East Siberian, Laptev and Kara seas, July 6 — October 2, 2019 are presented. Expatriates from the Pacific and Atlantic oceans made a significant contribution to the biomass of plankton communities in the Arctic seas. Plankton biomass was depended on the distance from the Bering Strait in the east, the Fram Strait and the Barents Sea in the west. A noticeable level of biomass was maintained by local Arctic plankton species. In megabenthos, crustaceans dominated in the Kara and Chukchi seas, and echinoderms in the Laptev and East Siberian seas. The biomass of demersal fishes (mostly Pacific cod, Saffron cod, yellowfin flounder, great sculpin and starry flounder) in the Karaginskaya subzone was 318 thousand tons, pollock biomass was 539–1349 thousand tons. The biomass of demersal fishes (mostly cod, great sculpin, and arrowtooth flounder) in the Olyutorsko-Navarinsky area was 768 thousand tons, pollock biomass 359–897 thousand tons. Representatives of Cottidae, Zoarcidae, Liparidae, Gadidae, Pleuronectidae, and Agonidae families comprised the bulk of catches in Arctic seas. Polar cod dominated in all seas, with exception of the Chukchi Sea, where pollock biomass increased many times compared to previous years. In the Laptev Sea, the abundance of Greenland halibut increased significantly. In the Chukchi Sea, Polar cod biomass was 117 thousand tons, flathead sole 43 thousand tons, pollock 897 thousand tons, snow crab 51 thousand tons. In the Kara Sea, Polar cod biomass was 171 thousand tons, snow crab 67 thousand tons. Significant differences in food composition and feeding intensity of pollock, Polar cod, capelin and Greenland halibut of different sizes revealed the existence of a complex trophic system. Anthropogenic pollution was assessed, the distribution of species-indicators of vulnerable marine ecosystems was studied, and materials for genetic research were collected.

Examining the ecological role of jellyfish in the Eastern Bering Sea

AbstractWithin the Eastern Bering Sea, the jellyfish Chrysaora melanaster has fluctuated widely over recent decades. We examined the role of C. melanaster as an ecosystem-structuring agent via application of ecosystem models representing inner-, mid-, and outer-shelf regions of comparable areal coverage. Chrysaora melanaster utilize 1% of total mid-shelf consumer production, or 1/4th the energy required by forage fish (capelin Mallotus villosus, Pacific herring Clupea pallasii, age-0 Pacific cod Gadus macrocephalus, age-0 walleye pollock Gadus chalcogrammus). Model simulations show the impacts of C. melanaster are broadly distributed across consumer groups with increasingly negative impacts with higher jellyfish biomass. Age-0 pollock represent the greater part of the forage fish biomass, and observed pollock biomass during low jellyfish years (2004–2007) was significantly greater than during high jellyfish years (2009–2014). However, sensitivity among consumer groups to observed jellyfish variability is small, within 5% of baseline (2004–2015) conditions. Estimates using similar models for the Coastal Gulf of Alaska (CGoA) and Northern California Current (NCC) suggest large differences in the role of scyphozoans among northern Pacific shelf ecosystems. Only 0.1% of total summer consumer production is required to support CGoA Chrysaora, while the coastal NCC population uses 19%.

Changes in the Biological Characteristics of Walleye Pollock Related to Demographic Changes in the East Sea During the Late 20th Century

AbstractWalleye pollock Gadus chalcogrammus (hereafter, pollock) was one of the predominant fish species in Korean waters during the 1970s and 1980s, but catches decreased rapidly beginning in the late 1980s and the fishery collapsed in the 2000s. The biological characteristics of fish species tend to depend on their biomass and environmental conditions. To investigate the changes in the biological characteristics of pollock (such as their size frequency, condition factor, and maturation) associated with biomass levels as well as the possible causes of the stock collapse as a result of environmental changes, we used data collected by the Korean National Institute of Fisheries Science (NIFS) during high (1976–1985) and low (1991–2000) periods of pollock biomass. Density‐dependent effects on the pollock population were evident in Korean waters: pollock were larger and heavier at length during the low‐biomass period. The gonadosomatic index also indicated that the peak spawning season was broad (from October to February) during the high‐biomass period and narrower (December and January) during the low‐biomass period. The population abundance and recruitment success of Korean pollock appear to have been affected by the warming of the ocean surface from the early 1990s. Thus, we suggest that the biomass fluctuations associated with environmental changes during their early life stages were responsible for the observed changes in the biological properties of pollock.

Combining bottom trawls and acoustics in a diverse semipelagic environment: What is the contribution of walleye pollock (Gadus chalcogrammus) to near-bottom acoustic backscatter in the eastern Bering Sea?

The abundance of walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea is estimated in part through fisheries-independent acoustic trawl (AT) surveys, which currently use acoustic backscatter data down to 3 m above the bottom. A large portion of adult pollock are demersal, and these estimates will become more accurate if the survey is extended closer to bottom. The purpose of this project was to assess the feasibility of extending the AT survey closer to the bottom by estimating the contributions of each demersal fish species to observed acoustic backscatter in the highly diverse near-bottom region. This was accomplished by fitting a regression model to simultaneously collected acoustic backscatter and bottom trawl catch data. Pollock were the dominant source of acoustic backscatter among demersal species, accounting for 85.9% ± 4.8% of acoustic backscatter (mean ± standard deviation). A method was developed to extend the AT survey to within 0.5 m of the bottom, and when applied to the 1994–2014 surveys, pollock biomass increased by a mean of 28% ± 9%.

Modelling spatially dependent predation mortality of eastern Bering Sea walleye pollock, and its implications for stock dynamics under future climate scenarios

Abstract Arrowtooth flounder (Atheresthes stomias) are an important predator of juvenile walleye pollock (Gadus chalcogramus) in the eastern Bering Sea (EBS) shelf and have increased 3-fold in biomass from 1977 to 2014. Arrowtooth flounder avoid the summer “cold pool” (bottom water ≤2°C) and variability in cold pool size and location has affected their spatial overlap with juvenile walleye pollock. Developing a method to account for the relationship between climate change and pollock mortality can highlight ecosystem dynamics and contribute to better assessments for fisheries management. Consequently, spatially resolved predation mortality rates were estimated within an age-structured walleye pollock stock assessment population model (based on spatial information on diet and abundance from trawl surveys), along with the effect of sea surface temperature (SST) on pollock recruitment. Projections of SST and cold pool area to 2050 were obtained (or statistically downscaled) from nine global climate models and used within an age-structure population model to project pollock abundance given estimated relationships between environmental variables and predator and prey spatial distributions, pollock recruitment, and maximum rate of arrowtooth flounder consumption. The climate projections show a wide range of variability but an overall trend of increasing SST and decreasing cold pool area. Projected pollock biomass decreased largely due to the negative effect of increased SST on pollock recruitment. A sensitivity analysis indicated that the decline in projected pollock biomass would be exacerbated if arrowtooth flounder increased their relative distribution in the EBS northwest middle shelf (an area of relatively high density of juvenile pollock) in warm years.

Euphausiids in the eastern Bering Sea: A synthesis of recent studies of euphausiid production, consumption and population control

Euphausiids are an important component of the eastern Bering Sea marine ecosystem. We synthesized information on the ecological roles of two species, Thysanoessa raschii, which predominates over the Middle and Inner Shelf Domains, and Thysanoessa inermis, which predominates over the Outer Shelf Domain. Although estimates of euphausiid biomass across the shelf are not well constrained, we estimated that, between April and July, 2004–2010, euphausiid biomass was 3.08–5.25gCm−2 on the outer shelf and 1.95–3.92gCm−2 on the middle shelf. Modeled estimates of euphausiid production, for spring and summer combined, varied between 0.043gCm−2d−1 and 0.051gCm−2d−1, depending on location, with a mean of 0.048gCm−2d−1. Recently reported field measurements of annual primary production over the southeastern Bering Sea in 2008–2009 vary between 0.06 and 6.65gCm−2d−1, with a mean of 1.262gCm−2d−1±2.049gCm−2d−1 in spring and summer combined, a level sufficient to support euphausiids, at least on an annualized basis. Walleye pollock (Gadus chalcogrammus, hereafter pollock) is the single most important consumer of euphausiids over the eastern Bering Sea shelf. We estimated that pollock consumed between 0.0042 and 0.019.7gCm−2d−1 of euphausiids, depending on year, with a mean of 0.011gCm−2d−1 in summer averaged over 1999–2009. This consumption is equivalent to between 17% and 29% of summer euphausiid production, depending on location.Over the period for which data were available (2004–2012), we observed a strong negative relationship between euphausiid biomass as determined in acoustic surveys and pollock biomass as estimated in the eastern Bering Sea pollock stock assessment (r2=0.82). During this time period, sea-surface temperature was the second strongest predictor of euphausiid biomass, (r2=0.63). However, for the period 2004–2010, bottom temperature (r2=0.94) was the strongest predictor, followed by pollock biomass from the pollock stock assessment (r2=0.82), and sea-surface temperature (r2=0.81). Mean pollock density in the acoustic surveys was not a powerful predictor of euphausiid biomass during either period. In spatially explicit multiple regression analyses for the periods 2004–2012 and 2004–2010 those formulations that included sea-surface and bottom temperatures as well as survey estimates of pollock had the greatest explanatory value. However, when either or both temperature terms were dropped, the explanatory value of the models dropped considerably. When pollock biomass was dropped from the models, there was little change in explanatory value compared to the full model. Euphausiid production and pollock consumption data coupled with a negative relationship between euphausiid biomass and stock assessment estimates of pollock biomass indicate a top-down predation effect. However, strong negative relationships between euphausiid biomass and water temperatures indicate the influence of a bottom-up mechanism. The apparent differences in these results may relate to the different spatial and temporal scales used to assess the pollock biomass used in the analyses. Alternatively, euphausiid biomass may be strongly controlled during a restricted portion of the year, such as spring, if critical food needs are not met in some years. We lack the data necessary to resolve these alternative hypotheses.

Variability in trophic positions of four commercially important groundfish species in the Gulf of Alaska

We examined trends in nitrogen stable isotope data as a proxy for trophic position (mean trophic level, TL) of commercial and survey catches as an ecosystem-based indicator of sustainability of four groundfish species in the Gulf of Alaska. From 2000 to 2004, walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus), arrowtooth flounder (Atheresthes stomias), and Pacific halibut (Hippoglossus stenolepis) were collected from the waters surrounding Kodiak Island, Alaska. Several analyses of covariance (ANCOVA) models were tested to detect variations in mean TL among years with fish length as a covariate. Best-fit models were selected using the Akaiki Information Criterion to estimate trends in mean TL of commercial catch using length-frequency data from onboard fishery observers for each target species. Then, linear regression models were used to estimate mean TL of commercial catch over 1990–2009 and the mean TL of population biomass over 1984–2007 based on length-frequency data and biomass estimates from trawl surveys conducted by National Marine Fisheries Service and from historical catch data. The TL of catch for each species except walleye pollock remained stable over the time frame of the study. Walleye pollock TLs became increasingly variable after 1999. Similar trends in mean TL were observed for the survey biomass of walleye pollock. Additionally, there was an observed decrease of the occurrence of higher TL Pacific halibut over time. While the decline had no impact on overall TL estimates during 1990–2009, a continued decline may affect mean TL in the future. Overall, length seems to be the most important factor in estimating a species’ TL. Therefore, including relationships between length of catch and TL estimates could lead to an early detection of TL declines that may be associated with unsustainable fishing mortality.

Dynamics of trophic structure for the epipelagic community in the western Bering Sea

Changes of quantitative composition and trophic structure of the nekton community in the western Bering Sea are considered for the last decades and role of pacific salmons in dynamics of trophic flows is evaluated in dependence on their abundance using the ecosystem model Ecopath. Two models are developed that describe trophic structure of the community in two cases: 1) low biomass of salmons and high biomass of walleye pollock (in 1980s, the year 1986 is analyzed as an example) and 2) decreased pollock biomass and increasing salmon biomass (in 2000s, the year 2006 as an example). Besides, a hypothetical situation is modeled with the salmon biomass multiplied by 1.5 relative to its level in 2006. Significant decrease of pollock abundance between 1980s and 2000s caused twofold reduction of total food consumption by nekton species; the heightened consumption by salmons and squids in the 2000s compensated only a small part of this reduction. However, the tenfold increase of salmons biomass changed their main diets with lowering of the prey trophic level from amphipods and squids to euphausiids, copepods, and pteropods. Now the salmons are the only numerous predator group of the fourth trophic level in the upper pelagic layer of offshore waters in the western Bering Sea. Due to their high trophic plasticity, they can feed by wide range of prey belonged to 2-3 rd trophic levels that supplies them by a large amount of food. Even in the modeled case of increasing of the salmons biomass in 1.5 times relative to the level of 2000s, the current level of forage resources is able to support their populations. There is concluded that carrying capacity of the western Bering Sea is excessively sufficient for pacific salmons in periods of their high abundance.

The spatial distribution of euphausiids and walleye pollock in the eastern Bering Sea does not imply top-down control by predation

Although euphausiids of the genus Thysanoessa ('krill') are a key zooplankton taxon in the Bering Sea ecosystem, the processes controlling variation in the standing stock of these animals are not well understood. Both forcing by temperature ('bottom-up') and predation ('top- down') have been proposed. If strong top-down forcing were present, a negative relationship would be expected between the standing stock of euphausiids and that of walleye pollock Gadus chalcogrammus, their single most important predator in the Bering Sea. We developed multiple regression models using survey data collected over several years to test the hypothesis of a negative relationship between local (mesoscale) densities of these animals. We used the models to evaluate the relative importance of pollock biomass and water temperature in predicting euphau- siid biomass, and found that temperature was a far better predictor, a result that is not consistent with dominant top-down control of euphausiids by pollock predation.

Foraging Responses of Black-Legged Kittiwakes to Prolonged Food-Shortages around Colonies on the Bering Sea Shelf

We hypothesized that changes in southeastern Bering Sea foraging conditions for black-legged kittiwakes (Rissa tridactyla) have caused shifts in habitat use with direct implications for population trends. To test this, we compared at-sea distribution, breeding performance, and nutritional stress of kittiwakes in three years (2008–2010) at two sites in the Pribilof Islands, where the population has either declined (St. Paul) or remained stable (St. George). Foraging conditions were assessed from changes in (1) bird diets, (2) the biomass and distribution of juvenile pollock (Theragra chalcogramma) in 2008 and 2009, and (3) eddy kinetic energy (EKE; considered to be a proxy for oceanic prey availability). In years when biomass of juvenile pollock was low and patchily distributed in shelf regions, kittiwake diets included little or no neritic prey and a much higher occurrence of oceanic prey (e.g. myctophids). Birds from both islands foraged on the nearby shelves, or made substantially longer-distance trips overnight to the basin. Here, feeding was more nocturnal and crepuscular than on the shelf, and often occurred near anticyclonic, or inside cyclonic eddies. As expected from colony location, birds from St. Paul used neritic waters more frequently, whereas birds from St. George typically foraged in oceanic waters. Despite these distinctive foraging patterns, there were no significant differences between colonies in chick feeding rates or fledging success. High EKE in 2010 coincided with a 63% increase in use of the basin by birds from St. Paul compared with 2008 when EKE was low. Nonetheless, adult nutritional stress, which was relatively high across years at both colonies, peaked in birds from St. Paul in 2010. Diminishing food resources in nearby shelf habitats may have contributed to kittiwake population declines at St Paul, possibly driven by increased adult mortality or breeding desertion due to high foraging effort and nutritional stress.

Walleye pollock: global overview

Walleye pollock is the second most extensively fished species in the world. The major fishing grounds include the Bering and Okhotsk Seas. Large-scale fishing started in the 1960s and continues to date with average annual landings over this 50-year period of 1.5 million tons. Yet over this period catches were characterized by considerable volatility. This volatility makes rational management of stock and planning of annual fishing activities difficult. The changes in annual catches correlate with the changes in the biomass of walleye pollock. Existing data suggest a close link between climate change in the northern Pacific and biomass, which allows quantitative estimates of future trends in the biomass, and consequently annual catch, of walleye pollock. Cooling of the northern Pacific is expected to increase the biomass in the Sea of Japan and decrease it in the Bering Sea and Sea of Okhotsk. The opposite is predicted to occur if the northern Pacific experiences warming.

Hydrography and biological resources in the western Bering Sea

The variability of temperature, salinity, dissolved oxygen and nutrients (phosphate and silicate) in the west Bering Sea in the Russian Exclusive Economic Zone (REZ) since 1950 and the influence of these factors on the distribution and dynamics of hydrobionts were studied. Since 1950, the sea surface temperature has been gradually increasing, although non-significant cooling occurred in the last decade. In contrast, in the 50–200m depth range, the temperature has been cooling. During the last 60 years, the salinity decreased by 0.30, 0.06–0.10 and 0.04 at the sea surface, at the 100–200m layer and at the depth of 500m, respectively, resulting in a strengthening of the vertical stability and weakening of the vertical water exchange. As a consequence, the oxygen concentrations at depths down to 1000m decreased during this period. Phosphate and silicate concentrations increased during the last 40 years. The water exchange with the North Pacific (based on the discharge through the Kamchatka Strait) from the mid-1960s to the early 1990s was 2-3-fold higher than in the 1950s or from the mid-1990s to 2010. During the periods of weakened water exchange, the herring population sharply increased, while during periods of strengthened water exchange, pollock biomass increased. The increase of codfishes, flounders and sculpin biomass at the sea shelf during the second half of the 20th century coincided with sea surface warming. Since 2007, the westward water transport from the Aleutian Basin was almost half that during 2002–06, while the northward stream from Near Strait noticeably increased. The populations of immature chum, sockeye and chinook in the REZ declined because of their weakened input from the US zone, and these species were distributed mainly in the northern and eastern Russian waters.Taking into account the cooling since the middle of the last decade, the change in the intensity and direction of the Aleutian Low and Siberian High trends, and the westward shift of the Aleutian Low, one can hypothesize the possible return of the Bering Sea climate to the conditions of the 1960–80s and the corresponding response observed in ichthyofauna.