Juvenile Walleye Pollock Research Articles

Feeding habits of pacific salmons in the eastern Bering Sea in 2003-2012

Change in prey for pacific salmons in the eastern Bering Sea is considered between the periods of relatively warm (2003-2006) and relatively cold (2007-2012) conditions on the base of data obtained in the frames of BASIS research program. In the warm period, mean biomass of the large-sized, medium-sized and small-sized fractions of zooplankton was 287.9, 258.2, and 109.2 mg/m 3 , respectively. In these conditions, fish prey was the basis of diet for pink salmon (70-68 % of ration), chum salmon (64-84 %) and sockeye salmon (45-87 %). Pink, chum, sockeye, and coho salmons had similar diet with the dominance of juvenile walleye pollock (similarity 70-90 %), in contrast to the diet of chinook salmon (40-70 % similarity with others). In the cold period, the large-sized zooplankton was much more abundant (1094.8 mg/m 3 on average) mostly due to higher abundance of Copepoda and Chaetognatha; euphausiids, hyperiids, and pteropods were more abundant, as well. In these conditions, zooplankton, mainly euphausiids Thysanoessa raschii and hyperiids Themisto libellula and T. pacifica , dominated in the diet of plankton-eating salmons: 86-92 % of the ration for pink salmon, 88-92 % for chum salmon, and 74-81 % for sockeye salmon; all these species and chinook juveniles had similar diet (70-80 % similarity). On the contrary, predatory salmons as coho and chinook adults still preyed on fish. Mean daily rations in the warm/cold periods are estimated as 12.1/9.4 % of body weight for sockeye salmon with the length 10-20 cm, 8.6/7.3 % for sockeye salmon 20-30 cm, 8.3/13.8 % for chum salmon 10-20 cm, 8.6/10.0 % for chum salmon 20-30 cm, and 8.8/10.3 % for pink salmon 10-25 cm. The prevalence of nekton or planktonic food does not affect on daily rhythm of feeding intensity: juvenile salmons feed mainly in the daytime, so they have the maximal index of stomachs fullness in the evening or early night (100-230 ‱, both in the warm and cold periods). Generally, the salmons prefer fish larvae and juveniles in conditions of low zooplankton abundance and large-sized zooplankton in conditions of its high abundance. Selectivity on euphausiids, hyperiids and pteropods is observed only in conditions of high zooplankton abundance and is an indicator of sufficient food supply.

Effect of environmental factors and prey species on daytime and nighttime vertical distribution of juvenile walleye pollock in and around Funka Bay

We examined factors affecting the vertical distribution of juvenile walleye pollock during “the second critical period” (i.e. the transition period of prey size) in and around Funka Bay, Hokkaido, Japan. During the day, small juveniles (<30 mm) were distributed mainly in shallow, warm, and low-salinity water within the bay, whereas larger ones (≥30 mm) occurred in deeper, cold and saline water. During the night, the large juveniles migrated to shallower depths. Outside the bay, small- and large-sized juveniles were distributed in water masses of similar physical properties during the daytime, whereas during the nighttime, most of the juveniles moved to shallower depth, and some juveniles dived to deeper water mass. Dietary analysis revealed that large juveniles fed on large copepods (≥0.5 mm in body width) more than small juveniles did. The composition of zooplankton in our survey area showed that large-sized copepods often occurred in deeper saline water. From these results, there is a possibility that juvenile pollock inside and outside Funka Bay occupied different water masses preferable for each feeding advantage.

Spatial match-mismatch between juvenile fish and prey provides a mechanism for recruitment variability across contrasting climate conditions in the eastern Bering Sea.

Understanding mechanisms behind variability in early life survival of marine fishes through modeling efforts can improve predictive capabilities for recruitment success under changing climate conditions. Walleye pollock (Theragra chalcogramma) support the largest single-species commercial fishery in the United States and represent an ecologically important component of the Bering Sea ecosystem. Variability in walleye pollock growth and survival is structured in part by climate-driven bottom-up control of zooplankton composition. We used two modeling approaches, informed by observations, to understand the roles of prey quality, prey composition, and water temperature on juvenile walleye pollock growth: (1) a bioenergetics model that included local predator and prey energy densities, and (2) an individual-based model that included a mechanistic feeding component dependent on larval development and behavior, local prey densities and size, and physical oceanographic conditions. Prey composition in late-summer shifted from predominantly smaller copepod species in the warmer 2005 season to larger species in the cooler 2010 season, reflecting differences in zooplankton composition between years. In 2010, the main prey of juvenile walleye pollock were more abundant, had greater biomass, and higher mean energy density, resulting in better growth conditions. Moreover, spatial patterns in prey composition and water temperature lead to areas of enhanced growth, or growth ‘hot spots’, for juvenile walleye pollock and survival may be enhanced when fish overlap with these areas. This study provides evidence that a spatial mismatch between juvenile walleye pollock and growth ‘hot spots’ in 2005 contributed to poor recruitment while a higher degree of overlap in 2010 resulted in improved recruitment. Our results indicate that climate-driven changes in prey quality and composition can impact growth of juvenile walleye pollock, potentially severely affecting recruitment variability.

Effect of region on the food-related benefits to age-0 walleye pollock (Theragra chalcogramma) in association with midwater habitat characteristics in the Gulf of Alaska

Abstract Wilson, M. T., Mier, K. L., and Jump, C. M. 2013. Effect of region on the food-related benefits to age-0 walleye pollock (Theragra chalcogramma) in association with midwater habitat characteristics in the Gulf of Alaska – ICES Journal of Marine Science, 70: . In the western Gulf of Alaska (GOA), we compared age-0 juvenile walleye pollock body weight, food habits, and expected growth rates between two regions during 3 years. The benefit to rearing off Kodiak Island vs. in the Semidi Islands vicinity increased among years with body length because larger fish consumed more euphausiids, and euphausiids were apparently more available in the Kodiak region. This explains previously observed regional variation in body size, condition, and growth rates within the GOA population and points to potential importance of the Kodiak region as a walleye pollock nursery. During September 2005, 2007, and 2009, Kodiak midwater was cooler and saltier than Semidi midwater, presumably due to a relatively narrow continental shelf and greater oceanic influence. Zooplankton composition differed between regions with the greater euphausiid population density off Kodiak being most relevant to walleye pollock food habits. As found historically, age-0 walleye pollock abundance was lower in the Kodiak region than in the Semidi region. However, the Kodiak fish were larger in both length and length-specific weight, and had fuller stomachs than Semidi fish, although differences were not significant in all years. Regional differences were most pronounced during the relatively warm year 2005 when fish were particularly large. Fish dietary mass was dominated by euphausiids, especially for Kodiak fish ≥55 mm standard length. Bioenergetics model output suggests that the greater euphausiid dietary proportion translates into faster growth of the Kodiak fish. The midwater habitat off Kodiak Island might therefore support above-average production of recruits to the GOA adult population due to better condition and likely survival of walleye pollock juveniles, especially when fish are large and thus able to more fully exploit euphausiids, but confirmation of above-average production of recruits depends on our ability to determine spatial structure in juvenile-adult population connectivity.

Nested scales of spatial heterogeneity in juvenile walleye pollock Theragra chalcogramma in the southeastern Bering Sea

We sought to characterize the distribution of juvenile walleye pollock Theragra chalcogramma in an area of intense predatorprey interactions and to describe habitat features that lead to their observed distributions. The distribution of juvenile walleye pollock around the Pribilof Islands in the southeastern Bering Sea in 2008 and 2009 was patchy, at spatial scales rang- ing from a few meters to several 10s of kilometers. These patches, and the spaces between them, were hierarchically nested with small, dense patches clustered together into larger aggregations which were further aggregated over the region sampled. Vertical physical habitat structure affected the vertical distribution of juvenile pollock similarly in both years. However, despite largely similar physical characteristics in both years, there were significant differences between years in the horizontal distribution, patch structure, and abundance of juvenile pollock. In neither year did biological or physical environment characteristics explain the broad-scale variability in the horizontal distribution of juvenile pollock, but at small horizontal scales a behavioral compo- nent was evident, with fish changing their group coherence as conditions changed despite consis- tent group sizes. It is clear that if we are to understand the processes that drive the distribution of juvenile pollock, we must consider multiple scales of heterogeneity. Only then will we begin to understand the role of pollock in the ecosystem and be able to predict the consequences of large sources of variability such as climate change, a critical question in the rapidly changing physical and biological environment of the Bering Sea.

Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 479:283-302 (2013) - DOI: https://doi.org/10.3354/meps10209 Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey Kelly J. Benoit-Bird1,*, Brian C. Battaile2, Chad A. Nordstrom2, Andrew W. Trites2 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon 97331, USA 2Marine Mammal Research Unit, University of British Columbia, Room 247, AERL, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada *Email: kbenoit@coas.oregonstate.edu ABSTRACT: Marine prey often occur in hierarchical mosaics whereby small, high-density patches are nested inside of larger, lower density aggregations. We tested the extent to which the foraging behavior of a marine predator (northern fur seal Callorhinus ursinus) could be explained by the hierarchical patch structure of a dominant prey species (juvenile walleye pollock Theragra chalcogramma) in the eastern Bering Sea. Comparing the movements of satellite-tracked fur seals with ship-based acoustic surveys of prey revealed that fur seals did not randomly search for prey, but instead showed deviations in the distribution of step-lengths (distances between their foraging patches) corresponding to the distances between aggregations of prey. Scales of prey distribution varied between Bering Sea shelf and deep-water slope habitats, while spatial scale distributions of fur seals showed corresponding changes, indicating that their search strategies were not innate patterns decoupled from the environment. Fur seals tended to avoid the smallest prey patches in both shelf and slope habitats. They also avoided prey patches that were separated by large distances. Fur seals responded to several levels of prey patchiness simultaneously, resulting in strong correlations between predator and prey over the entire range of aggregation scales observed in juvenile pollock. Our results indicate that, despite having a varied diet, fur seal foraging paths were defined by juvenile pollock aggregations. The presence of hierarchical, scale-dependent aggregation in both predator and prey provides new insights into fur seal behavior and a means to predict the dynamics of their interactions with prey. KEY WORDS: Patchiness · Spatial scale · Predator?prey · Foraging behavior · Hierarchical · Northern fur seal · Juvenile walleye pollock Full text in pdf format PreviousCite this article as: Benoit-Bird KJ, Battaile BC, Nordstrom CA, Trites AW (2013) Foraging behavior of northern fur seals closely matches the hierarchical patch scales of prey. Mar Ecol Prog Ser 479:283-302. https://doi.org/10.3354/meps10209 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 479. Online publication date: April 08, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Resiliency of juvenile walleye pollock to projected levels of ocean acidification

As atmospheric concentrations of CO2 rise, the pH of high-latitude oceans is pre- dicted to decrease by 0.3 to 0.5 units by 2100. Several biological consequences of ocean acidifica- tion across this pH range have already been documented in invertebrates and tropical marine fishes. However, little work has been done examining potential responses of the temperate and boreal marine fish species that support major fisheries. In 2 experiments, we examined the growth responses of juvenile walleye pollock Theragra chalcogramma at ambient and 3 elevated CO2 lev- els. In a short-term experiment with yearlings, CO2 treatment had no significant effect on growth or condition after 6 wk of rearing. Elevated CO2 levels (>450 µatm) increased the rate of otolith deposition, but did not affect otolith elemental composition. In a second experiment, growth in length of sub-yearlings over 12 wk at 8°C was 7.2% faster in the 2 higher CO2 treatments (>1200 µatm) than in the lower CO2 treatments (<900 µatm). Growth of sub-yearlings measured during 11 subsequent weeks of rearing at 2.5°C did not differ among CO2 treatments. There was no effect of CO2 treatment on condition factor following either phase of the experiment. Sub- yearling consumption rates were not directly affected by CO2 treatment, confirming that growth at elevated CO2 levels is not maintained through compensatory feeding. While not exhaustive of potential interactive environmental factors, these experiments demonstrate a general resiliency of growth energetics in juvenile walleye pollock to the direct effects of CO2 changes predicted for the Gulf of Alaska and Bering Sea in the next century.

Thermal effects on swimming activity and habitat choice in juvenile Pacific cod (Gadus macrocephalus)

The behavioral responses of fishes to temperature variation have received less attention than physiological responses, despite their direct implications for predator–prey dynamics in aquatic ecosystems. In this paper, we describe the temperature dependence of swimming performance and behavioral characteristics of juvenile Pacific cod (Gadus macrocephalus; 75–125 mm total length). Maximum swimming speeds increased with temperature and body size. Routine swimming speeds of Pacific cod in small groups of similarly sized fish (N = 6) increased with body size and were 34 % faster at 9 °C than at 2 °C. The response to temperature was opposite that previously described for juvenile walleye pollock (Theragra chalcogramma), reflecting species-specific differences in behavioral responses. In a separate experiment, we demonstrated the effect of temperature on habitat selection of juvenile Pacific cod: Use of an artificial eelgrass patch in a 5-m-long laboratory tank was significantly greater at 9 °C than at 2 °C. These results illustrate that temperature affects a range of behavioral traits that play important roles in determining the frequency and outcomes of predator–prey interactions.

Winter energy allocation and deficit of juvenile walleye pollock Theragra chalcogramma in the Doto area, northern Japan

Seasonal energy allocation and deficits of marine juvenile fishes have considerable effects on their survival. To explore the winter survival mechanism of marine fishes with low lipid reserves in their early life, juvenile walleye pollock Theragra chalcogramma were collected along the continental shelf of northern Japan over a 2-year period, and energy allocation and deficit patterns were compared between wild and laboratory-starved fish. Contrary to expectations, wild fish generally continued to accumulate protein mass and concurrently tended to reduce lipid mass from late autumn through winter. The most plausible explanation for the continuous structural growth is that juvenile pollock give priority to reducing mortality risk from size-selective predators under quasi-prey-limited conditions. Exceptionally, inshore small fish reduced both constituents during a winter. The inshore fish consumed 2.5 times more lipid energy than protein energy in November–December, but protein was more important than lipids as a source of energy in December–January and in February–March. However, dependence upon protein reserves was lower for the wild fish than for the laboratory-starved fish, suggesting milder nutritional stress of the wild fish than that observed in the starvation experiment. Moreover, the lipid contents of mortalities in the starvation experiment were mostly <1%, whereas few wild fish had such lipid contents in the field. These results suggest that juvenile pollock are able to avoid both starvation and predation by accumulating protein reserves.

Spatial variation in the small-scale distribution of juvenile walleye pollock (Theragra chalcogramma) in the southeastern Bering Sea

Juvenile walleye pollock (Theragra chalcogramma) is one of the primary prey items for bird and mammal predators in the Bering Sea and supports a large commercial fishery. An understanding of the abundance and distribution of juvenile pollock is needed to estimate the effects that change in these parameters may have on pollock predators and adult pollock abundance and distribution. During the summers of 2008 and 2009, surveys were conducted in three topographic zones (Middle Shelf, Outer Shelf, and Slope) near the Pribilof Islands in the southeastern Bering Sea. Multi-frequency (38, 70, 120, and 200) acoustic sampling occurred during the entire cruise duration with frequent environmental data sampling (e.g., temperature, salinity, dissolved oxygen, and chlorophyll a fluorescence) and targeted fish tows. These data showed that juvenile walleye pollock were primarily found in clusters of small, dense aggregations giving them a leopard spot appearance in the acoustical output. In both years, juvenile pollock distribution was highly variable on small spatial scales and was related to biological and physical features of the water column. These differences in juvenile walleye pollock distribution are likely to affect the use of habitat by predators and may have implications for future sampling.

Modeling the diet of humpback whales: An approach using stable carbon and nitrogen isotopes in a Bayesian mixing model

AbstractHumpback whales are considered generalist predators, feeding on schooling fish, and zooplankton, but variability likely exists among regional feeding aggregations. We explored the diet of one feeding aggregation of humpback whales near Kodiak Island, Alaska, through analysis of the stable carbon (δ13C) and nitrogen (δ15N) isotope ratios of their skin and regional prey sources. Humpback whales were sampled during the summer feeding season over 3 yr (n= 93; 2004–2006). Prey samples were collected from the same region during trawl surveys conducted between 2003 and 2005. Isotope values of humpback whale skin and prey were entered into a Bayesian dietary mixing model to estimate feasible contributions of prey to humpback diets. Diet results indicated that humpbacks feed heavily on euphausiids, but also consume juvenile walleye pollock, capelin, and Pacific sand lance. The diet of humpback whales in 2004 was the most diverse, while diets in 2005 and 2006 showed a higher proportion of euphausiids. Our results reveal annual differences in humpback diets from the Kodiak region due to either individual prey preferences or prey availability. Application of a Bayesian mixing model to stable isotope analysis improves description of regional diets and comparison of these diets to resource availability and quality.

Recruitment forecast models for walleye pollock Theragra chalcogramma fine-tuned from juvenile survey data, predator abundance and environmental phase shifts

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 417:237-248 (2010) - DOI: https://doi.org/10.3354/meps08817 Recruitment forecast models for walleye pollock Theragra chalcogramma fine-tuned from juvenile survey data, predator abundance and environmental phase shifts Tianyang Zhang1, Kevin M. Bailey2, Kung-sik Chan1,* 1Department of Statistics and Actuarial Science, University of Iowa, 263 Schaeffer Hall, Iowa City, Iowa 52242, USA 2Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE, Seattle, Washington 98115, USA *Corresponding author. Email: kchan@stat.uiowa.edu ABSTRACT: Forecasting recruitment of marine fishes from environmental effects acting upon larvae has proven difficult due to multiple, nonlinear, interacting factors influencing larval survival. We used another approach, which circumvents the period of high egg and larval mortality, and instead improves forecasts from juvenile abundance indices. We compared several statistical recruitment forecast models and demonstrate that an increasing abundance of predators on juvenile walleye pollock Theragra chalcogramma, particularly the arrowtooth flounder Atheresthes stomias, which now dominates the groundfish biomass in the Gulf of Alaska, and autocorrelation caused by intercohort interactions strongly affect pollock recruitment during the juvenile phase. Furthermore, the weight of predictor variables changes with threshold criteria, which are linked to phase shifts in the marine environment. Our results indicate that forecasting recruitment of marine fishes can be improved by considering factors that influence survival after the juvenile period, but also needs to account for changes in community structure and phase shifts in the environment, as opposed to only environmental correlates. Inclusion of these factors is consonant with biological knowledge of the species. KEY WORDS: Population dynamics · Fisheries · Environment · Ocean research · Regime shift · Ecological community · Fish recruitment · Forecast Full text in pdf format PreviousNextCite this article as: Zhang T, Bailey KM, Chan Ks (2010) Recruitment forecast models for walleye pollock Theragra chalcogramma fine-tuned from juvenile survey data, predator abundance and environmental phase shifts. Mar Ecol Prog Ser 417:237-248. https://doi.org/10.3354/meps08817 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 417. Online publication date: November 04, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

The influence of pelagic habitat selection and interspecific competition on productivity of juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus) in the Gulf of Alaska

AbstractHere we investigate processes affecting productivity of capelin and walleye pollock in the Gulf of Alaska. We examine pelagic habitat selection by comparing the distribution of juvenile fish and their prey with oceanographic properties and we evaluate the potential for interspecific competition by comparing diets and measures of foraging. The primary field study was conducted in Barnabus Trough, Kodiak Island, Alaska, during September 2005. The distribution of fish was assessed acoustically and trawls were used to collect individual fish for stomach content analyses. Physical and biological data were collected with conductivity–temperature–depth probes and zooplankton tows. Age‐0 pollock were distributed in cool waters offshore of a mid‐trough front, coincident with the distribution of euphausiids, their preferred prey. In contrast, capelin and their prey (copepods) were distributed throughout the trough. We observed that sympatric capelin (occurring with pollock) often had reduced foraging success compared to allopatric capelin (occurring alone). Results of a bioenergetic model also suggest that the exclusion of capelin from foraging on euphausiids can have negative consequences for capelin growth.

Foraging behavior of incubating and chick-rearing thick-billed murres Uria lomvia

The varying demands associated with egg incubation and chick-rearing are known to have a corresponding effect on the foraging behavior of seabirds. We deployed data loggers on incu- bating and chick-rearing thick-billed murres Uria lomvia to examine differences in their diving behavior and characteristics of habitats used for foraging. To compare diets of incubating and chick- rearing birds we collected their stomach contents using a water offloading technique. We found that incubating birds performed longer foraging trips than chick-rearing birds (incubating: 19.0 ± 7.2 h; chick-rearing: 9.9 ± 5.6 h). Incubating birds foraged in the offshore stratified water masses (sea sur- face temperature (SST) > 9°C) and frequently dived to the depth of the thermocline (20 to 50 m). Chick-rearing birds spent more time foraging in the inshore, well-mixed water masses (SST 60 m. Small juvenile walleye pollock Theragra chalcogramma, squid and euphausi- ids were the dominant prey of incubating and chick-rearing birds. Distributions of these small prey were commonly associated with the thermocline, while larger fish, which parents brought back to feed their chicks, were distributed below the thermocline. Results suggest that incubating murres mainly foraged at shallow depths near the thermocline with higher concentrations of small prey, while chick-rearing murres feed their chicks large prey caught on deep dives while feeding them- selves on small prey caught on shallow dives.

Low nekton abundance in the western Bering Sea according to trawl survey data and model estimates

Complex trawl surveys were conducted in the upper epipelagic zone of the western Bering Sea and adjacent Pacific waters in the summer and fall seasons of 2002–2006. The abundance of small nekton (micronekton) was estimated using two independent methods: traditional trawling and a mathematical model of selective feeding by fish. According to the trawl data, total micronekton density varied from 1 to 158 (average 40) mg/m3 on the northwestern Bering Sea shelf and from 6 to 151 (37) mg/m3 in deep-water areas of the southwestern Bering Sea and adjacent Pacific waters. According to model calculations, micronekton density was higher—72–193 (141) mg/m3 on the shelf and 78–507 (228) mg/m3 in the deep-water part of the studied area. Both trawl and model data showed that small nekton on the northwestern shelf mostly consisted of larval and juvenile walleye pollock, as well as small fish species, such as capelin and Pacific sand lance. In the deepwater areas, mesopelagic fish and squid (northern lampfish, northern smoothtongue, and boreopacific gonate squid), which migrate to the surface at night, juvenile Atka mackerel, and shortarm gonate squid dominated among micronekton. The advantages and disadvantages of both the trawl and model methods for calculating the abundance of small fish and squid were considered. Comparison of abundance estimates for mass fish species, obtained through trawl and model methods, enabled us to analyze trawl catchability coefficients and propose a more differentiated division of micronekton into size classes than had been done earlier. A function that characterizes the dependence of the catchability coefficient (CC) on body length was offered for juvenile Atka mackerel. This equation can be also used for evaluation of CC for other fishes that have similar size and behavior.

Use of a multibeam sonar to characterize fish aggregations in the northern Pacific.

Acoustic observations to describe small-scale spatial patterns or aggregations of fish have traditionally been made in two dimensions using vertical echosounders. Recently, multibeam sonars have been utilized as a tool to study the three-dimensional structure of fish aggregations. This study describes small-scale spatial patterns of juvenile walleye pollock, Theragra chalcogramma, with data collected using a calibrated ME70 multibeam sonar. The ME70 has reduced side-lobes and a very high dynamic range, which allow better characterization of the water column backscatter than typical multibeam systems. For this study, the sonar was configured with a 120 deg swath composed of 31 narrow bandwidth beams, ranging from 70–120 kHz. Data were collected during winter 2009 in the Gulf of Alaska and summer 2009 in the Bering Sea to evaluate whether seasonal and geographical differences existed in the juvenile fish aggregation patterns. Size- and shape-related aggregation descriptors (e.g., height, length, volume, and surface area) are described, and the variability in these metrics as a function of time and space is explored. Particular challenges for using the ME70 as a tool to describe the small-scale spatial patterns of marine animals are highlighted.

Seasonal variations of shelf circulation in Hidaka Bay, Hokkaido, Japan, with an interpretation of the migration route of juvenile walleye pollock

The outer shelf of Funka Bay, located at the bay head of Hidaka Bay, is a recognised main winter spawning ground for walleye pollock (Theragra chalcogramma), whose newly hatched juveniles migrate eastward along the Hidaka shelf to the nursery ground located in the Doto area. To examine the seasonal change of the coastal current along this migration route, four current moorings were deployed along the shelf in Hidaka Bay from April, 2004 to June, 2006. Since these mooring sites were close to the coast, the circulation was estimated after removal of the wind-driven component. It was found that the winter coastal flow, forced by a north-westerly monsoon wind, is clockwise along the Hidaka shelf. However, this flow is weak due to the superimposition of the opposite Coastal Oyashio flow trapped on the shelf. In summer, a bifurcation of the coastal current occurs along the north-eastern Hidaka shelf with a south-eastward flow, associated with the Tsugaru Gyre, and a north-westward flow, consisting of a branch from this gyre. Our results provide a new understanding of the migration of juvenile walleye pollock: (1) very slow transport of juveniles along the Hidaka shelf from winter to spring, and (2) selection of earlier spawning survivors due to the bifurcated flow in early summer.

Winter lipid depletion of juvenile walleye pollock Theragra chalcogramma in the Doto area, northern Japan

Seasonal variation in body size and nutritional condition of juvenile walleye pollock Theragra chalcogramma was examined to elucidate the mechanism underlying their first-winter survival on the continental shelf of the Doto area, northern Japan, based on monthly samples collected over 2 years. Stored lipid mass was highest during autumn, but 93% (2004) and 80% (2005) of lipids were exhausted by the onset of winter. Lipid levels in the winter of 2004 remained low (7-14% of the autumnal maximum), and there was reduced growth rate until the spring, whereas in 2005 lipid levels were higher and more variable (10-46% of the maximum) and some growth occurred. An analysis of the allometric relationships between body size and stored energy showed that larger individuals accumulated disproportionately more energy in the autumn, but the advantage disappeared prior to the winter. In January 2004, stored lipid energy was low throughout the Doto continental shelf relative to the continental slope area. These results suggest that winter feeding opportunities on the shelf are severely limited but not completely absent. Previous studies have shown that winter temperatures on the shelf are lower than those in the slope area. It is possible that juvenile T. chalcogramma survive winter on the shelf without a high level of pre-winter lipid storage because the occasional feeding in the cold shelf water benefits energy conservation.

Swimbladders under pressure: anatomical and acoustic responses by walleye pollock

Abstract Horne, J. K., Sawada, K., Abe, K., Kreisberg, R. B., Barbee, D. H., and Sadayasu, K. 2009. Swimbladders under pressure: anatomical and acoustic responses by walleye pollock. – ICES Journal of Marine Science, 66: 1162–1168. Pressure influences echo intensities from fish through changes in swimbladder volumes and surface areas. Volume reduction is expected to correspond to Boyle’s law (volume α pressure−1), but the effects of deforming the dorsal-aspect surface area of the swimbladder on the target strength (TS) of a physoclist have not been quantified at geometric scattering frequencies. Dorsal and lateral radiographs of three juvenile walleye pollock (Theragra chalcogramma) in a pressure chamber were used to image swimbladders from ambient to a maximum of 4.9 × 105 Pa (i.e. 5 atm). Radiographs from an additional 16 0-group, 21 juvenile, and 28 adult fish were used to compare swimbladder sizes and shapes across life-history stages. The directional elliptical eccentricity, which describes the relative compression in dorsal and lateral planes, was formulated to define swimbladder shapes and their variation. As pressure increased, dorsal-aspect surface areas of the swimbladder decreased at a constant rate. Swimbladder-volume reductions were similar among individuals, but less than those predicted by Boyle’s law. Compression was greatest in the dorsal–ventral plane and asymmetric anterior to posterior. The Kirchhoff-ray mode backscatter model predicted that TS at 38 and 120 kHz decreased by ∼4 dB as pressure increased from ambient to 4.9 × 105 Pa.

Benthic-pelagic trophic interactions within the fish assemblage in the western Bering Sea shelf area according to stomach content analysis and ratios of C and N stable isotopes

The composition, abundance, diet and trophic status of zooplankton, bottom invertebrates, fish and nekton were analyzed based on the data collected by the staff of the TINRO-Center during complex bottom trawl catches on the Bering Sea shelf in the fall of 2004. The stomach contents of mass fish species were analyzed and the nitrogen and carbon isotopic composition of 36 mass species of plankton, benthos, nekton and nektobenthos, which together make up the basis of pelagic and bottom communities, was determined. It was found that zooplankton noticeably differ from benthic invertebrates in carbon isotopic composition: δ13C values in zooplankton varied from −20.3‰ to −17.9‰; in benthos—from −17.5‰to −13.0‰; and in fish—from −19.2‰ (juvenile walleye pollock) to −15.3‰ (saffron cod). The levels of 13C isotope in the tissues of fish depended mostly on the share of pelagic or benthic animals in their diet. δ15N values in the studied species ranged from 8.6‰ (in sea urchins) to 17.2‰ (in large Pacific cods), which corresponds to a trophic level of 2.8. Obviously the δ15N values reflect the degree of predation and generally show the ratio of primary, secondary and tertiary consumers in a fish’s diet. Trophic interactions manifest a high degree of interdependence between benthic and pelagic communities (even without taking into account such lower components of the food web as phytoplankton, bacteria, and protozoa) occurring in most nektonic species that depend on both bottom and pelagic food.