Juvenile Pollock Research Articles

Size, diet, and condition of age-0 Pacific cod (Gadus macrocephalus) during warm and cool climate states in the eastern Bering sea

The revised Oscillating Control Hypothesis for the Bering Sea suggests that recruitment of groundfish is linked to climatic processes affecting seasonal sea ice that, in turn, drives the quality and quantity of prey available to young fish for growth and energy storage during their critical life history stages. We test this notion for age-0 (juvenile) Pacific cod (Gadus macrocephalus) by examining the variability in size, diet, and energetic condition during warm (2003–2005), average (2006), and cool (2007–2011) climate states in the eastern Bering Sea. Juvenile cod stomachs contained high proportions of age-0 walleye pollock (by wet weight) during years with warm sea temperatures with a shift to euphausiids and large copepods during years with cool sea temperatures. Juvenile cod were largest during years with warm sea temperatures and smallest during years with cool sea temperatures. However, energetic status (condition) of juvenile cod was highest during years with cool sea temperatures. This result is likely linked to the shift to high quality, lipid-rich prey found in greater abundance on the shelf and in the stomach contents of juvenile cod during cool years. Our examination of juvenile cod size, diet, and energetic status provided results that are similar to those from studies on juvenile pollock, suggesting that the common mechanisms regulating gadid recruitment on the eastern Bering Sea shelf are climate state, prey quality and quantity, and caloric density of gadids prior to winter.

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.

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.

Interannual variation in diets of walleye pollock in the Doto area, in relation to climate variation

Interannual variation in the diet of walleye pollock Theragra chalcogramma along the southeastern coast of Hokkaido Island (Doto area) was analyzed using >15 000 specimens col- lected in the periods 1989�1992 and 1995�2011, in relation to oceanographic variables. The area experienced a warm period from 1998 to 2002, which was preceded and followed by cold periods. An apparent shift in diet occurred between the first cold period and the warm period. Appendic- ularians (larvaceans) became an important summer prey for small-sized pollock (≤300 mm SL) in addition to copepods and euphausiids, which were also important before the onset of the warm period. In medium- and large-sized pollock, an abrupt decrease in mesopelagic fishes (mainly myctophid Diaphus theta) was observed in stomach contents 3 yr after the 1997/1998 climate regime shift, possibly reflecting a recruitment failure of D. theta. Appendicularians, cannibalism and anchovy Engraulis japonica compensated for the disappearance of mesopelagic fishes from the diet. During autumn and winter, a similar shift from micronekton (mainly D. theta and firefly squid Watasenia scintillans) to euphausiids (exclusively Euphausia pacifica) and cannibalism was found between 2000 and 2001. Although diet changes lagged the onset of the warm period, con- dition factor increased immediately after the regime shift. This inconsistency may be attributable to the bioenergetic response of juvenile pollock, which attain better growth at warmer tempera- tures. Thus, in the Doto area, the effect of climate variation on the survival of pollock seemed to be limited, due to their flexible feeding behaviors.

1970-1990년대 동해에서 어획된 명태(Theragra chalcogramma)의 체장에 따른 체급별 어획 마릿수 추정

Walleye pollock Theragra chalcogramma fisheries in Korean waters have changed dramatically during the last three decades: the highest catches occurred in 1981, followed by continuous decreases through the 1990s, ending with a complete collapse of the population in the 2000s. The major spawning ground of walleye pollock is located in North Korean waters, and some juveniles (called nogari in Korean, <300 mm) migrate to the south for feeding and growth. Since the 1960s, Korean fishermen have often caught juveniles, and the weight (metric tons) of juvenile catch was recorded from 1975-1997. However, because the walleye pollock were not aged, the population age structure was not delineated. We developed a model to estimate the number of walleye pollock of each size class based on catch statistics of adults and juveniles, the catch proportion of each size class, and length and weight information on specimens collected by Danish-seine and drift-gill-net fisheries. The model results demonstrated that the recruitment size of walleye pollock was consistently within the 200-250mm size class, and the highest number of this size class occurred in 1981, although values greatly fluctuated interannually. The number of juvenile pollock was 10.4 times higher than that of adult pollock during 1975-1997. The total yield of juvenile pollock was 0.95 million tons, which was equivalent to about 68.2% of total pollock production. The number of juvenile pollock caught during the same period, however, was 16 billion, comprising about 91.2% of the total number caught. Such high fishing pressure on juvenile pollock is considered one of the main factors causing the collapse of the pollock population.

Climate and Demography Dictate the Strength of Predator-Prey Overlap in a Subarctic Marine Ecosystem

There is growing evidence that climate and anthropogenic influences on marine ecosystems are largely manifested by changes in species spatial dynamics. However, less is known about how shifts in species distributions might alter predator-prey overlap and the dynamics of prey populations. We developed a general approach to quantify species spatial overlap and identify the biotic and abiotic variables that dictate the strength of overlap. We used this method to test the hypothesis that population abundance and temperature have a synergistic effect on the spatial overlap of arrowtooth flounder (predator) and juvenile Alaska walleye pollock (prey, age-1) in the eastern Bering Sea. Our analyses indicate that (1) flounder abundance and temperature are key variables dictating the strength of flounder and pollock overlap, (2) changes in the magnitude of overlap may be largely driven by density-dependent habitat selection of flounder, and (3) species overlap is negatively correlated to juvenile pollock recruitment when flounder biomass is high. Overall, our findings suggest that continued increases in flounder abundance coupled with the predicted long-term warming of ocean temperatures could have important implications for the predator-prey dynamics of arrowtooth flounder and juvenile pollock. The approach used in this study is valuable for identifying potential consequences of climate variability and exploitation on species spatial dynamics and interactions in many marine ecosystems.

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.

Distribution of fish and macrozooplankton in ice-covered and open-water areas of the eastern Bering Sea

The eastern Bering Sea shelf is a productive ecosystem with extensive commercial fisheries. Although the area is well-studied during summer months, little is known about the abundance and distribution of fish and macrozooplankton during periods of seasonal ice cover. The use of an icebreaker during the Bering Sea Ecosystem Study (BEST) provided a platform for spring acoustic surveys of fish and zooplankton in ice-covered areas for the first time. Icebreaker measurements were complemented with observations from conventional vessels during spring and summer. In spring, very little backscatter from fish (dominated by walleye pollock, Theragra chalcogramma) was observed in the ice-covered northern areas where near-bottom waters were cold (<∼0.5°C), including areas where walleye pollock are abundant in summer. The majority of fish were observed within 40km (and often slightly inside) the ice edge over similar seafloor depths as in summer. Together, these observations suggest that pollock, a dominant component of the ecosystem, shift their distribution to a more restricted geographic area in spring, following the ice edge southeast along the bathymetry, away from areas of cold water and extensive ice cover, then reoccupying these areas in summer. In contrast, acoustic backscatter attributed to zooplankton (likely dominated by euphausiids) was more evenly distributed, and less restricted by water temperature and ice cover. The implications of this seasonal shift in fish distribution are uncertain, but this may affect predator–prey interactions by reducing overlap of pollock with euphausiids, an important prey source, while increasing overlap of adult and juvenile pollock and potentially increasing cannibalism.

Active acoustic examination of the diving behavior of murres foraging on patchy prey

During the 2008 and 2009 breeding seasons of murres Uria spp., we combined visual observations of these predators with active acoustics (sonar), fish trawls, zooplankton net tows, and hydrographic measurements in the area surrounding breeding colonies in the southeastern Bering Sea. We acoustically detected thousands of bubble trails that were strongly correlated with the number of visually detected murres, providing a new tool for quantitatively studying the for- aging ecology of diving birds. At the regional scale, the number of acoustically detected bubble trails, which served as a proxy for diving murre abundance, was related to the combined availabil- ity and vertical accessibility of squid, krill, and pollock. There were, however, no clear relation- ships at this scale between diving murres and any individual prey taxon, highlighting the impor- tance of prey diversity to these animals. Individual krill patches targeted by murres had higher krill density and were located shallower than the mean depth of krill patches, but were similar in total krill abundance and overall size. The diving depth of murres within krill patches was highly correlated to the depth of the upper edge of these patches, whereas murres found outside of krill patches showed a depth distribution similar to that of juvenile pollock. Throughout the study area, murres showed strong diel patterns in their diving behavior in response to the diel migrations of their prey. These results suggest that murres select prey with specific patch characteristics imply- ing effective information gathering about prey by murres. The high proportion of diving murres in aggregations and their consistent inter-individual spacing support the hypothesis that intra- specific local enhancement may facilitate foraging in these predators.

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.

Active acoustic examination of the diving behavior of murres foraging on patchy prey

Van Holliday used sound to study nearly every type of organism living in the ocean, contributing to our understanding of acoustics as well as marine ecological processes. To honor Van, I will talk about the one animal group that I could not find in his long publication list—seabirds. We combined visual observations of murres with active acoustics, fish trawls, zooplankton net tows, and hydrographic measurements in the area surrounding breeding colonies in the southeastern Bering Sea. We detected thousands of unique acoustic targets that were strongly correlated with the number of visually detected murres, providing a new tool for quantitatively studying the foraging ecology of diving birds. Diving murre abundance was related to the combined availability and vertical accessibility of squid, krill, and pollock. Individual krill patches targeted by murres had higher krill density and were shallower than mean values but were similar in total krill abundance and overall size. Murres found outside of krill showed a depth distribution similar to that of juvenile pollock. The high proportion of diving murres in aggregations and their consistent inter-individual spacing support the hypothesis that intra-specific local enhancement may facilitate foraging in these predators.

Climate change in the southeastern Bering Sea: impacts on pollock stocks and implications for the oscillating control hypothesis

Concern about impacts of climate change in the Bering Sea prompted several research programs to elucidate mechanistic links between climate and ecosystem responses. Following a detailed literature review, Hunt et al. (2011) (Deep-Sea Res. II, 49, 2002, 5821) developed a conceptual framework, the Oscillating Control Hypothesis (OCH), linking climaterelated changes in physical oceanographic conditions to stock recruitment using walleye pollock (Theragra chalcogramma) as a model. The OCH conceptual model treats zooplankton as a single box, with reduced zooplankton production during cold conditions, producing bottom-up control of apex predators and elevated zooplankton production during warm periods leading to top-down control by apex predators. A recent warming trend followed by rapid cooling on the Bering Sea shelf permitted testing of the OCH. During warm years (2003–06), euphausiid and Calanus marshallae populations declined, post-larval pollock diets shifted from a mixture of large zooplankton and small copepods to almost exclusively small copepods, and juvenile pollock dominated the diets of large predators. With cooling from 2006–09, populations of large zooplankton increased, post-larval pollock consumed greater proportions of C. marshallae and other large zooplankton, and juvenile pollock virtually disappeared from the diets of large pollock and salmon. These shifts in energy flow were accompanied by large declines in pollock stocks attributed to poor recruitment between 2001 and 2005. Observations presented here indicate the need for revision of the OCH to account for shifts in energy flow through differing food-web pathways due to warming and cooling on the southeastern Bering Sea shelf.

Seasonal abundance and distribution of pelagic and demersal fishes in southeastern Alaska

We quantified seasonal and interannual variability of pelagic and demersal fishes available to marine predators in southeastern Alaska focusing on prey of the Steller sea lion ( Eumetopias jubatus). Estimates of abundance were determined from echo integration mid-water trawl and demersal longline surveys. The dominant species were walleye pollock ( Theragra chalcogramma) (average biomass 9057 t), Pacific hake ( Merluccius productus) (1715 t), Pacific herring ( Clupea pallasii) (1176 t), Pacific halibut ( Hippoglossus stenolepis) (average catch rate 524 kg per 1000 hooks), Pacific cod ( Gadus macrocephalus) (177 kg per 1000 hooks), sablefish ( Anoplopoma fimbria) (120 kg per 1000 hooks), and sandpaper skate ( Bathyraja interrupta) (26 kg per 1000 hooks). Of these species, seasonal differences in species abundance were detected for walleye pollock ( p = 0.03), Pacific cod ( p = 0.001) and sablefish ( p < 0.001) with walleye pollock the most abundant and widespread species year-round. Herring, hake, and juvenile (120–350 mm) and adult (>350 mm) pollock are pelagic species. Adult and juvenile pollock and hake were found in open water, while herring, young-of-the-year (<120 mm) and the smallest juvenile pollock were found in bays. Herring of all ages concentrate in dense schools. Pollock and hake form scattered layers throughout open water with juvenile pollock shallower than adult pollock and adult pollock shallower than hake. Halibut, sablefish, skates, Pacific cod and arrowtooth flounder are demersal species; sablefish were deeper than the other demersal species. These seasonal and annual changes in prey availability affected prey selection of sea lions which shifted their diet in response.

Length-selective retention of walleye pollock, Theragra chalcogramma, by midwater trawls

AbstractWilliams, K., Punt, A. E., Wilson, C. D., and Horne, J. K. 2011. Length-selective retention of walleye pollock, Theragra chalcogramma, by midwater trawls. – ICES Journal of Marine Science, 68: 119–129. Midwater trawls are commonly used during acoustic surveys of fish abundance to determine species and length compositions of acoustically sampled fish aggregations. As trawls are selective samplers, catches can be unrepresentative of sampled populations and lead to biased abundance estimates. Length-dependent retention of walleye pollock was estimated using small recapture nets, so-called pocket nets, attached to the outside of the trawl. Experimental haul sets comprising eight hauls each were conducted in the Gulf of Alaska in 2007 and 2008 and in the eastern Bering Sea (EBS) in 2007. Pocket-net catches were then modelled by fitting parameters for selectivity and escapement location along the trawl. Within- and between-haul variability was jointly estimated using hierarchical Bayesian methods. There was significant undersampling of juvenile (&lt;25 cm) pollock, with the length-at-50%-retention (L50) estimated between 13.5 and 26.1 cm among haul sets. In the EBS set, L50 values were correlated with light level, escapement being greater at night. Trawl selectivity may be a significant source of error in acoustic-survey estimates of the abundance of pollock.

Influence of size on the sources of energy consumed by overwintering walleye pollock ( Theragra chalcogramma)

The critical size hypothesis proposes the risks of winter mortality are greatest for small individuals in fish populations. This results from size-dependent predation and starvation of wintering fishes. We extend this idea to walleye pollock ( Theragra chalcogramma) and argue size-dependence leads to different wintering strategies among subadult life stages. We sampled fish quarterly, measured their size, growth and energy allocation. By comparing the contributions of endogenous energy sources (storage and structure) to metabolic demand in young-of-the-year (YOY) and juvenile pollock over winter we determined how length influenced their winter foraging requirement. From this we inferred the relative risks of starvation and predation for different sized fish. Young-of-the-year (≤ 170 mm) experienced high starvation risk and relied on foraging to meet routine metabolic cost. Consequently, they consumed relatively little storage or structural energy over winter. In contrast, juveniles (> 170 mm and ≤ 270 mm) responded to the risk of predation by consuming significant amounts of protein and structural lipids while minimizing foraging. Consistent with these observations we found YOY maximized growth in fall while juveniles maximized energy storage. In spring, YOY resumed growth earlier than juveniles. These data indicate the critical size and period hypothesis can be extended to walleye pollock.

Investigation of foraging habits and prey selection by humpback whales (Megaptera novaeangliae) using acoustic tags and concurrent fish surveys

AbstractTags containing acoustic time‐depth transmitters (ATDT) were attached to four humpback whales near Kodiak, Alaska. Tags allowed for whale dive depths to be recorded in real time. Acoustic and mid‐water trawl surveys were conducted concurrent with tagging efforts within the study area to quantify available fish resources and describe potential prey selection by humpback whales. Recorded dives were grouped through visual assessment and t‐tests. Dives that indicated likely foraging occurred at a mean maximum depth of 106.2 m with 62% of dives occurring between 92 m and 120 m. Acoustic backscatter from fish surveys was attributed to potential humpback prey based on known target strength values and 10 net tows. Capelin comprised 84% of the total potential prey abundance in the region followed by age 0 (12%) and juvenile pollock (2%), and eulachon (&lt;1%). Although horizontally segregated in the region, both capelin and age 0 pollock were distributed at depths exceeding 92 m with maximum abundance between 107 m and 120 m. The four‐tagged humpbacks were found to forage in areas with greatest capelin densities but bypassed areas of high age 0 pollock abundance. The location and diving behavior of tagged whales suggested that whales were favoring capelin over pollock as a prey source.

Rise and fall of jellyfish in the eastern Bering Sea in relation to climate regime shifts

A steep increase in jellyfish biomass, primarily Chrysaora melanaster, over the eastern Bering Sea shelf was documented throughout the 1990s. Their biomass peaked in summer 2000 and then declined precipitously, stabilizing at a moderate level after 2001. The onsets of the outburst and decline coincided with transitions between climatic regimes. Specifically, 1989 marked the beginning of a period of moderate temperatures in the eastern Bering Sea, after the warm conditions of the late 1970s through the 1980s. Very warm conditions came to the eastern Bering after 2000, as evidenced by decreased ice cover in winter and increased total heat content and surface water temperatures in summer. We examined the relationships between jellyfish biomass and temperature, ice cover, atmospheric variables, current patterns, zooplankton biomass, and associated fish biomass in two regions of the Middle Shelf Domain (SE and NW) by use of Generalized Additive Models (GAM). We found density-dependent interactions within and between jellyfish biomass in the two regions related to the flow regime, and demonstrated a linkage between biophysical indices and jellyfish biomass. In particular, ice cover (SE and NW), sea-surface temperature in spring (SE) and summer (NW), and wind mixing (SE) all influenced jellyfish biomass. In addition, the importance of juvenile pollock biomass (SE) and zooplankton biomass (NW) suggest that jellyfish biomass was sensitive to the availability of prey. Since most climate models suggest continued warming is likely in the Bering Sea, the jellyfish populations may remain at moderate levels there but will likely shift northward into the Arctic Ocean.

The effect of oceanographic variability and interspecific competition on juvenile pollock (Theragra chalcogramma) and capelin (Mallotus villosus) distributions on the Gulf of Alaska shelf

Abstract Results from this study suggest that small-scale variability in the Alaska Coastal Current (ACC) and competition between juvenile pollock and capelin are potential mechanisms affecting the distribution and abundance of fishes in the Gulf of Alaska (GOA). Fish distributions in Barnabus Trough, off the east coast of Kodiak Island, were assessed using acoustic data collected with a calibrated echosounder during August–September 2002 and 2004. Trawl hauls were conducted to determine the species composition of the fish making up the acoustic backscatter. Oceanographic data were collected from moorings, conductivity–temperature–depth (CTD) probes, trawl-mounted microbathythermographs (MBT) and expendable bathythermographs (XBT). National Centers for Environmental Prediction (NCEP) reanalysis data were used to assess area winds, and information on regional transport was derived from current meters deployed on moorings north and south of Kodiak Island. The distribution of water-mass properties and fish during 2002 showed variability at the temporal scale of weeks. Juvenile pollock (age-1 and age-2) were initially most abundant in warm, low-salinity water on the inner shelf, whereas capelin were distributed primarily on the outer shelf in cool, high-salinity waters. During a 2-week period juvenile pollock distribution expanded with the offshore expansion of warm, low-salinity water, and capelin abundance in outer-shelf waters decreased. We hypothesize that wind-driven pulsing of the ACC resulted in increased transport of warm, low-salinity water through the study area. In 2004, warm, low-salinity water characterized the inner shelf and cool, high-salinity water was found on the outer shelf. However, the distribution of water-mass properties did not show the weekly scale variability observed in 2002. Area winds were consistently toward the southwest during 2004, such that we would not expect to see the wind-driven pulsing of ACC water that occurred in 2002. Age-1 and age-2 pollock were not observed in Barnabus Trough in 2004. Instead, the midwater acoustic backscatter was composed of capelin mixed with age-0 pollock, and these capelin were not restricted to the outer-shelf waters, but were found primarily in warm, low-salinity inner-shelf waters that had been previously occupied exclusively by age-1 and age-2 pollock. We suggest that this is consistent with inner-shelf waters being preferred foraging habitat for juvenile pollock and capelin. Further study of the mechanisms linking climate change with variability in the ACC is needed, as are studies of the potential for competition between juvenile pollock and capelin.