Juvenile Walleye Pollock Research Articles

Rate of energy depletion and overwintering mortality of juvenile walleye pollock in cold water

The winter energy deficit and mortality of juvenile walleye pollock at extremely cold temperature were examined by field observations and laboratory experiments. In the Doto area, along the northern coast of Japan, juvenile walleye pollock resided on the continental shelf despite extremely cold temperatures (mean 0·4° C) during the latter half of winter (March to April). Measurements of the rate of energy depletion (equivalent to the routine metabolic rate) revealed that juvenile walleye pollock consumed 37% less energy at 0·5° C than at 2·0° C, suggesting an energetic benefit of residence in cold water (<1·0° C) over the shelf during winter. Prior to the starvation experiments, temperatures and ration level in the holding tanks were adjusted to create two different body condition groups of fish. Under the thermal condition of the latter half of winter (0·5° C), fish with a mean condition factor of 0·6 and 0·5 suffered 19·1 and 74·5% mortality, respectively, at the end of the experiments (after 56 days). The residual analysis of total body energy demonstrated that the cause of mortality was mainly associated with the depletion of energy reserves. When a logistic regression model for mortality derived from the experiments was applied to wild fish collected in March, the estimated overwintering mortality in 2004 and 2005 was 25·4 and <2·3%, respectively, assuming no feeding during the winter. Considering that juvenile walleye pollock feed during winter as shown in previous studies, intense overwintering mortality induced by energy depletion is improbable during the latter half of winter in the Doto area.

Daily and sub-daily otolith increments of larval and juvenile walleye pollock, Theragra chalcogramma (Pallas), as validated by alizarin complexone experiments

Walleye pollock ( Theragra chalcogramma) were reared from eggs to the juvenile life stage to study daily increment formation in the sagittae otoliths, which are routinely used for age and growth analyses. The apparent deposition of sub-daily growth increments becomes problematic for determining fish age from the late larval stage throughout the juvenile (young-of-the-year) development stage. Otolith marking experiments were conducted to determine interpretation criteria to differentiate between daily and sub-daily increments. Immersion of larval and transforming walleye pollock in 25 mg/l of alizarin complexone (ALC) for 6 h once a week produced a fluorescent mark on the day of staining. Evidence of six well defined and equally spaced increments counted between the weekly ALC marks validated the deposition of daily increments. The daily increments gradually increased in width as the fish/otolith grew. The criteria for determining the presence of sub-daily increments between the daily increments were (1) weak optical definition and (2) a sudden change in incremental distance that lasted for one or two increments and were approximately <0.5 μm in width. Growth problems that occurred during the experiments were identified on otoliths as reductions in daily incremental widths and optical definition, which continued for several days. Otoliths from field-collected fish have also shown similar changes in daily increment properties during the juvenile stage, which may be an indicator of an environmental influence. The criteria for defining different increment types help to resolve our current age determination issues for late larval and early juvenile stage walleye pollock from the Gulf of Alaska.

Optimum temperature for growth of juvenile walleye pollock Theragra chalcogramma

Food consumption (FC), specific growth rate (SGR), and gross growth efficiency (GGE) were measured for juvenile walleye pollock (initial size 112–153 mm fork length) fed euphausiids at temperatures ( T) between 2 and 16 °C. FC increased up to 12 °C (7.3% of body weight) and was slightly lower at 16 °C. SGR also increased up to 12 °C (2% d − 1 ) but rapidly decreased at 16 °C. The optimum temperatures for FC and SGR were 12.3 °C and 11.5 °C, respectively. GGE was similar (27.3–29.9%) for temperatures in the range of 2–9 °C and decreased at higher temperatures. GGE in terms of energy (GGE = − 1.37 T + 38.41) and the energy density of fish was higher at lower temperatures, suggesting that residence of juvenile pollock in cold water induces energy storage. In the Doto continental-shelf area, juvenile pollock were distributed over a wide range of temperatures (5.4–15.0 °C) during the summer. This suggests that factors other than water temperature may affect the summer distribution of juvenile pollock, although temperature would be the most important factor in a food-unlimited environment.

Interannual variability in hatching period and early growth of juvenile walleye pollock, Theragra chalcogramma, in the Pacific coastal area of Hokkaido

ABSTRACTJuvenile walleye pollock of the Japanese Pacific population were collected from the Funka Bay [spawning ground; 16–64 mm fork length (FL)] in spring and the Doto area (nursery ground; 70–146 mm FL) in summer. Hatch dates were estimated by subtracting the number of otolith daily increments from sampling dates, and their early growth was back‐calculated using otolith radius–somatic length relationships. Interannual change of the hatching period was observed during 2000–02, and the peaks ranged from mid‐February in 2000 to early‐April in 2002. In 2000, when a strong year class occurred, early life history of the surviving juveniles could be characterized by early hatching and slower growth in the larval stage (&lt;22 mm length). Higher growth rate in 2001 and 2002 did not always lead to good survival and recruitment success. Even though their growth was slow in 2000, the larvae hatched early in the season had larger body size on a given date than faster‐growing larvae hatched in later season in 2001 and 2002. Bigger individuals at a certain moment may have advantage for survival. The delay of hatching period may result in higher size‐selective mortality, and as a necessary consequence, back‐calculated growth in 2001 and 2002 could shift towards higher growth rate, although abundance of such a year class would be at the lower level. Variability in spawning period, early growth and their interaction might have a strong relation to larval survival through cumulative predation pressure or ontogenetic changes in food availability.

Thermal effects on behavior of juvenile walleye pollock (Theragra chalcogramma): implications for energetics and food web models

The behavioral responses of fishes to temperature variation have received little attention despite their direct implications to bioenergetics-based models of production and encounter-based models of food web dynamics. Behavioral characteristics of juvenile walleye pollock (Theragra chalcogramma), a pelagic marine zooplanktivore, were examined between 2 and 9 °C in large arenas. Routine swim speed, path sinuosity, and schooling cohesiveness of fish in small groups were described from overhead video observations. In a separate experiment, maximum swimming speeds were measured in a recirculating flume. Routine and maximum swimming speeds had contrasting responses to temperature demonstrating a behavioral rather than physiological regulation of activity level. Routine swim speed was 48% faster at 2 °C than at 9 °C. This result is inconsistent with the assumption of a constant activity multiplier for metabolism incorporated into most bioenergetics models of fish growth. Increased swim speed, along with the reduced path sinuosity observed at low temperatures, may reflect kinetic aspects of habitat selection. Group cohesion increased at low temperatures, with nearest neighbors averaging 32% closer at 2 °C than at 9 °C. These results demonstrate that representative models of energy flow through marine food webs depend on an improved understanding of the behavioral as well as physiological responses of fishes to thermal variation.

Temperature and prey quality effects on growth of juvenile walleye pollock Theragra chalcogramma (Pallas): a spatially explicit bioenergetics approach

A bioenergetics model for juvenile age‐0 year walleye pollock Theragra chalcogramma was applied to a spatially distinct grid of samples in the western Gulf of Alaska to investigate the influence of temperature and prey quality on size‐specific growth. Daily growth estimates for 50, 70 and 90 mm standard length (LS) walleye pollock during September 2000 were generated using the bioenergetics model with a fixed ration size. Similarities in independent estimates of prey consumption generated from the bioenergetics model and a gastric evacuation model corroborated the performance of the bioenergetics model, concordance correlation (rc) = 0·945, lower 95% CL (transformed) (L1) = 0·834, upper 95% CL (transformed) (L2) = 0·982, P &lt; 0·001. A mean squared error analysis (MSE) was also used to partition the sources of error between both model estimates of consumption into a mean component (MC), slope component (SC), and random component (RC). Differences between estimates of daily consumption were largely due to differences in the means of estimates (MC= 0·45) and random sources (RC= 0·49) of error, and not differences in slopes (SC= 0·06). Similarly, daily growth estimates of 0·031–0·167 g day−1 generated from the bioenergetics model was within the range of growth estimates of 0·026–0·190 g day−1 obtained from otolith analysis of juvenile walleye pollock. Temperature and prey quality alone accounted for 66% of the observed variation between bioenergetics and otolith growth estimates across all sizes of juvenile walleye pollock. These results suggest that the bioenergetics model for juvenile walleye pollock is a useful tool for evaluating the influence of spatially variable habitat conditions on the growth potential of juvenile walleye pollock.

Effect of ocean conditions on the cross‐shelf distribution of walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus)

AbstractAcoustic trawl surveys were conducted in 2000 and 2001 in two troughs located off the eastern coast of Kodiak Island in the Gulf of Alaska as part of a multiyear, multidisciplinary experiment to examine the influence of environmental conditions on the spatial distribution of adult and juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Continuous underway sea surface temperature samples and water column profiles collected in 2000 and 2001 showed the presence of a sharp shelf‐break front in Chiniak Trough and a mid‐trough front in Barnabas Trough. At distances &lt;22 km from shore, the water column was well mixed, whereas a well‐defined mixed layer was present beyond approximately 22 km from shore. Satellite drifter tracks in Barnabas Trough entered along the upstream edge of the trough and appeared to follow the frontal boundary across the middle portion of the trough. A storm in 2001 weakened stratification and cooled surface water temperature by 1.6–2.1°C. Wind mixing associated with the storm event mixed subsurface chlorophyll a to the surface and enhanced nutrients in the surface waters. The storm event revealed spatial partitioning of summer production in Barnabas Trough, with production concentrated in regions inside the mid‐trough front. In contrast, post‐storm summer production was distributed throughout Chiniak Trough. The spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat characteristics. Acoustic survey data identified four acoustic sign types: age‐1 pollock, adult pollock, capelin, capelin–age‐0 pollock mix. The spatial distribution of these four sign types appears to be influenced by the oceanographic and topographic features of the two troughs. Adult pollock were broadly distributed throughout Chiniak Trough, whereas adult pollock were aggregated on the coastal side of the frontal system in Barnabas Trough. In 2000, capelin occurred with age‐0 pollock. In Chiniak Trough, capelin were most abundant along steep topographic gradients at the edges of the trough and in a deep region near Cape Chiniak, whereas the capelin–age‐0 mix (2000) or capelin (2001) concentrations were observed in slope water intrusions over the outer shelf in Barnabas Trough. Results suggest that habitat selection of walleye pollock and capelin are controlled by different processes. Capelin distributions appear to be limited by oceanographic conditions while other factors appear to be more important for pollock.

Comparing acoustic model estimations and in situ target strength measurements of juvenile walleye pollock (Theragra chalcogramma)

Recently, many acoustic surveys for the juvenile walleye pollock have been done around the Hokkaido, Japan. Dependence of target strength on fish length is a most important factor in the conversion of acoustic data to fish biomass. However, the relation between target strength and length of juvenile walleye pollock is not well investigated. This study examined the target strength length relationship (3–10 cm) for the echo integration measurements of the juvenile walleye pollock at 38 kHz from acoustic theoretical model predictions and in situ measurements. A prolate spheroid model was used to compute the target strength of the fish relative to its orientation. Swimbladder morphologies of live juvenile walleye pollock caught by set net were obtained from transmission of light or soft x‐ray, and converted their morphological parameters. Biological samplings using framed mid‐water trawl were carried out near the spawning area of walleye pollock. During fish samplings, in situ target strength measurements wer...

Swimbladders under pressure: Anatomical and acoustic responses by walleye pollock

Pressure influences echo intensities of swimbladdered fish through changes in swimbladder volumes and surface areas. Volume reduction is expected to correspond to Boyle’s law (volume ∝pressure−1) but the resulting deformation in swimbladder surface area will largely determine target strength at geometric scattering frequencies. Dorsal and lateral radiographs of juvenile walleye pollock (Theragra chalcogramma) in a pressure chamber were used to image swimbladders from ambient to a maximum of 5‐atm pressure. As pressure increased, dorsal swimbladder surface areas decreased at a constant rate among the three individuals. Swimbladder volume reduction rates were similar among individuals but less than that predicted by Boyle’s law. Compression of swimbladders occurred dorsal‐ventral, anterior‐posterior, and laterally. Resulting swimbladder shapes became more spindlelike as pressure increased. KRM predicted target strengths at 38 and 120 kHz decreased more rapidly at 3 atm above ambient than at lower pressures. [Work supported by ONR and NRIFE.]

Foraging habitats based on the diet of female northern fur seals (Callorhinus ursinus) on the Pribilof Islands, Alaska

AbstractScats (fecal samples) collected between 1987 and 2000 on northern fur seal Callorhinus ursinus rookeries of St Paul (n=2968) and St George Islands (n=1203), Alaska, were used to examine the relationship between breeding sites and food habits of adult female seals. On the basis of the frequency of occurrence (FO) and per cent minimum number of individual prey (%MNI) in scats, juvenile walleye pollock Theragra chalcogramma and gonatid squid Gonatopsis borealis/Berryteuthis magister and Gonatus madokai/Gonatus middendorffi were the dominant prey species consumed overall. Other primary prey (FO&gt;5%) included Pacific sand lance Ammodytes hexapteus, Pacific herring Clupea pallasi, northern smoothtongue Leuroglossus schmidti, Atka mackerel Pleurogrammus monopterygius, Pacific salmon (Oncorhynchus spp.) and other squid of the Gonatus genus. We identified five rookery complexes from a cluster analysis of the FO of primary prey in scats. Rookery complexes were separated geographically and each was further defined by characteristic patterns in the representation of prey types typically associated with specific hydrographic domains. Diet differences were observed among rookeries on the north and south side of St George Island and on the east, south and south‐west side of St Paul Island. The rookery clusters observed in this study provide evidence of resource partitioning among adult female northern fur seals and have important implications for fur seal conservation and management.

Seasonal and Geographic Variation in Condition of Juvenile Walleye Pollock in the Western Gulf of Alaska

AbstractBody length, length‐specific weight, and length‐specific whole‐body energy content (WBEC) of juvenile walleye pollock Theragra chalcogramma were examined for evidence of seasonal and geographic variation. Identifying the impact of winter on these measures of fish condition was of primary concern, but we also sought to determine whether condition varies among different geographic regions. The 2000 year‐class of walleye pollock was sampled in the western Gulf of Alaska during four seasons (the autumn of 2000 and the winter, summer, and autumn of 2001), and the study area was divided into three regions (Kodiak, Semidi, and Shumagin). In each region, length‐specific weight decreased from autumn to winter by up to 17%, subsequently increasing into the summer by 13–20%. Significant seasonal changes in length‐specific WBEC varied by region but reflected the same trends as length‐specific weight. Walleye pollock from the Kodiak region were in better condition than those from other regions, as evidenced by larger lengths in winter (18‐mm difference) and summer, a 5.9% higher length‐specific weight in autumn of 2000, and higher length‐specific WBEC in autumn of 2000 (0.783 kJ/g wet weight, or 18.3%, higher) and in the following winter (0.607 kJ/g, or 14.1%, higher). The poorer condition of juvenile walleye pollock in winter may be widespread through the Gulf of Alaska, but spatial heterogeneity in habitat can also greatly influence the size and condition of juveniles. The impact of such variability on growth and mortality remains uncertain.

Oceanographic conditions structure forage fishes into lipid-rich and lipid-poor communities in lower Cook Inlet, Alaska, USA

Forage fishes were sampled with a mid-water trawl in lower Cook Inlet, Alaska, USA, from late July to early August 1996 to 1999. We sampled 3 oceanographically distinct areas of lower Cook Inlet: waters adjacent to Chisik Island, in Kachemak Bay, and near the Barren Islands. In 163 tows using a mid-water trawl, 229 437 fishes with fork length <200 mm were captured. More than 39 species were captured in lower Cook Inlet, but Pacific sand lance Ammodytes hexapterus, juvenile Pacific herring Clupea pallasi, and juvenile walleye pollock Theragra chalcogramma comprised 97.5% of the total individuals. Both species richness and species diversity were highest in warm, low- salinity, weakly stratified waters near Chisik Island. Kachemak Bay, which had thermohaline values between those found near Chisik Island and the Barren Islands, had an intermediate value of species richness. Species richness was lowest at the Barren Islands, an exposed region that regularly receives oceanic, upwelled water from the Gulf of Alaska. Non-metric multidimensional scaling (NMDS) was used to compute axes of species composition based on an ordination of pairwise site dissimilarities. Each axis was strongly rank-correlated with unique groups of species and examined separately as a function of environmental parameters (temperature, salinity, depth), area, and year. Oceanographic parameters accounted for 41 and 12% of the variability among forage fishes indicated by Axis 1 and Axis 2, respectively. Axis 1 also captured the spatial variability in the upwelled area of lower Cook Inlet and essentially contrasted the distribution of species among shallow, nearshore (sand lance, her- ring) and deep, offshore (walleye pollock) habitats. Axis 2 captured the spatial variability in forage fish communities from the north (Chisik Island) to the south (Barren Islands) of lower Cook Inlet and essen- tially contrasted a highly diverse community dominated by salmonids and osmerids (warmer, less saline) with a fish community dominated by Pacific sand lance (colder, more saline). Axis 3 reflected the negative spatial association of capelin Mallotus villosus and Pacific cod Gadus macrocephalus. Correlations of year with Axes 1 and 3 indicate that from 1996 to 1999 the forage fish community sig- nificantly decreased in lipid-poor gadids (walleye pollock and Pacific cod), and significantly increased in lipid-rich species such as Pacific sand lance, Pacific herring, and capelin.

Movement of juvenile walleye pollock, Theragra chalcogramma, from a spawning ground to a nursery ground along the Pacific coast of Hokkaido, Japan

AbstractThe movement of juvenile walleye pollock, Theragra chalcogramma, from the Funka Bay spawning ground to the Doto nursery ground along the Pacific coast of Hokkaido was observed during spring through summer of 2000 and 2001. A positive relationship between the size of the juveniles and the position of the eastern front of their distribution indicated a relationship between the hatching period and the timing of the eastward movement of juveniles toward the Doto area. Based on the bathymetric distribution of juvenile pollock on the continental shelf in Hidaka Bay, the coastal current is suggested to play a role in the easterly movement of juveniles. However, further supporting evidence from physical and biological oceanographic studies is necessary to confirm this hypothesis.

Size‐dependent, Spatial, and Temporal Variability of Juvenile Walleye Pollock (Theragra chalcogramma) Feeding at a Structural Front in the Southeast Bering Sea

Abstract. The waters surrounding the Pribilof Islands are an important nursery ground for juvenile walleye pollock (Theragra chalcogramma), an important forage fish in the pelagic food web of the productive Bering Sea shelf region. The diet of juvenile pollock was studied in two consecutive years along a transect line crossing from a well‐mixed coastal domain, through a frontal region to stratified water farther offshore. Variability in stomach fullness was high and evidence for increased feeding intensity in the front was weak. Prey diversity and prey size generally increased with increasing fish size, shifting from predominantly small copepods to larger, more evasive prey items such as euphausiids, crab megalopae and fish. The diet of the fish reflected changes in the relative abundance of copepods and euphausiids in the prey fields between years. Juvenile pollock showed increased feeding rates at dusk, and stomach fullness as well as prey condition were generally lowest just before sunrise; however, the proportion of euphausiids increased in the diet of pollock caught at night, suggesting that some food was also ingested during darkness. Juvenile pollock and their euphausiid prey both vertically migrated above the thermocline at night, although each had a different daytime depth.

Collaborative research on the pelagic ecosystem of the southeastern Bering Sea shelf

Two Bering Sea marine research programs collaborated during the final years of the 1990s to forge advances in understanding the southeastern Bering Sea pelagic ecosystem. Southeast Bering Sea Carrying Capacity, sponsored by NOAA Coastal Ocean Program, investigated processes on the middle and outer shelf and the continental slope. The Inner Front Program, sponsored by NSF, investigated processes of the inner domain and the front between the inner and middle domains. The purposes of these programs were to (1) increase understanding of the southeastern Bering Sea ecosystem, including the roles of juvenile walleye pollock, (2) investigate the hypothesis that elevated primary production at the inner front provides a summer-long energy source for the food web, and (3) develop and test annual indices of pre-recruit pollock abundance. The observations occurred during a period of unusually large variability in the marine climate, including a possible regime shift. Sea-ice cover ranged from near zero to one of the heaviest ice years in recent decades. Sea-surface temperatures reached record highs during summer 1997, whereas 1999 was noted for its low Bering Sea temperatures. Moreover, the first recorded observations of coccolithophore blooms on the shelf were realized in 1997, and these blooms now appear to be persistent. The programs' results include an archive of physical and biological time series that emphasize large year-to-year regional variability, and an Oscillating Control Hypothesis that relates marine productivity to climate forcing. Further investigations are needed of the confluences of interannual and even intra-seasonal variability with low-frequency climate variability as potential producers of major, abrupt changes in the southeastern Bering Sea ecosystem.

A comparison of the functional ecology of visual vs. nonvisual foraging in two planktivorous marine fishes

Some visually foraging planktivorous fish will facultatively engage in nonvisual foraging when opportunities arise or necessity dictates. Yet, little is known about the ecology of nonvisual foraging. We examined the roles of prey size, fish size, and prey density on the nonvisual foraging of walleye pollock, Theragra chalcogramma (40–100 mm total length), and sablefish, Anoplopoma fimbria (66–89 mm), in the laboratory. Both species were size selective, disproportionately consuming large prey just as they do during visual foraging. Large prey were encountered more often, presumably because they were more easily detected by the fish's lateral-line system. When foraging visually, larger fish consumed more prey, but during nonvisual foraging, there was no foraging advantage to greater fish size. Unlike visual detection distances, lateral-line detection distances may not increase appreciably with fish size. Lastly, prey density influenced nonvisual prey consumption. Walleye pollock were characterized by a type I functional response, whereas sablefish were characterized by a type II functional response. Models of planktivore foraging typically assume negligible foraging by particulate feeders below their visual foraging thresholds. On the basis of this study and field data, we suggest that foraging models for particulate feeders, such as juvenile walleye pollock and sablefish, should account for nonvisual size-selective foraging.

Trawl stress and escapee vulnerability to predation in juvenile walleye pollock: Is there an unobserved bycatch of behaviorally impaired escapees?

Recent studies suggest that mortality of undersized fishes escaping through trawl codends may range from 0 to 100%, with mortalities of 10 to 30% being common. These values may be low, as they do not account for fishes which become behaviorally compromised by their passage through the trawl and ultimately succumb to predators. The goal of this study was to simulate in the laboratory the stressors associated with trawl passage and determine if they degrade the behavioral capabilities of juvenile walleye pollock Theragra chalcogramma to avoid predation. In the first of 2 experiments, groups of Age 1 yr+ walleye pollock were subjected to 3 treatments: (1) controls: no stressor; (2) swim/escape: forced swimming for 90 min at 0.33 m s -1 in a towed net, followed by escape through 8 cm square mesh; (3) swim/crowd/escape: forced swimming followed by 3 min of crowding, followed by escape. To evaluate the effect of these treatments on pollock behavior, a sable- fish Anoplopoma fimbria (48 to 53 cm) was placed in an observation arena with the group and pol- lock anti-predator behavior was quantified. Beginning immediately after simulated trawling and for up to 24 h afterwards, pollock exposed to both trawl-stressor treatments were less likely to avoid the predator than controls, allowing it to approach closer. They were also less able to form a cohesive shoal, and in the case of the swim/crowd/escape treatment, swam more slowly than control fish. To determine if trawl-stressed fish are more vulnerable to predation, in a second experiment I mixed control and swim/crowd/escape pollock together and then subjected them to predation by a 48 to 60 cm lingcod Ophiodon elongatus, observing the behavior and enumerating the number of pollock consumed in each treatment. Lingcod concentrated attacks upon solitary individuals or those strag- gling behind the shoal, were more likely to lunge at pollock that did not move away when approached, and were more successful the closer the pollock at lunge initiation. As a result, trawl- stressed pollock were consumed in greater numbers than controls. On the basis of these results, it is reasonable to expect that juvenile walleye pollock passing through trawls suffer behavioral deficits, subjecting them to elevated predation risk. If this is a generic effect, these results suggest that there may be a significant bycatch associated with many commercial trawl fisheries which is generally unrecognized, unmeasured, and unaccounted for in current stock-assessment models.

Fat content and fatty acid composition of forage fish and invertebrates in Prince William Sound, Alaska: factors contributing to among and within species variability

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 241:161-181 (2002) - doi:10.3354/meps241161 Fat content and fatty acid composition of forage fish and invertebrates in Prince William Sound, Alaska: factors contributing to among and within species variability Sara J. Iverson1,*, Kathryn J. Frost2, Shelley L. C. Lang1 1Department of Biology, Dalhousie University, 1355 Oxford St., Halifax, Nova Scotia B3H 4J1, Canada 2Alaska Department of Fish and Game, Division of Wildlife Conservation, 1300 College Road, Fairbanks, Alaska 99701, USA *E-mail: sara.iverson@dal.ca ABSTRACT: We determined the fat content and fatty acid composition of 26 species of fish and invertebrates (n = 1153) that are primary forage species of piscivorous seabirds and marine mammals in Prince William Sound (PWS), Alaska. Flatfish, shrimps and octopus had the lowest average fat contents (~1.0%), although some cods, as well as juvenile walleye pollock Theragra chalcogramma, Pacific herring Clupea harengus pallasi and pink salmon Oncorhynchus gorbuscha also ranged as low as 0.5 to 0.7% fat. The highest fat contents were found in eulachon Thaleichthys pacificus (25%), adult herring (21%) and the squid Berrytheuthis magister (5 to 13%). Within species, fat content varied mostly with season, but also with size. Fatty acid signatures generally distinguished forage species, with up to 95% of individuals correctly classified using either discriminant or classification and regression tree (CART) analyses. Discriminant plots provided insight into the relationships between fatty acid signatures of different species. Species with similar life histories and diets clustered closer together, while those with the greatest differences in ecology differed most in their fatty acid patterns. Within some species, changes in fatty acid signatures were apparent with increasing size and were consistent with known dietary shifts reported from stomach contents analyses. Furthermore, fatty acid signatures of Age 0 (yr) pollock and herring in PWS were consistent with previous stomach contents analysis that indicated annual differences in the timing of dietary changes from eating zooplankton to piscivory. Overall, when size/age classes were taken into account, species classification using fatty acid signatures was improved. Our findings have important implications for evaluating diets and food web interactions of fish stocks, as well as at higher trophic levels. Despite individual variation within species, our results indicate that fatty acid signatures accurately characterize forage species in this ecosystem, and consequently can be used to study and perhaps estimate the species composition of diets of their predators. KEY WORDS: Fatty acids · Forage fish · Food webs · Trophic interactions Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 241. Online publication date: October 04, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Prey selection and dietary overlap among zooplanktivorous jellyfish and juvenile fishes in Prince William Sound, Alaska

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 210:67-83 (2001) - doi:10.3354/meps210067 Prey selection and dietary overlap among zooplanktivorous jellyfish and juvenile fishes in Prince William Sound, Alaska Jennifer E. Purcell1,*, Molly V. Sturdevant2 1University of Maryland Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA 2NOAA/NMFS, Alaska Fisheries Science Center, Auke Bay Laboratory, 11305 Glacier Highway, Juneau, Alaska 99801, USA *Present address: Shannan Point Marine Center, 1900 Shannon Point Rd., Anacortes, Washington 98221, USA. E-mail: purcell@hpl.umces.edu ABSTRACT: The potential for competition for food among zooplanktivorous species of gelatinous zooplankton and fishes has been the subject of much speculation. Here, we evaluate the dietary overlaps among 2 scyphomedusan species, Aurelia labiata and Cyanea capillata, 1 ctenophore species, Pleurobrachia bachei, 1 hydromedusan species, Aequorea aequorea var. albida, and juveniles (Age 0) of 4 forage fish species, walleye pollock Theragra chalcogramma, Pacific sandlance Ammodytes hexapterus, Pacific herring Clupea pallasi, and pink salmon Oncorhynchus gorbuscha. Zooplankton samples collected in July-August 1995 to 1998 showed great similarity among years (86 to 96%), which allowed valid dietary comparisons between pelagic coelenterates and fishes collected in different years during that period. The predators fell into 2 groups, one that ate primarily crustacean prey (A. labiata, P. bachei, juvenile walleye pollock, sandlance, and herring), and another that ate mostly larvaceans (C. capillata, A. aequorea, and juvenile pink salmon). Species within the first group showed significant positive selection for crustacean prey and significant negative selection for larvaceans. The opposite trends were observed in the second group. Dietary overlaps were higher among crustacean-eating species (63 ± 13%) and larvacean-eating species (66 ± 10%) than when comparing crustacean- with larvacean-eating species (24 ± 14%). Dietary overlaps among pelagic coelenterate species (41 ± 21%) and among fish species (42 ± 25%) were similar, while overlaps between pelagic coelenterate and fish species averaged 50 ± 21%. These pelagic coelenterate and fish species were collected together in 45 purse seine sets taken in Prince William Sound in July 1999. We conclude that the diets of pelagic coelenterate and forage fish species overlap substantially, and that the species co-occur spatially and temporally. Therefore, the potential for competition for prey exists for these zooplanktivores in Prince William Sound. KEY WORDS: Aurelia · Competition · Ctenophore · Cyanea · Herring · Pollock · Salmon · Sandlance · Zooplankton Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 210. Online publication date: January 26, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.

Diel vertical migration and interaction of zooplankton and juvenile walleye pollock (Theragra chalcogramma) at a frontal region near the Pribilof Islands, Bering Sea

The diel vertical migration of age-0-walleye pollock (Theragra chalcogramma Pallas) and their principal prey organisms were examined at a productive frontal region in the Bering Sea using 38 kHz acoustic measurements and net samples. Small copepods dominated the catch of depth-stratified plankton tows. Two copepod species (Calanus marshallae and Metridia pacifica), euphausiids and chaetognaths, exhibited strong diel vertical migrations, although the magnitude and timing of the migrations varied among taxa. Age-0 pollock dominated midwater trawl catches (92% by number) that targeted layers of strong acoustic backscatter. Distributions of target strengths (TS) recorded within the layers corresponded well with predicted values based on empirical length/TS relationships for age-0 walleye pollock. Juvenile pollock in these layers migrated from daytime maxima at 40 m depths to less than 20 m at night. The proportion of large copepods, euphausiids, and chaetognaths in the diet of juvenile pollock increased with increasing fish size, but prey composition did not change significantly throughout the diel period. 2000 International Council for the Exploration of the Sea