Robert J. Foy

Winter Feeding and Changes in Somatic Energy Content of Age-0 Pacific Herring in Prince William Sound, Alaska

Abstract During the winter of 1996–1997, somatic energy content and diets of age-0 Pacific herring Clupea pallasi from Prince William Sound, Alaska, were examined. From October to March, the standard length of age-0 recruits continued to increase, indicating either growth or mortality of smaller fish. The whole-body energy content averaged 4.4 kJ/g wet weight for fish captured in October and increased to 5.0 kJ/g wet weight in November. Thereafter, somatic energy continually declined, reaching 3.9 kJ/g wet weight in March. The December–March decline in nutritional status of the recruits verified that the energy derived from feeding was not enough to meet metabolic needs. The energy density of taxa found in the diets remained steady through the winter and increased in March. Total zooplankton biomass, however, decreased as the mean water temperature decreased. Estimated assimilation rates were lower for smaller fish than larger fish and decreased as winter progressed. We concluded that juvenile herring rel...

Spatial and Temporal Variability in Juvenile Pacific Herring, Clupea pallasi, Growth in Prince William Sound, Alaska

We examined the spatial and temporal variability of juvenile Pacific herring, Clupea pallasi, growth within Prince William Sound, Alaska. Pacific herring, ranging from post-larval to mature fish, were collected from four spatially segregated bays between October 1995 and March 1998. Linear growth equations from each bay were similar. However, growth rates and wet weight-at-length, reflecting condition, of juvenile Pacific herring cohorts varied seasonally and annually. The short term spatial variability in juvenile Pacific herring growth suggested that each bay was a unique nursery area. The physical and biological conditions within each bay appeared to dictate Pacific herring growth rate.

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

Abstract Body 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 b...

Temporal and Ontogenetic Variability in Trophic Role of Four Groundfish Species—Walleye Pollock, Pacific Cod, Arrowtooth Flounder, and Pacific Halibut—around Kodiak Island in the Gulf of Alaska

Abstract Changes in trophic position estimates of commercial fishery catches are used as an ecosystem-based indicator for sustainability, but often these estimates do not incorporate species-specific seasonal feeding dynamics and ontogenetic diet changes. Using stable isotope analysis, we obtained a fine-scale resolution of ontogenetic and temporal (interannual and seasonal) variations in the trophic roles of four commercially and ecologically important groundfish species in the Gulf of Alaska: walleye pollock Theragra chalcogramma, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and Pacific halibut Hippoglossus stenolepis. For each groundfish taxon, the nitrogen stable isotope signature (δ15N) increased with total length. In contrast, the lipid-normalized carbon stable isotope signature (δ13C′) significantly varied with size-class only for walleye pollock. There were species-specific differences in trophic position; adult Pacific cod fed at the highest trophic position, and wall...

Seasonal Differences in Prey Availability Around a Steller Sea Lion Haulout and Rookery in the Gulf of Alaska

Abundance and distribution of fish biomass were surveyed around a Steller sea lion (Eumetopias jubatus) haulout (nonbreeding) and rookery (breeding) site in the Gulf of Alaska to test the hypothesis that seasonal occupation of either site was related to the availability of prey. The haulout and rookery are located 30 nmi (55.56 km) apart at Long Island and Marmot Island in the Central Gulf of Alaska region where the Steller sea lion population is slowly recovering from a severe decline. Surveys conducted in May and November of 2002 (just before and after the breeding season) showed significantly higher prey energy density (total fish biomass density × energy content; kJ nmi) around the Long Island haulout than around the Marmot Island rookery. A survey conducted in July of 2002 (during breeding season) showed prey energy densities that were not significantly different between Long Island and Marmot Island but that were more concentrated in a single area by Marmot Island. Major prey species groups in all surveys were arrowtooth flounder, walleye pollock, cod, and soles; all are known prey of Steller sea lions in this area. Steller sea lion counts at Long Island during nonbreeding seasons from 2000 to 2004 correlated significantly with midwater prey energy densities. Steller sea lion counts at Marmot Island over the same period did not correlate with midwater prey energy densities in either breeding or nonbreeding seasons. The results of the study indicate that prey availability may be an important factor in the choice of haulout sites by Steller sea lions, and the higher prey availability at rookery sites provides some advantage.