Juvenile Pacific herring (Clupea pallasi) feeding ecology in Prince William Sound, Alaska

Abstract

Pacific herring (Clupea pallasi) are commercially exploited along the Asiatic and North American Pacific Ocean continental shelves. In Prince William Sound (PWS), Alaska, herring were commercially important until a year class failures in 1993. A noticeable lack of life history information on juveniles was available in PWS to use for studies addressing the failed recruitment. This study describes the seasonal herring feeding ecology in PWS nursery areas from 1996 to 1998. Zooplankton from 535 vertical tows and herring diet data from 3,282 stomach contents were collected from Eaglek, Simpson, Whale and Zaikof Bays. Zooplankton species composition was dominated by small calanoid copepods, cyclopoids, invertebrate eggs, and adult euphausiids in March prior to the spring phytoplankton bloom. Small calanoid copepods, especially Pseudocalanus spp., were dominant during the peak abundance. Oikopleurans were abundant from August to October. The zooplankton density peaked at 1,234 to 5,594 individuals m-3 between June and July 1996. Zooplankton density was significantly lower in 1997 than 1996. Seasonal density and diversity were found to vary among and within the four bays. The abundance of prey in herring diets was correlated to the timing and degree of zooplankton prey availability. Feeding was highest at 1,192 items per fish in July 1996 and decreased until winter (December to March) when the number of empty stomachs ranged from 70 to 90 %. Lower zooplankton densities in 1997 were reflected in significantly lower abundances of prey in 1997 diets. Prey selectivity was negatively correlated with zooplankton densities among months. Diel and ontogenetic feeding trends as well as differences between feeding depths were noted. Assimilation rates of smaller herring were closer to basal metabolic rates and herring less than 3 g had insufficient energy reserves to survive the winters of 1995-1996 and 1996-1997. These patterns suggest that juvenile herring are dependent on an abundance of prey to successfully feed and have enough energy reserves to overwinter. The effects of increased temperatures on zooplankton fluctuations and changes in herring condition may have had population level consequences in PWS. Successful feeding when prey abundance and composition was highly variable reveals herring’s adaptability to multiple environments.