Hypotheses concerning the decline and poor recovery of Pacific herring in Prince William Sound, Alaska

Abstract

This paper updates previous reviews of the 1993 stock decline of Pacific herring (Clupea pallasi) in Prince William Sound, Alaska, and focuses on hypotheses about subsequent poor recovery. Recent age structured assessment modeling with covariate analysis indicates that the population dynamics of the sound’s herring are influenced by oceanic factors, nutrition, and, most substantially, hatchery releases of juvenile pink salmon. For the 1993 decline, poor nutrition remains the most probable cause with disease a secondary response. Concerning poor recovery, we examined 16 potential factors and found three to be causal: oceanic factors, poor nutrition, and hatchery releases of juvenile pink salmon. Absences of strong year classes at both Sitka and Prince William Sound after 1993 indicate the action of large-scale ocean processes. Beyond regional-scale environmental factors, two factors specific to the sound influence the population dynamics of herring and are likely impeding recovery. First, pink salmon fry releases have increased to about 600 million annually and may disrupt feeding in young herring, which require adequate nutrition for growth and overwintering survival. Juvenile pink salmon and age-1 herring co-occur in nearshore areas of bays in late spring and summer, and available data on dietary overlap indicates potential competition between the age-1 juvenile herring and juvenile pink salmon. Field studies demonstrate that juvenile herring reduce food intake substantially in the presence of juvenile pink salmon. Second, overwintering humpback whales may consume potentially large amounts of adult herring, but further studies must confirm to what extent whale predation reduces herring biomass.