Winter variability in the diets of groundfish inhabiting a subarctic sound with a focus on Pacific herring and walleye pollock piscivory

Piscivorous groundfish have been implicated as a major hindrance to Pacific herring (Clupea pallasii) population recovery within Prince William Sound, Alaska (PWS); however, the key piscivores have not been identified. To address this knowledge gap, groundfish were collected from PWS herring nursery fjords (2009-2012) during winter, a period when juvenile herring energy levels decrease, suppressing predator avoidance. Three species, Pacific cod (Gadus macrocephalus), walleye pollock (G. chalcogrammus), and big skate (Beringraja binoculata) were identified as key herring consumers based on diet frequency of occurrence. Their stomach contents were examined to document any variability in prey occurrences and to explore whether pollock or other prey types could mediate herring consumption. For Pacific cod, there was strong evidence of seasonality in herring consumption, with herring occurring in 7% and 50% of stomachs in November and March, respectively. Walleye pollock and big skate diets were more influenced by body size and longitude, respectively. Pacific cod consumed notably more adult herring (ages 3+) than the other piscivores, big skate consumed herring ages 3+ and younger, while walleye pollock was a major consumer of age 0 herring. As groundfish body size increased, all three species consumed larger, age 2+ pollock, suggesting pollock presence mediates herring overwintering mortality and that pollock cannibalism is an important dietary pathway in PWS. By identifying the key groundfish consumers of herring and pollock in PWS, and the spatial, temporal, and biological variables that influence the occurrence of these forage fishes in their diets, this research contributes to a larger body of knowledge seeking to explain the mechanisms responsible for their lack of recovery in PWS and offers novel insight into the importance of pollock as a forage fish in PWS.