Considering predation by humpback whales in the mystery of the failed recovery of Pacific herring in Prince William Sound, Alaska

Abstract

In Prince William Sound (PWS) in the early 1990s, biomass of an important forage species, Pacific herring (Clupea pallasii), crashed and has since failed to recover. This thesis explores the role that predation by humpback whales (Megaptera novaeangliae) may have in limiting this recovery. This was done by modeling the historic (1980-2009) relationship between these two species using existing data. Mark-recapture estimates of abundance were combined to form the first time series of humpback whale abundance for this area. This time series was then integrated into the age-structured assessment model used by Alaska Department of Fish and Game to manage herring biomass in PWS by adding annual estimates of humpback whale abundance as a covariate to herring mortality. The current (2009) humpback whale population estimate was 191 (SE 32) representing a rate of increase of 4.5% annually. Humpback whale abundance helped to explain variation in mortality of older (ages 5+) herring only, with humpback whales accounting for as much as one-third of all mortality in these ages. However, retrospective analyses of the updated model show that recruitment, and not predation on adult herring, is the most influential factor limiting the recovery of herring in PWS.