Top-Down Control of Pacific Herring by Predators in Lynn Canal, Southeast Alaska

As humpback whale populations recover they play an uncertain role in the structuring of marine ecosystems. We have observed seabirds and pinnipeds capitalizing on the foraging efforts of humpback whales during the winter months in Alaskan waters. The feeding methods used by humpback whales are known to disrupt the school structure of euphausiids and Pacific herring, making them available to other predators with limited diving abilities. These associations are persistent (months) and predictable (spatially and temporally) which likely facilitates a long-term foraging strategy for multiple species during winter months when herring are enriched in lipid content. We examined the association between humpback whales and the winter schooling behavior of herring in Lynn Canal, Alaska. We measured herring abundance, distribution and school structure through monthly hydroacoustic surveys between November and February during the winters of 2007-2008 and 2008-2009. We simultaneously estimated the number of whales present in the area for each month. Results suggest the spatial and temporal distribution of humpback whales are strongly associated with herring schools during the early stages of the winter in both years.  Humpback whale foraging activity correlated with dispersed schooling behavior of herring. During early winter when whales were more numerous, herring were spread over a larger area and were more dispersed throughout the water column. As winter progressed and humpback whales began their southerly migration out of the region, the spatial and vertical extent of herring decreased as they consolidated into dense schools occupying the deep channel habitats in Lynn Canal. This schooling behavior is presumed to be a metabolically beneficial strategy during winter months when prey is scarce and fish are maturing. We hypothesize that humpback whale foraging facilitates commensal predation which may have profound effects on winter survival of other herring predators. As humpback whale numbers increase, this may represent an important mechanism of top-down control on herring populations.