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Habitat Assessment & Marine Chemistry Program

Significance of Humpback Whale Predation on Over-Wintering Pacific Herring in Prince William Sound and Southern Lynn Canal

Figure 1, euphausids and humpback whale
Figure 2.  Euphausids dimple the water’s surface attempting to escape from a humpback whale.  A day later the same whale switched prey and fed on herring.  Photo by John Moran
 

Two stocks of Pacific herring (Clupea pallasii), one in Prince William Sound (PWS) and the other in southern Lynn Canal, have been struggling in population numbers since 1982. To survive winter months when little prey is available, Pacific herring lower their energy requirements by seeking out bays and trenches with cooler water and little current. On these over-wintering grounds, herring form large, dense aggregations vulnerable to predation. The spatial and temporal consistency of these herring schools provides a predictable, high energy food resource for marine mammals such as humpback whales (Megaptera novaeagliae) (Fig. 2) and Steller sea lions (Eumetopias jubatus).

In Prince William Sound, herring have been classified as “not recovered” by the Exxon Valdez Oil Spill Trustee Council. The PWS herring fishery was thriving prior to the 1989 oil spill. The fishery closed from 1993 to 1997, opened for 2 years, and has remained closed since 1999. The fishery openings produced little in the way of catch. The Lynn Canal herring stock has failed to recover despite closure of the commercial herring fishery since 1982. Currently the Lynn Canal stock is being evaluated for listing under the Endangered Species Act. In both of these regions, humpback whales, often in signficant numbers, have been observed feeding heavily on herring during fall and winter.

The Habitat and Marine Chemistry Program, in cooperation with the University of Alaska Southeast, University of Alaska Fairbanks, and the ADF&G, is investigating humpback whale predation as a possible factor in the failed recovery of the two herring stocks. We are quantifying the impact of whale predation on recovery of these herring stocks through a series of semimonthly boat-based surveys and quarterly aerial surveys.

Survey efforts include counting humpback whales and estimating abundance using photographic identification and determining prey through visual observations, trawling, and acoustics. The aerial surveys identify bays where there are focal groups of whales and are used to determine subsequent boat survey locations. We are estimating herring biomass using hydroacoustics during winter.

Ultimately, we will use whale abundance estimates in a bioenergetic model to determine numbers of herring consumed (and energy content consumed). The estimated numbers of herring consumed will be incorporated into an age-structured model to assess the significance of whale predation on herring recovery.

To date we have completed our first winter of field work (2006-07) focusing on Sawmill Bay in PWS and southern Lynn Canal (herring sampling and assessments; whale photoidentification). Next year, we will repeat our sampling effort and will expand our efforts into Sitka Sound. Comparisons with Sitka Sound will provide a contrast of a healthy herring stock experiencing whale predation with the depleted stocks of Lynn Canal and PWS. Portions of this research are funded by two projects funded by the Exxon Valdez Trustee Council.

By John Moran
 

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