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Habitat and Marine Chemistry Program

Humpback Whales in Seymour Canal, Southeast Alaska: Numbers and Forage Base

Figure 2, humpback whale
Figure 2.  A humpback whale "flick feeding" in Seymour Canal.  With the breakdown of the thermocline in November, euphausiids were present at the surface simplifying prey identification.  Photo by John Moran.

 
 

As part of continuing research on the effect of predation on Pacific herring, an estimated 240 humpack whales were observed in November
(Fig. 2), with 210 photographically identified between Juneau and Seymour Canal. These observations are an extension of work in Sitka, Lynn Canal, and Prince William Sound to determine the forage base of humpback whales in fall and winter and whether they are impacting herring stocks, possibly to detrimental levels.

John Moran (ABL) in collaboration with Jan Straley of the University of Alaska Southeast completed a 10-day research cruise to Seymour Canal during mid-November 2009. The large concentration of humpback whales at this time of year is consistent with earlier work by Straley where well over 100 whales have been present in a 40-km stretch of Seymour Canal in November for a number of years.

These late-season humpbacks are not new to Seymour; Straley has been keeping track of them since 1979. Why are they there? Seymour Canal has high concentrations of euphausiids. Researchers with ABL's Habitat and Marine Chemistry Program are trying to understand why the area is so productive by looking at parameters such as oceanographic features, euphausiid energetics, and predator abundance.

Research from Seymour Canal will complement humpback whale foraging data collected in Prince William Sound, Lynn Canal, and Sitka Sound. In Prince William Sound and Lynn Canal, herring have been identified as the primary prey for humpbacks. Sitka Sound has both euphausiids and abundant herring stocks. Euphausiid-filled Seymour Canal provides some contrast on how these late season whales are using different foraging strategies to fuel up before migrating to lower latitude breeding areas.

Direct observations of whale predation are often difficult to assess, and prey type is often inferred from acoustic signal. However, recent analysis of fatty acid and stable isotope analysis from whale blubber and prey samples confirm our field observations—whales in Prince William Sound are feeding at a higher trophic level on herring.

The impact of whale predation on the struggling herring stocks of Prince William Sound and Lynn Canal is not trivial. For example, whales in Prince William Sound have the capacity to consume between 18% and 32% of the current spawning stock biomass between September and March; basically they have replaced a former commercial fishery.

No commercial fishery for herring has been permitted in Lynn Canal since the 1970s, and only two fisheries have been permitted in Prince William Sound in the last 17 years.

By John Moran
 

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