Abundance and population structure of seasonal gray whales in the Pacific Northwest, 1998-2008

The existence of a small number of eastern North Pacific gray whales that spend the spring, summer and fall feeding in coastal waters of the Pacific Northwest has been known for some time and localized and short-term studies have examined aspects of the natural history of these animals. We report the results of an 11-year (1998-2008) collaborative study examining the abundance and the population structure of these animals conducted over a number of regions from Northern California to British Columbia using photographic identification. Some 12,679 identifications representing 872 unique gray whales were obtained. Gray whales seen after 1 June (after the northward migration) were more likely to be seen repeatedly and in multiple regions and years and 1 June was used as the seasonal start date for the data included in the abundance estimates. Gray whales using the Pacific Northwest in summer and fall include two groups: 1) whales that return frequently and account for the majority of the sightings and 2) apparent stragglers from the migration seen in only one year, generally for shorter periods and in more limited areas. Abundance estimates for whales present in summer and fall using three different methods and different geographic scales revealed the abundance of animals to be at most a few hundred individuals. The proportion of calves documented was generally low but varied dramatically among years and may have been biased downward by weaning of calves prior to much of the seasonal effort. Observations of calves returning to the Pacific Northwest in subsequent years documents one possible mechanism for recruitment. The results we present will be valuable in assessing the impacts of potential resumption of a gray whale hunt by the Makah Tribe, currently proposed to target migrating whales by hunting prior to 1 June.

Citation:

Calambokidis, J., J.L. Laake, A. Klimek. 2010. Abundance and population structure of seasonal gray whales in the Pacific Northwest, 1998-2008. Report SC/62/BRG32 submitted to IWC Scientific Committee, June 2010. 50 pp.