Abundance of blue whales off the US West Coast using photo identification

Blue whales (Balaenoptera musculus) are considered endangered and their populations were depleted by whaling throughout most of their range. Blue whales feed off California from May through November (Dohl et al. 1983) and migrate to waters off Mexico and Central America in winter and spring (Calambokidis et al. 1990, Stafford et al. 1999, Mate et al. 1999, Chandler et al. 1999). Photographic identification of blue whales has revealed that animals identified off California have been seen as far north as the Queen Charlotte Islands (Calambokidis et al. 2004) and the Gulf of Alaska and as far south as the Costa Rica Dome (Chandler et al. 1999).

Blue whales occur in both coastal and offshore waters of the US West Coast. Accurate estimates of blue whale abundance off California using capture-recapture methods have only been possible when at least one representative sample is obtained from surveys systematically covering both inshore and offshore waters (Calambokidis and Barlow 2004). This is because a portion of the blue whale population feeds offshore beyond the reach of small-boat surveys. Although there is interchange between inshore and offshore areas, there is not complete mixing. To avoid heterogeneity of capture probabilities which would bias capture-recapture estimates, past estimates have relied on at least one sample coming from identifications obtained during systematic ship surveys covering both coastal and offshore waters, similar to that obtained by Southwest Fisheries Science Center (SWFSC) on the Collaborative Survey of Cetacean Abundance and the Pelagic Ecosystem (CSCAPE) cruise in 2005 (Forney 2007). 


Calambokidis, J., A Douglas, E Falcone, and L Schlender.  2007. Abundance of blue whales off the US West Coast using photo identification. Final report to SWFSC. 13pp.  Available from www.cascadiaresearch.org

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