Gray whale photographic identification in 1998-2003: Collaborative research in the Pacific Northwest

A collaborative effort to photographically identify of gray whales in the waters of the Pacific Northwest from California through Alaska from late spring through fall was conducted from 1998-2003. This report summarizes these results and provides new insights about the movements, abundance and survival of gray whales in the Pacific Northwest. Each year between 1,159 and 1,499 photographic identifications of gray whales were obtained. Surveys were most numerous along the south and west coasts of Vancouver Island and just north of Vancouver Island, British Columbia. Using all identification photographs, 600 unique whales were identified. We focused our analysis on 477 gray whales identified after 1 June to exclude whales that were seen during the course of the northward migration to the Bering Sea. Individual whales were commonly photographed in more than one region of the Pacific Northwest during the same year and between years including movements from the southernmost sampled areas of California and northernmost areas around Kodiak, Alaska. Gray whales were most likely to be re-sighted in adjacent regions indicating fidelity at a scale smaller than the entire Pacific Northwest but larger than a single region for most whales. Assessing the degree and scale of site fidelity is further complicated by its dynamic and temporal nature.  Whales that were seen in more years were seen in more regions, so our ability to assess fidelity is limited by the timeframe of the observations.    
Abundance of gray whales in the Pacific Northwest and sub-regions was estimated with closed and open population capture-recapture models.  The well-known Petersen estimator for closed populations was used with adjacent years of photographs. The annual estimates for northern California to SE Alaska ranged from 261 to 298 and for Oregon to British Columbia (excluding Alaska and California), 197 to 256. Analysis of data collected from northern California to northern British Columbia (PCFA – Pacific Coast Feeding Aggregation) using open population models demonstrated a lack of geographic and demographic closure. Most whales seen for the first time were transients and were never seen again probably because they never returned (rather than mortality). Whales were more likely to return in a following year if they stayed for a longer time in their first year as measured by minimum residency tenure (MRT) (i.e., time between first and last dates photographed within a year). MRT was also a useful predictor for the probability that a “returning(resident)” whale would be seen the following year.  We propose that the mechanism for these relationships is related to foraging success or failure of whales.  Whales visiting this feeding area during and following the migration may join the feeding aggregation depending on the success they have in locating food. The average annual survival rate of resident (returning) whales was 0.97 (SE=0.012).  Annual abundance estimates of gray whales in the PCFA ranged from 200 to 225 during 2001-2003.  An analysis of data from Oregon to southern Vancouver Island yielded lower estimates of abundance for this smaller region from 137 to 153 during 2001-2003.

Citation:

Calambokidis, J., R. Lumper, J. Laake, M. Gosho, and P. Gearin. 2004. Gray whale photographic identification in 1998-2003: Collaborative research in the Pacific Northwest. Final report to National Marine Mammal Laboratory, Seattle, WA. Available from Cascadia Research (www.cascadiaresearch.org), 218½ W Fourth Ave., Olympia, WA  98501.  48pp