Abundance and population structure of humpback whales in the North Pacific Basin
Calambokidis, J., G.H. Steiger, J.M. Straley, T.J. Quinn II, L.M. Herman, S. Cerchio, D.R. Salden, M. Yamaguchi, F. Sato, J. Urbán R., J. Jacobsen, O. von Ziegesar, K.C. Balcomb, C.M. Gabriele, M.E. Dahlheim, M. Higashi, S. Uchida, J.K.B. Ford, Y. Miyamura, P. Ladrón de Guevara P., S.A. Mizroch, L. Schlender and K. Rasmussen. 1997. Abundance and population structure of humpback whales in the North Pacific Basin. Report to Southwest Fisheries Science Center, National Marine Fisheries Service, La Jolla, California. 71pp.
This project examined the abundance and population structure of humpback whales in the North Pacific and was the result of the collaboration of researchers from 16 independent studies. Locations sampled included three wintering regions (Mexico, Hawaii and Japan) and feeding areas from California to the Aleutian Islands. Photographs taken between 1991 to 1993 (later expanded to include 1990 for Mexico) were selected because samples throughout the entire North Pacific were the largest and the most complete during this period. Of the 6,414 photographs, 3,650 were selected based on photographic quality for the sample used in the comparison (including photographs from Mexico from 1990). Two matching teams made independent comparisons of the entire collection to identify resightings of the same whale. Several methods to evaluate their success in finding photographic matches revealed that 93-99% of matches were correctly found. A total of 2,712 (2,993 without inter-regional duplicates) different individual whales were determined to be present in the sample.
The study provided new insights into the movements and population structure of humpback whales in the North Pacific. Although there was some interchange among the three wintering regions, it occurred only at a very low rate. While there was considerable mixing among the three subareas sampled off the Hawaiian Islands, interchange was more structured among the subareas in Mexico and Japan. In all three wintering regions, migrations from multiple feeding areas were documented. Whales identified at some feeding areas showed a clear preference for particular wintering regions (whales that fed off southeastern Alaska tended to migrate to Hawaii and whales that fed off California to Mexico) while at other feeding areas, animals tended to travel to multiple wintering areas. Whales identified off British Columbia, for example, showed a similar rate of interchange with all three wintering regions. Whales showed a strong site fidelity to specific feeding areas, although the near continuous distribution of whales along their feeding range and the limited sampling of most areas makes defining these feeding areas difficult.
Abundance estimates were determined using several geographically stratified capturerecapture models. Two models (Darroch and Hilborn) that incorporate migration rates among wintering areas, yielded estimates of approximately 6,000 humpback whales (4,000 for Hawaii, 1,600 for Mexico, and 400 for Japan). Probable sources of downward bias to these estimates included a skewed sex ratio towards males sampled on the wintering areas and other sources of heterogeneity of capture probabilities such as from geographic sampling bias. Alternate Petersen capture-recapture estimates of the abundances of humpback whales in the wintering areas using whales initially captured on the feeding areas yielded slightly higher estimates for Hawaii (5,200) and dramatically higher estimates for Mexico (4,200). The disparity in the estimates for Mexico are likely the result of the uneven sampling among the three subareas and the stratification of movements among them. The true abundance for Mexico may be between these values which would be consistent with the estimates of 2,200-2,800 from other studies using a larger time period (Urban et al. 1994, In prep.). The best estimate of the humpback whale population in the North Pacific using data from this study was 6,010 (SE=474) based on the average of the estimates from the Darroch method. Adjustments for the effects of sex bias and use of the alternate estimate for Mexico suggests that true abundance could be as much as 2,000 whales higher (to a total of 8,000). Expansion of this methodology to a more representative sampling of the North Pacific would improve this estimate. Nevertheless, this study shows that the North Pacific humpback whale population is well above the rough estimates of 1,400 that were made at the end of whaling in the 1960s.