Population Estimates of Humpback Whales in the Gulf of the Farallones, California

We examined population estimates of humpback whales (Megaptera novaeangliae) based on photo-identification and aerial surveys in the Gulf of the Farallones, California. Population estimates for 1986-8 were made within years, between years and among years; methods used to estimate population size included the total number of individuals, the rate of discovery, mark-recapture estimators (Petersen estimator with the Chapman and Bailey modifications and Jolly-Seber method) and aerial line-transect calculations. We found a number of violations of mark-recapture assumptions in our data; the most serious was the lack ofa defined closed population. Heterogeneity of capture probability was observed, particularly in the occurrence of a sub-group of whales that was more likely than others to return to the region. Assuming that within-year estimates were accurate, 22% and 17% of whales present in 1986 and 1987, respectively, did not return the following year; 54% and 35% of whales present in 1987 and 1988, respectively, had not been present the previous year. All abundance estimates based on photo-Identification, including the number of individuals identified, were higher than estimates from aerial line-transects even when an attempt was made to correct the latter for whales submerged and undetected. Estimates of abundance based on between-year samples at a feeding area may be seriously biased unless whales in the entire region are sampled randomly.

Citation:

Calambokidis, J., J.C. Cubbage, G.H. Steiger, K.C. Balcomb, and P. Bloedel. 1990. Population Estimates of Humpback Whales in the Gulf of the Farallones, California. Report to the International Whaling Commission (special issue 12): 325-333.