Population estimates of humpback whales in the Gulf of the Farallones, California
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.
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.