Reproductive Rates of Humpback Whales off California
Steiger, G.H. and J. Calambokidis. 2000. Reproductive rates of humpback whales off California. Marine Mammal Science 16:220-239.
From 1986 to 1996 we examined the reproductive rates, calving rates, and reproductive histories of mature females as part of photo-identification studies of humpback whales that feed off California, Oregon, and Washington during summer and fall. Annual reproductive rates were measured by two methods: proportion of all whales that were calves based on sightings (0.6% - 5.9% per year, mean = 3.6 %, SD =1.6) and based on individually identified animals (1.1% - 8.0% per year, mean = 4.1%, SD = 1.8). The reproductive rate based on sightings varied significantly by year (G test, P < 0.001), region (G test, P < 0.001), and by month (G test, P < 0.05). Seventy-nine sexually mature females were identified with 97 calves out of a total of 844 known individuals over the 11 yr study. Mother-calf separation on the feeding grounds was recorded in several instances. The apparent reproductive rates of this population are considerably lower than rates 4%-15% reported from other feeding areas for this species. Our estimates are likely biased downward because this population has been increasing at about 5% per year. Calves may have been missed due to early weaning and because of our sampling from small boats late in the season. We also found evidence of geographic segregation of mother-calf pairs within our large study area. Despite these factors, we conclude the reproductive rate of this population appears to be lower than has been reported in other areas.