Population Structure of Female and Immature Sperm Whales (Physeter macrocephalus) off the Galápagos Islands

The structure of the population of female and immature sperm whales (Physeter macrocephalus) in the region of the Galapagos Islands was studied using individual photographic identifications of 1285 animals collected between 1985 and 1989. Population parameters were estimated using a maximum-likelihood mark–recapture estimate permitting emigration from the study area in which identifications are collected and then reimmigration back into it. Because permanent associations among whales violated assumptions of independence, confidence intervals for the estimates were constructed using Monte-Carlo population simulation. The analysis suggested that there is a population of very approximately 200 whales in the study area around the islands at any time. These were part of a larger population numbering between 2600 and 5300 individuals (95% confidence interval). An average of 39–94% (95% confidence interval) of the whales left the study area in any month, with a similar number immigrating.