Density-dependent habitat selection and the modeling of sperm whale (Physeter macrocephalus) exploitation

The monitoring and management of sperm whale (Physeter macrocephalus) popul ations have proved problematic. Studies of living animals indicate that mov ements are largely determined by resource availability, thus suggesting tha t density-dependent habitat selection may be a realistic framework within w hich to study sperm whale populations. A model, in which animals migrate be tween 2 + 2 degrees squares at rates that depend on relative resource avail ability, was used to examine the effects of whaling on measures of sperm wh ale abundance. The model simulated four types of whaling: shore-based whali ng, pelagic open-boat whaling by many boats, pelagic whaling by a fleet bas ed around one factory ship, and pelagic whaling by a fleet sequentially exp loiting different parts of the study area. Catch per unit effort was found to have little relationship with population size in any part of the study a rea for shore-based whaling and for pelagic whaling when the study area was sequentially exploited. Thus, in these circumstances, catch per unit effor t should not be used as a measure of depletion. To give a reasonable assess ment of depletion, visual or acoustic surveys must extend well beyond the a reas being exploited.