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

The monitoring and management of sperm whale (Physeter macrocephalus) populations have proved problematic. Studies of living animals indicate that movements are largely determined by resource availability, thus suggesting that density-dependent habitat selection may be a realistic framework within which to study sperm whale populations. A model, in which animals migrate between 2 x 2°squares at rates that depend on relative resource availability, was used to examine the effects of whaling on measures of sperm whale abundance. The model simulated four types of whaling: shore-based whaling, pelagic open-boat whaling by many boats, pelagic whaling by a fleet based around one factory ship, and pelagic whaling by a fleet sequentially exploiting 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 area for shore-based whaling and for pelagic whaling when the study area was sequentially exploited. Thus, in these circumstances, catch per unit effort should not be used as a measure of depletion. To give a reasonable assessment of depletion, visual or acoustic surveys must extend well beyond the areas being exploited.