Indications of density-dependent effects from comparisons of sperm whale populations

Abstract

Physeter macrocephalus were studied in three areas with very different recent whaling intensities: off the Galapagos Islands, where there seems to have been little recent whaling; off the Seychelles and Amirantes Islands, where animals were regularly caught by passing Soviet whalers; and off mainland Ecuador, adjacent to the area whaled very heavily from Paita, Peru. Sperm whale densities off the Galapagos were 4.2 times higher than off the Seychelles and 2.4 times higher than off mainland Ecuador. Feeding success was significantly higher off the Seychelles. Calving rates showed the same trend, but differences were not significant. Length distributions of females and immatures were similar off the Galapagos and Seychelles but larger animals were found off mainland Ecuador. There were very low densities of large, mature males in all areas. Results, apart from the low feeding success and calving rate found off mainland Ecuador, are consistent with general models of density-dependent processes. The discrepancies may be due to migration of whales to mainland Ecuador from less exploited regions. -Authors