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

Abstract

Standardized studies of living sperm whales Physeter macrocephalus were carried out in 3 areas with very different recent whaling intensities. These were 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 about 4. 2 times higher than off the Seychelles and 2.4 times higher than off mainland Ecuador. Feeding success, as indicated by defecation rates, was significantly higher off the Seychelles than in the other 2 areas. Calving rates showed the same trend, but differences between areas were not significant. Length distributions of females and immatures were similar off the Galapagos and Seychelles but rather larger animals were found off mainland Ecuador. There were very low densities of large, mature males in all areas. Our results, with the exception of 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.

Coda vocalizations recorded in breeding areas are almost entirely produced by mature female sperm whales (Physeter macrocephalus)

“…Length measurement distributions (from 7.7 to 17.8 m) from photographs of sperm whales off the Galápagos from 1985 to 1987 (834 measurements; Waters and Whitehead 1990) and off Ecuador during 1991 (343 measurements; Kahn et al 1993) were compared with 2641 acoustic length measurements of coda producers from the same areas during the same periods (Fig. 2).…”

“…We then compared the length distributions of coda producers in groups off the Galápagos from 1985 to 1987 and off Ecuador during 1991 with photographic measurements of whales from the same groups during these studies Kahn et al 1993). The photographic measurement technique is that of Gordon (1990) and requires photographs being taken perpendicular to the whales that show its back and the horizon.…”

“…The photographic measurement technique is that of Gordon (1990) and requires photographs being taken perpendicular to the whales that show its back and the horizon. The analytical methods are described and the data summarized by Waters and Whitehead (1990) and Kahn et al (1993).…”

Geographical variation in the body size of adult female sperm whales (Physeter macrocephalus) – an example of McNab’s resource rule?

“…They concluded that females appeared to be smaller (by some 0.8 m) at every age when compared with other sperm whale populations (Ohsumi 1971, Gambell 1972. A nearly identical size distribution (8-11 m) was found for 79 females and immature sperm whales measured photogrammetrically off the Seychelles in 1990 (Kahn et al 1993).…”

Data on biology and exploitation of West Atlantic sperm whales, Physeter macrocephalus (Cetacea: Physeteridae) off the coast of Paraíba, Brazil

“…KAHN et al (1993) mention that during the last 40 years of sperm whaling the preference for catching mature males ended up changing the adult sex ratio in some areas (CLARKE et al 1980). It has been suggested that this change resulted in a disruption of the sperm whale's mating system, leading to a reduction in pregnancy rates, thus lessening the potential of the population for recovery (CLARKE et al 1980, MAY & BEDDINGTON 1980, WHITEHEAD 1987.…”