Original Articles

Population estimates for bottlenose dolphins Tursiops truncatus in Natal and Transkei waters


Abstract

Population estimates of bottlenose dolphins Tursiops truncatus on the Natal (five replicates) and Transkei (two replicates) coasts are made from aerial inshore strip-transects conducted between 23 April and 5 May 1985. Counts of dolphin schools and numbers were made in a 1 km wide strip-transect flown northwards along the water's edge, and a narrow transect flown southwards 600 m offshore. Three independent tests failed to detect any drop in sighting efficiency across this strip, although the power of the tests was low because of small sample sizes. Count variability between transects was high, probably reflecting movement of dolphins beyond the strip surveyed and failure to sight all the schools within the strip. The average count over a number of transects provides an index of population size for determination of future population trends, but this index was estimated with poor precision. Nevertheless, estimates of average density from the counts indicate that densities of bottlenose dolphins on the Natal South Coast are an order of magnitude less than those off the Natal North Coast and the coast of Transkei. The maximum counts on transects over areas "preferred" by bottlenose dolphins provide underestimates of absolute abundance (population censuses) of 433 and 219–249 for the Natal North and Natal South coasts respectively. Low estimates on the Natal South Coast may reflect offshore movements due to water turbidity. If this is not the case, available data on catches of dolphins in shark nets along the Natal coast suggest that present catches probably exceed the level that the South Coast population can sustain, and so would be causing this population to decline. The Natal South Coast should be accorded urgent priority for future studies on the species, and specific recommendations for such studies are made.

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