The distribution and abundance of the humpback dolphin (Sousa chinensis) along the Natal coast, South Africa.

Abstract

Populations of the humpback dolphin in Natal, South Africa, are subject to increasing pressures including capture in the shark nets and habitat degradation, and concern has been raised about the status of the population. A minimum of 95 humpback dolphins were caught in the shark nets during the period from 1980 to 1992. Capture and sighting records of the Natal Sharks Board revealed a relatively high occurrence of humpback dolphins at Richards Bay. Elsewhere, in southern Natal, the infrequent sightings and captures were attributed to a seasonal occurrence of dolphins, possibly due to temporary movements away from resident areas. Sighting rates reported by the Natal Sharks Board has decreased by 55%from 1984-86 to 1990-92 and may reflect a decrease in the population. In a photo-identification study, searches took place in ten search areas in Natal. The sighting rates in the different areas revealed a relatively high density of humpback dolphins occurring in north central Natal, from the Tugela River to the St. Lucia estuary (including Richards Bay). This distribution correlated significantly with the turbidity of the water and the width of the inshore continental shelf, and was inversely related to the density of bottlenose dolphins. Within the northern Tugela Bank region, higher densities of dolphins were found surrounding the five river mouths and estuaries. The Natal population was estimated to be between 161 to 166 animals (95% confidence limits 134 to 229). The annual mortality due to shark net captures approximates 4,5%of the population. Various evidence, including a high mortality rate and a decrease in the annual sighting per unit effort reported by the Natal Sharks Board suggest that the humpback dolphin population in Natal is vulnerable and may be decreasing in size. A proposal is made to reduce the capture rate by relocating shark nets away from the Richards Bay harbour.