Trends in harvests and pup numbers of the South African fur seal: implications for management

Abstract

Management action for the South African fur seal Arctocephalus pusillus pusillus varied considerably in the 1980s, as did numbers of pups, bulls and cows killed, their products (pelts, bull genitalia, meat and oil) and product prices. Since 1900, 2,7 million seals have been killed, the majority in Namibia. Most of these (92%) were pups. Numbers of pups have fluctuated, but-the average annual rate of growth has been approximately 3 per cent since 1972. Approximately two-thirds of the present seal population is on the Namibian coastline, and the remainder on the South African coast. In 1985, there was a large decrease in pup numbers, which was partially attributable to disturbance effects associated with bull-sealing, and to the removal of cows in preceding years. Estimated rates of sealing over the past two decades show that the rate at which pups have been killed has decreased from above 40 per cent to a few per cent, whereas that of bulls has increased but has not exceeded 8 per cent. The rate at which cows have been killed is negligible. Simulation modelling shows that a population : pup ratio used to provide rough estimates of total population size from pup numbers should be over 5:1, higher than the 4:1 or 4,49:1 previously used. In addition, variability in annual trends in both pup numbers and population size is much greater if there is disturbance associated with sealing. The analyses in this paper provide two recommendations for consideration by managers of the South African fur seal stock. First, a long-term management plan needs to be adopted and adhered to. Second, bull-sealing, if it involves disturbance at the colony, makes determination of the real extent of reduction of pup production difficult. Managers should be aware of this.