Estimates of daily ration for the South African (Cape) fur seal

Abstract

An increasing population size of South African (Cape) fur seals Arctocephalus pusillus pusillus has caused concern about increasing fish consumption by these marine predators. This has led to a controversy regarding possible competition between seals and commercial fisheries and, therefore, the need for accurate estimates of consumption rates by seals. Seven published allometric equations, considered applicable to South African fur seals of different ages, provide estimates of daily ration in terms of either biomass or energy ingestion. The energy ingestion equations are converted to equations of biomass ingestion using the calorific content of prey. The calorific contents of 14 of the major species in the diet were estimated using bomb calorimetry and ranged between 3,65 and 7,73 kJ·g−1. The various estimates of daily ration produced by the consumption equations are compared. The equation for biomass ingestion by growing marine mammals (Equation 23 of Innes et al. 1987) is, at present, considered the most appropriate single equation for all South African fur seals.