Comparison of the abundance of pilchard in Cape gannet diet and commercial catches off the Western Cape, South Africa

Abstract

A hypothesis is presented that, in the absence of quota restrictions, commercial catches of pilchard Sardinops ocellatus in the western Cape and the abundance of pilchard in the diet of Cape gannet Morus capensis obtained at three South African colonies are directly related to pilchard biomass when the latter is low. There were significant correlations (p < 0,05) between the abundance of pilchard in gannet diet at Lambert's Bay and Malgas Island combined, and the total catch and CPUE at annual, though not monthly, time-scales. Trends in levels of correlation are discussed in relation to short-term variability, sampling intensity, statistical chance and the distribution of pilchard. Gannet diet and commercial catches reflect regional, but not local, changes in biomass. The 1983 pilchard year-class was relatively strong. Gannet diet is suggested to be a reliable monitor of the trend in pilchard biomass at low pilchard biomasses, a time at which direct surveys are least reliable.