Acoustic study on the abundance and distribution of anchovy spawners and recruits in South African waters

Abstract

Direct surveys of Cape anchovy Engraulis capensis spawner and recruit biomass in South African coastal waters were made with a 38-kHz calibrated echo-sounder and custom-built digital echo-integrator. The spawning surveys were conducted each November between 1983 and 1985, predominantly on the Agulhas Bank. Recruit surveys were conducted in autumn 1985 and 1986, mainly on the West Coast. The spawning fish were widely distributed across the continental shelf, whereas recruit concentrations were highest close inshore. Employing a target strength per kilogramme expression for herring Clupea harengus, the spawning biomass in 1984 was estimated at 1,06 million tons and that in 1985 at 0,98 million tons. Similarly, recruit biomass in May/June 1985 and June 1986 was estimated at 310 and 466 thousand tons respectively. From an analysis of error in the latter two estimates, it was considered approximately 80 per cent certain that the recruitment in 1986 was greater than in 1985. The estimates of spawning biomass are 3–4 times higher than earlier estimates made by Virtual Population Analysis (VPA) of commercial catch data, and suggest that the stock is less seriously threatened by present catch levels than was indicated by VPA. It is concluded from the distribution and age structure of the spawning population that the VPA estimates are invalid because of the failure of commercial vessels to sample the major part of the adult population.