ACOUSTIC AND EGG-PRODUCTION ESTIMATES OF SOUTH-AFRICAN ANCHOVY BIOMASS OVER A DECADE - COMPARISONS, ACCURACY, AND UTILITY

The spawning biomass of South African anchovy (Engraulis capensis) has been estimated acoustically and by the daily egg-production method ev ery year since 1984. Recruit biomass has been estimated acoustically e very year since 1985. The estimates are evaluated, chiefly through com parisons between the acoustic and egg-production estimates. The mean o f the acoustic spawner biomass estimates, obtained using a recently de veloped target-strength expression, agreed to within 10% of the egg-pr oduction mean, supporting the expression. The results are consistent w ith those from a population dynamics model which indicated relatively little bias in the acoustic estimates of spawner biomass compared to t he corresponding recruit estimates, which the model indicated to be su bstantially negatively biased. The coefficient of variation in estimat es of spawner biomass, obtained by scaling the current acoustic estima te by the average ratio between previous egg production and acoustic e stimates, was typically 21%. It is considered that the value of furthe r egg-production estimates in estimating anchovy biomass is diminishin g, and that effort should be concentrated on improving the accuracy of direct acoustic estimates. Ways in which the different estimates have been used to manage the fishery over the past 10 years are briefly de scribed and their utility discussed. It is concluded that the survey r esults have reliably reflected the major changes in anchovy recruit an d spawner biomass that have occurred between 1984 and 1994, and have p rovided valuable information for managing the fishery over this period . (C) 1996 International Council for the Exploration of the Sea.