Acoustic and egg-production estimates of South African anchovy biomass over a decade: comparisons, accuracy, and utility

Abstract

The spawning biomass of South African anchovy (Engraulis capensis) has been estimated acoustically and by the daily egg-production method every year since 1984. Recruit biomass has been estimated acoustically every year since 1985. The estimates are evaluated, chiefly through comparisons between the acoustic and egg-production estimates. The mean of the acoustic spawner biomass estimates, obtained using a recently developed target-strength expression, agreed to within 10% of the egg-production mean, supporting the expression. The results are consistent with those from a population dynamics model which indicated relatively little bias in the acoustic estimates of spawner biomass compared to the corresponding recruit estimates, which the model indicated to be substantially negatively biased. The coefficient of variation in estimates of spawner biomass, obtained by scaling the current acoustic estimate by the average ratio between previous egg production and acoustic estimates, was typically 21%. It is considered that the value of further egg-production estimates in estimating anchovy biomass is diminishing, 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 described and their utility discussed. It is concluded that the survey results have reliably reflected the major changes in anchovy recruit and spawner biomass that have occurred between 1984 and 1994, and have provided valuable information for managing the fishery over this period.