Simulating the effects of side-aspect fish orientation on acoustic biomass estimates

Abstract

Abstract Boswell, K. M., Roth, B. M., and Cowan, Jr J. H. 2009. Simulating the effects of side-aspect fish orientation on acoustic biomass estimates. – ICES Journal of Marine Science, 66: 1398–1403. Simulation methods were used to investigate the effect of side-aspect fish orientation on estimates of fish biomass derived from acoustic data. Three population-level length distributions (n = 1000 individuals) were simulated (identical, uniform, and age class) and applied to a target strength (TS) function of fish length previously derived for Gulf menhaden (Brevoortia patronus). Systematic manipulation of the TS–length coefficients strongly affected the biomass estimates. Based on simulations, biomass estimates decreased markedly from the true side-aspect to head- or tail-on orientation, with estimates of population biomass decreasing by 80% when orientation deviated by as little as 30° from true side-aspect. Simulated data indicate that biomass estimates generated from surveys may be grossly inaccurate if fish orientation is not taken into account.