A combined acoustic and trawl survey for efficiently estimating fish abundance

When fish are patchily distributed, unstratified trawl surveys give highly variable abundance estimates. If the patches are small and mobile, it is impractical to use pre-stratified or multi-stage adaptive designs to reduce variability. Based on recent trawl surveys for mackerel icefish Champsocephalus gunnari, we have modified the survey design and analysis to reduce estimation variance at minimal cost, using concomitant qualitative acoustic data. We have also developed new confidence interval procedures that should be accurate even for small samples. In computer simulations, the new method substantially outperformed a standard trawl survey.