Gillnet catches as an estimate of fish abundance: a comparison between vertical gillnet catches and hydroacoustic abundances of Baltic Sea herring (Clupea harengus) and sprat (Sptattus sptattus)

Abstract

We compared the catches in a series of six vertical gill nets (6.25- to 18.75-mm bar mesh) with estimates of fish abundance derived from hydroacoustics. A total of 47 combined acoustic and gillnet surveys were collected between July and November 1987–1992 in archipelago areas of the northern Baltic Sea. Fish abundance determined by hydroacoustics was significantly correlated with gillnet catches but explained only 28% of the variance in catch per unit effort corrected for gillnet selectivity and fish swimming speed. Visibility (measured as Secchi disk depth) significantly decreased the likelihood of fish being caught in gill nets, while the effect of temperature was positive but nonsignificant. These data imply that gillnet catches will be a biased indicator of changes in fish abundance over time or space, if visibility and possibly also temperature change as well.