GILLNET CATCHES AS AN ESTIMATE OF FISH ABUNDANCE - A COMPARISON BETWEEN VERTICAL GILLNET CATCHES AND HYDROACOUSTIC ABUNDANCES OF BALTIC SEAHERRING (CLUPEA-HARENGUS) AND SPRAT (SPRATTUS-SPRATTUS)

We compared the catches in a series of six vertical gill nets (6.25- t o 18.75-mm bar mesh) with estimates of fish abundance derived from hyd roacoustics. 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 w as significantly correlated with gillnet catches but explained only 28 % of the variance in catch per unit effort corrected for gillnet selec tivity and fish swimming speed. Visibility (measured as Secchi disk de pth) significantly decreased the likelihood of fish being caught in gi ll nets, while the effect of temperature was positive but nonsignifica nt. These data imply that gillnet catches will be a biased indicator o f changes in fish abundance over time or space, if visibility and poss ibly also temperature change as well.