STOCK ASSESSMENT OF PELAGIC 3-SPINED STICKLEBACK (GASTEROSTEUS-ACULEATUS) - A SIMULTANEOUS UP-LOOKING AND DOWN-LOOKING ECHO-SOUNDING STUDY

Hydroacoustics is a suitable method for estimating the stock size and spatial distribution of pelagic fish species with low fishing mortalit y. Pelagic three-spined stickleback (Gasterosteus aculeatus) is such a species. Its spatial distribution and abundance were studied by hydro acoustics and trawling in the Gulf of Bothnia in August 1991, Upward a nd downward looking transducers were used in this round-the-clock surv ey. Sticklebacks were in the surface layers at night, and during the d ay they migrated down to 10-20 m depth. The thermocline was at a depth of 13 m. Sticklebacks had a very patchy horizontal distribution, with the highest biomass densities being found just north of the Quark are a (up to 14-28 t NM(-2)) and the lowest in the southern parts of the B othnian Sea. The areal density of sticklebacks was lowest in the south ern Bothnian Sea (124 x 10(3) specimens NM(-2)) and highest in the Bot hnian Bay (3000 x 10(3) specimens NM(-2)). The estimated total biomass of three-spined stickleback was about 25 000 tons in the pelagic area s of the Gulf of Bothnia.