Stock assessment of pelagic three-spined stickleback ( Gasterosteus aculeatus): A simultaneous up- and down-looking echo-sounding study

Abstract

Hydroacoustics is a suitable method for estimating the stock size and spatial distribution of pelagic fish species with low fishing mortality. Pelagic three-spined stickleback ( Gasterosteus aculeatus) is such a species. Its spatial distribution and abundance were studied by hydroacoustics and trawling in the Gulf of Bothnia in August 1991. Upward and downward looking transducers were used in this round-the-clock survey. Sticklebacks were in the surface layers at night, and during the day 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 area (up to 14–28 t NM −2) and the lowest in the southern parts of the Bothnian Sea. The areal density of sticklebacks was lowest in the southern Bothnian Sea (124 × 10 3 specimens NM −2) and highest in the Bothnian Bay (3000 × 10 3 specimens NM −2). The estimated total biomass of three-spined stickleback was about 25 000 tons in the pelagic areas of the Gulf of Bothnia.