Spatial and temporal distribution of the demersal fish fauna in a Baltic archipelago as estimated by SCUBA census

Abstract

ABSTRACT: A quantitative investigation of the demersal fish fauna of a 160 km2 archipelago area in the northern Baltic proper was carried out by SCUBA census technique. Thirty-four stations covering seaweed areas, shallow soft bottoms with seagrass and pond weeds, and deeper, naked soft bottoms down to a depth of 21 m were visited at all seasons. The results are compared with those obtained by traditional gill-net fishing. The dominating species are the gobiids (particularly Pornatoschistus rninutus) which make up 75 % of the total fish fauna but only 8.4 % of the total biomass. Zoarces viviparus, Cottus gobio and Platichtys flesus are common elements, with P. flesus constituting more than half of the biomass. Low abundance of all species except Z. viviparus is found in March-April, gobies having a maximum in September-October and P. flesus in November. Spatially, P. rninutus shows the widest vertical range being about equally distributed between surface and 20 m depth. C. gobio aggregates in the upper 10 m. The Mytilus bottoms and the deeper soft bottoms are the most populated areas. The former is characterized by Gobius niger, Z. viviparus and Pholis gunnellus which use the shelter offered by the numerous boulders and stones. The latter is totally dominated by P. minutus. Annual mean values for the total demersal fish fauna are 2.1 ind m-2 and 4.3 g m-2 fresh weight. The maximum biomass value, l7 g m-* fresh weight, was measured on very exposed Mytilus bottoms. Comparisons with other geographical areas such as shallow soft bottoms in the North Sea, seagrass beds in the northeastern USA and Texas and Spartina marsh ponds in North Carolina show these figures to be surprisingly high.