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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, l 7 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.

In this paper the hypothesis is put forward that Baltic herring (Clupea harengus membras) spawning time, spring or autumn, is determined by feeding conditions during the adult phase and thus not genetically fixed. The present "absence" of autumn spawners is thought to be the result of improved feeding conditions during the latest decades as a result of the eutrophication of the Baltic Sea. During two spawning ground studies carried out in 1978 and 1982 unusually high mortality rates were noted for eggs in situ. In 1982, during 4 wk close to peak of spawning, the mortality increased substantially, especially for eggs among filamentous algae. A significant difference was noted between eggs on coarser algae and those among filamentous algae (p < 0.001). During this period the average mortalities were 33 and 75%, respectively. Very low levels of oxygen were measured at night among the filamentous algae. An increase in the amounts of this type of algae as a response to the eutrophication might constitute a new hazard to the reproductive success of the Baltic herring.

Three‐spined sticklebacks from the northern Baltic proper, caught in 1968 and 1969, were examined with regard to number of plates, body length, head length in relation to body length, and the percental proportions of the forms trachurus, leiurus (=gymnurus), and semiarmatus. A total number of 1791 specimens were examined. It is shown that the number of asymmetric specimens, regarding the number of plates on the right and left side of the body, is higher than numbers shown earlier, and that there is a significant difference in relative head length between females and males, and, that smaller fish displayed proportionally longer heads than adults. Semiarmatus specimens accounted for about 80 per cent of the samples and only 2 leiurus specimens were found.

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