HighlightsWe modelled the surface water salinity in the Baltic from the 1960s to 2100.We studied possible changes in distribution areas of predominant plant, invertebrate and fish species.The results suggest a critical shift in the salinity range 5–7, which is a bottleneck for both marine and freshwater species distribution and diversity.This foreseen salinity change is likely to have large impacts on marine ecology, it́s monitoring, modelling as well as fisheries.

Two methods of determining the age of herring (Clupea harengus L.) from the Bothnian Sea, in the northern Baltic Sea, were compared. Ages were estimated from whole otoliths, which is a routine method for herring age determination, and from neutral red stained otolith cross sections. There was disagreement between the methods, especially in specimens with more than five annual rings. The agreement between the determinations of different age readers was better with the cross section method and, besides, it is clear that with the whole otolith method there is considerable negative bias in old fish. Thus, more herring were classified as more than 15 years old with the cross section method. The study indicates that conventional methodology may generate considerable errors in age distributions, especially in samples which mainly consist of old fish. However, the differences in age reading produced relatively small changes in growth rate estimates, because of the slow growth of the old fish, that are most prone to age determination errors.

The environmental factors affecting the deposition and mortality of the eggs of the springspawning Baltic herring were studied in the inner Archipelago Sea of south-western Finland. On four spawning grounds, 27 study squares (area 1 m') were surveyed by divers. In each square, one quantitative egg sample was taken and the following data were recorded: depth, temperature, bottom quality of the square and under the eggs, egg substrate, cover (%) of plants and/or the bivalve Myrilus edulis, and the heterogeneity of theenvironment, expressed as the total number of all bottom materials, plants and Mytilus found in the square.Eggs were found in the depth zone of I 4 m. Their density ranged from 6OOO to 2.3 millions of eggs per square metre. Egg number had a significant negative correlation with both depth ( r = -0.545, P

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