Reproductive success in relation to salinity for three flatfish species, dab (Limanda limanda), plaice (Pleuronectes platessa), and flounder (Pleuronectes flesus), in the brackish water Baltic Sea

The reproductive success and thus abundance and distribution of dab, plaice, and flounder in the Baltic Sea, a large brackish water area, is restricted by salinity. By measuring spermatozoa mobility and fertilisation rates at different salinities and determining the salinity at which eggs are neutrally buoyant, the salinity requirements for successful egg development were assessed. The results were used for the evaluation of potential spawning areas and for stock discrimination by analyses of differences in the salinity requirements of fish from different areas (ICES Subdivisions (SD) 23 28). The results suggest that there are two stocks of dab and successful reproduction may occur in the Sound (SD 23) and, occasionally, in the Arkona and Bornholm basins (SD 24 and SD 25). Opportunities for successful reproduction of plaice exist regularly in the Arkona and Bornholm basins and occasionally in the Gdansk and Gotland basins (SD 26 and SD 28). No differences in salinity requirements for fish from SD 24-28 suggest one stock of plaice in the Baltic proper. There are two different types of flounder, one with demersal eggs and the other with pelagic eggs. The former, constituting one distinct stock, may reproduce successfully as far north as the Bothnian Sea and the Gulf of Finland (SD 30 and SD 32), up to the 6 psu isohaline. For flounder with pelagic eggs, opportunities for the eggs to obtain neutral buoyancy suggest that successful reproduction may occur regularly in the Sound, the Arkona, and Bornholm basins as well as in the Gdansk and Gotland basins, and that there are three stocks of flounder with pelagic eggs. (C) 2002 International Council for the Exploration of the Sea.