MEPS 152:261-271 (1997)  -  doi:10.3354/meps152261

The brackish water of the Baltic Sea, with decreasing salinity from the west to the east and from the south to the north, is a borderline area for cod Gadus morhua. The major stock, the eastern Baltic cod, has decreased substantially during the last decade. A discussion concerning the possibility of immigration of cod and the potential of stock interactions has evolved. The present study was conducted in order to elucidate if differences in salinity requirements for successful spawning exist between the eastern Baltic cod and the western Belt Sea cod, and if adaptation to ambient salinity is possible. Activation of the spermatozoa occurred at >=11 to 12 psu (practical salinity units) for Baltic cod and at >=15 to 16 psu for Belt Sea cod. Neutral egg buoyancy was obtained at 14.5 ± 1.2 psu and at 20 to 22 psu respectively. Transfer of fish from marine to brackish water conditions showed that these characteristics remained essentially the same; activation of spermatozoa occurred at >=14 to 15 psu and neutral egg buoyancy at 19 to 21 psu. The results suggest that these characteristics are specific to populations. Regarding hydrographic conditions in the spawning areas of cod in the Baltic, the results suggest that stock interactions may be possible in the western Baltic spawning areas where salinity requirements for both stocks are fulfilled, but not in the eastern spawning areas as low salinity prevents successful spawning of Belt Sea cod. Salinity and oxygen conditions in the Baltic vary with highly irregular saline water inflows. Periods of stagnation without inflows may, due to decreasing salinity, e.g. <15 psu during the last stagnation period, act as an ecological barrier separating the stocks.

Gadus morhua · Stock discrimination · Salinity · Spawning · Spermatozoa · Fertilization · Egg buoyancy · Baltic Sea