VARIATION IN EGG BUOYANCY OF BALTIC COD GADUS-MORHUA AND ITS IMPLICATIONS FOR EGG SURVIVAL IN PREVAILING CONDITIONS IN THE BALTIC SEA

In the Baltic Sea successful spawning of cod is restricted to the deep basins, and egg buoyancy is regarded as a major factor that allows eg gs to avoid the stressful oxygen conditions that often prevail in the deep layers. In this study eggs of 3 to 5 yr old spawners (the basis o f the spawning stock of Baltic cod Gadus morhua L. nowadays) maintaine d neutral buoyancy at a salinity of 14.5 +/- 1.2 ppt, with great varia tion among eggs from different females. Egg buoyancy was significantly correlated with yolk osmolality and chorion thickness and weakly corr elated with egg size. Measurements of egg buoyancy in successive batch es from the same female indicated that egg buoyancy increased with bat ch number. The significance of neutral buoyancy for cod egg survival i n prevailing conditions in the Baltic was evaluated by comparing the a bility of eggs to achieve neutral buoyancy in the Baltic cod spawning areas during June of the 6 years 1977-1980, 1982 and 1987. The results suggest that the ability of eggs to maintain neutral buoyancy should be included in attempts to find explanations for fluctuations in recru itment of Baltic cod.