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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 eggs 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 of the spawning stock of Baltic cod Gadus morhua L. nowadays) maintained neutral buoyancy at a salinity of 14.5 i 1.2 ppt, with great variation among eggs from different females. Egg buoyancy was significantly correlated with yolk osmolahty and chorion thickness and weakly correlated with egg size. Measurements of egg buoyancy in successive batches from the same female indicated that egg buoyancy increased with batch number. The significance of neutral buoyancy for cod egg survival in prevailing conditions in the Baltic was evaluated by comparing the ability 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 recruitment of Baltic cod.

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