The ability of Baltic cod eggs to maintain neutral buoyancy and the opportunity for survival in fluctuating conditions in the Baltic Sea

In the brackish water of the Baltic Sea successful spawning of Baltic cod Gadus morhua is restricted to the Bornholm, Gdansk and Gotland basins below the halocline, occurring at 50–80 m depth. Due to irregular mixing of the deep water, stagnant conditions occur regularly accompanied with unfavourable oxygen conditions. In avoiding stressful oxygen conditions maintenance of egg buoyancy is considered a major limiting factor for successful spawning of Baltic cod. Batches of eggs were incubated experimentally in a density gradient column. Egg specific gravity changed during development, decreasing from the time of gastrulation, then increasing prior to hatching. The changes in specific gravity varied among egg batches from different females and were related to egg quality, egg size and ambient salinity. Eggs achieve different specific gravity depending on incubation salinity. Initial egg specific gravity together with the ability of eggs to gain and maintain buoyancy up to hatching, determine larval specific gravity and the depth where hatching will occur, and thus opportunities for larval survival, avoiding stressful oxygen conditions and developing at favourable feeding conditions.