Cod Populations In The Eastern Baltic Sea And The White Sea: The Environmental Extremeness As A Factor Of Two Unique Reproduction Strategies Formation

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Two populations of numerous cod stocks in the Northeastern Atlantic Ocean – the eastern Baltic cod Gadus morhua callarias and the White Sea cod Gadus morhua maris-albi – are inhabiting the intra-continental seas, where reproduction conditions are close to the extreme ones to the species as a whole. Some aspects of their reproductive strategy are considered with reference to the determining influence of the environment conditions extremeness. The unique feature of the White Sea cod consists in coincidence of its reproduction peak with the hydrological winter season and negative water temperatures. The biological sense of this adaptation is to avoid summer decline of the surface salinity leading to decrease of eggs buoyancy. As a result of adaptation to maturation and reproduction at the extreme temperature conditions, the White Sea cod strategy is directed to reduction of expenses for power-consuming functions: fecundity and growth rate. The reproductive strategy of the Baltic cod is under the combined impact of salinity and oxygen content variability and the reproductive volume is the integral index of this impact. Highly significant duration of the Eastern Baltic cod spawning and shift of mass spawning period from spring to summer after the prolonged absence of the North Sea water inflows became the result of reproduction association with the restricted near-bottom water volume. As a result, it is assumed that salinity is a general factor, which to a significant extent controls reproduction and eggs survival of two populations considered.