Fecundity of flounder, Pleuronectes flesus, in the Baltic Sea — Reproductive strategies in two sympatric populations

Two sympatric flounder populations with different reproductive strategies, offshore spawning at 10–20 psu producing pelagic eggs and coastal spawning at 5–7 psu with demersal eggs respectively, inhabit the brackish water Baltic Sea. Salinity governs the reproductive success by irregular saline water inflows and hence stock abundance and distribution. The potential fecundity (the standing stock of vitellogenic oocytes in the pre-spawning ovary) was assessed for fish sampled at five locations (two for offshore spawners and three for coastal spawners) along the salinity gradient (ICES SD 25, 27/28, 28 and 29). Multiple linear regression analysis resulted in r2-values of 0.698–0.894 for the respective sampling location with somatic weight or total fish length as the main predictor, and otolith weight (proxy for age) and oocyte density (by gravimetric counting) as additional predictors. Analysis by univariate regressions using GLM revealed significantly higher fecundity for coastal spawning- than offshore spawning flounder (∼ 69% more oocytes for an intermediate sized fish) but no intra-population differences. Similarly, gonad dry weight was significantly higher for coastal spawning flounder. Further, growth estimations indicated higher growth in offshore spawning- than in coastal spawning flounder suggesting that the coastal spawning population allocate relatively more resources into reproductive growth than somatic, potentially a result of strong selection for high fecundity of flounder producing demersal eggs due to poor egg survival for this spawning strategy. Earlier studies on fecundity of Baltic flounder are reviewed and discussed with consideration of inter-population differences questioning the idea of in general higher fecundity of flounder in the Baltic Sea.