Genetic baseline for conservation and management of sea trout in the northern Baltic Sea

The genetic structure of nine wild, seven hatchery-reared, and one presumably mixed, sea trout (Salmo trutta L.) populations sampled from watersheds along the Swedish Baltic Sea coast was analysed using ten microsatellite loci. DNA-information was evaluated as a baseline for mixed stock analysis (MSA) and individual assignment (IA). A clear genetic structure with distinct populations was identified (global F ST = 0.066), with significant regional differentiation. Average gene diversity (H e) was similar among samples of wild and reared trout, whereas levels of heterozygote deficiency differed significantly (wild: H e = 0.70, F IS = 0.075; reared: H e = 0.69, F IS = 0.022). The high F IS found in wild samples indicates presence of within-river sub-structuring. Evaluation with realistic-fishery simulations indicated that the baseline resolution was sufficient for MSA, at least at a regional level. Hierarchical MSA and IA analyses of real catches from two coastal fisheries showed that populations from the northern region contributed about 90 % to the catch. Analysed individually, the two fisheries differed in catch compositions despite a short geographic distance among sites. One fishery mainly caught sea trout from a small wild population whereas the other fishery was dominated by reared sea trout. Stock composition analysis is a valuable tool for refining exploitation rate estimates for individual sea trout populations in mixed coastal fisheries, as well as for investigating migration patterns in the Baltic Sea.