Atlantic salmon (Salmo salar L.) in the border of distribution range: patterns of osteological variation

Osteological differentiation and developmental stability of Atlantic salmon juveniles were studied in eight rivers of the White, Barents and the Baltic seas basins. Mean values and fluctuating asymmetry (random deviations from perfect bilateral symmetry, measure of developmental instability, FA) were measured on skull bones using shape of the bones and number of teeth and sensory pores. Shape of skull bones allows correct identification of juveniles from different rivers with probability as high as 80%. Number of teeth and sensory pores allow only 46%. The most pronounced differences were found between three samples from the Pechora River basin, i.e. northeastern border of the species range, and five other samples. Pattern of osteological differentiation of samples does not coincide with genetic differentiation. Based on genetic markers, salmon from the Baltic Sea basin differ considerably from fish of the White and Barents seas basins. Morphological data show similarity between the Baltic and the White seas salmon and differences from Pechora River (the Barents Sea basin) fish. These results suggest that inter-population osteological differences in Atlantic salmon are mostly determined by local conditions in rivers. Eastern populations also differ from all others by a higher FA, indicating their lower developmental stability. A significant negative correlation between FA and winter temperature suggest that lower winter temperatures, directly or indirectly, are stressful factor causing decrease in developmental stability of salmon near the eastern border of their distributional range.