The skull bones possess many characteristic features that are regarded as the main criteria of species, forms, and population differentiation. The performed measurements allowed not only to estimate the bone shapes, proportions, the extent of variability ranges but also to perceive the variability dynamics of particular bones of the individuals aged from 0+ to adults. Osteological analysis of 4 skull bones obtained from both juvenile and adult salmon and sea trout individuals revealed a lot of differences between studied forms. The dissimilarities in cranium structure between salmon and sea trout were visible in juveniles already but were more significant in adults. Mesethmoideum of both juvenile and adult salmon individuals was much more streamlined than the sea trout’s one. The praemaillare bone of juvenile individuals of salmon lacked the small process that was present in sea trout. Besides, in juveniles and adult salmon this bone was narrower. Vomer of juvenile and adult salmon differed in plate shape, the narrowing below the plate and the length of denticulated section on the corpus. The characteristic differences in os glossohyale structure concerned the length of denticulated section that was longer in sea trout, and the teeth alignment.

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