Assessment of the Population of the European Grayling under Limited Data

The state of populations of European grayling Thymallus thymallus (Linnaeus, 1758) has been assessed and compared in two Ural tributaries of the Pechora River. The grayling inhabiting the Shchugor River (the territory of Yugyd Va National Park) has been satisfactorily preserved with minimal shifts in population characteristics. In the Bolshaya Synya River, protected only at the river source, the grayling population decreased mostly by the fish of the maximum age (up to 7+), the share of repeatedly spawning individuals has decreased as well (4.0–4.8%), and the overall mortality rate has increased. Deterioration of the grayling population in the Bolshaya Synya River was due to intensive illegal fishing. Under limited initial data, using a set of traditional indicators appeared a more universal assessment approach, while the length-based indicators (LBIs) method has only limited application.