Field protocol for assessing small obstacles to migration of brown trout Salmo trutta, and European grayling Thymallus thymallus:: a contribution to the management of free movement in rivers

Between 1996 and 2004, adult brown trout, Salmo trutta L. (n = 40) and European grayling, Thymallus thymallus (L.) (n = 39) were radio-tracked in three southern Belgium rivers to assess their capabilities to bypass various obstacles. During their upstream migrations individuals encountered different types of physical obstacles and successfully passed some under variable environmental conditions. The obstacles cleared by the fish were characterised based on a simple topographical description protocol and compared with tracking data. The ability of trout and grayling to pass different typologies of physical obstacles in natural river systems is discussed in the context of enabling their free movement in rivers.