Barriers effect on lotic fish fauna

Sammanfattning: Barriers in running (lotic) waters are known to have serious impacts on fish fauna that would otherwise naturally colonialize them. The two most important effects barriers have on lotic fish fauna are the blockage of fish migration routes and the reduction of water velocity. These effects can in turn change the conditions for survival for species using lotic habitats. In this study I have investigated how barriers effect lotic fish fauna and whether the number of barriers blocking a river section from a migration pool is an important factor affecting these communities. To investigate this, a specially designed net, The Nordic multi-mesh Stream Survey Net (NSSN), for lotic watercourses was used to sample fish in the Swedish river Hedströmmen during July 2015. The number of species caught in the net was noted, the number of individuals per species counted, and in addition the body length was measured for European perch (Perca fluviatilis). In contrast to our predictions, number of species did not differ between unblocked and blocked sections in the river. However, there were differences in abundance (i.e. total number of individuals), although species differed in this respect. Seven out of the total eleven species were unevenly distributed between the two site categories (blocked and unblocked), in general showing a greater abundance in unblocked sites, thus following the predictions of this study. For European perch there were also differences in body length between the sites, with mean length being larger in blocked sites, proving different ecological implications depending on whether the site is obstructed or not. These results thus indicate that many fish species in lotic waters are affected by barriers.