Effects of artificial instream cover on the density of 0 + brown trout

The effect of instream cover on the density of juvenile sea trout, Salmo trutta L., in two streams in southern Sweden was studied. One of the streams, Vallkärrabäcken, was narrow (2–3 m) and had few sympatric species, whereas the other stream, Braån, was wider (6–8 m) and had many sympatric species. Three treatments were used: (1) addition of artificial vegetation; (2) an undisturbed control; and (3) removal of natural vegetation. Only the first two treatments were tested in Braån, whereas all three treatments were tested in Vallkärrabäcken. The 0 + trout density was higher in sections containing artificial vegetation than in control sections, which in turn had higher densities than sections where natural vegetation was removed. Moreover, the effect of adding artificial vegetation on trout density was most pronounced at the end of the summer. The results indicate that instream cover from submerged macrophytes may be important for maintaining high 0 + trout densities in small and medium‐sized streams where other types of instream cover are in short supply.