Stream habitat use and diet of juvenile (0+) brown trout and grayling in sympatry

Electrofishing survey data and experiments carried out in a semi-natural st ream were used to test the hypothesis that interactions between underyearli ng (0+) trout and grayling during their first summer affect habitat use and diet. The survey data revealed a general difference in habitat use, with 0 + grayling being more common than 0+ trout in large streams and deeper sect ions. It was also found that in the presence of trout, finer substrate and shallower sections were utilised more by grayling. Field experiments were c arried out with three treatments; trout alone, grayling alone and the two s pecies together. In both the July and September experiments grayling tended to occupy deeper sections than trout. Trout abundance was higher in shallo w areas in sympatry, while no such difference was found in allopatry. The r ate of disappearance of grayling from the study sections was significantly higher in sympatry in July, while no difference was found in September. In July the size difference between species and the degree of dietary overlap were small and nonsignificant, respectively, indicating that the two specie s were strongly competing.