BENTHIC ASSEMBLAGES IN UTAH HEADWATER STREAMS WITH AND WITHOUT TROUT

In a series of high-gradient streams along the Wasatch Front in northern Utah, perlid stoneflies were more abundant in benthic samples from 14 fishless streams than in 9 similar streams which contained trout. Smaller-bodied perlodid stoneflies were more abundant in samples from streams containing trout. Herbivorous insect abundances varied substantially within groups of streams containing and lacking trout; multivariate analysis of variance using the six most abundant herbivorous taxa indicated no significant difference between the two groups of streams. Similarly, total numbers of invertebrates did not differ between streams with and without fish. Sites above and below the upstream limit of trout in three streams showed patterns in invertebrate abundances similar to that seen at the whole-stream scale. Trout may have no effect on the overall density of stream benthos because of their negative direct effects on invertebrate predators.