Predation and the prey community of a headwater stream

SUMMARY 1. Predatory, net‐spinning larvae of the caddis Plectrocne‐mia conspersa (Curtis) were abundant in the acid headwaters of some southern English streams where fish were absent, but were scarce or absent downstream where brown trout (Salmo trutta L.) occurred. 2. Field enclosure experiments showed that both underyearling and older brown trout reduced the density of P. conspersa. However, whereas small trout affected the overall density of caddis, older fish reduced that of large larvae only. 3. Although P. conspersa is itself an important invertebrate predator there was little evidence of an indirect effect of brown trout on the prey of P. conspersa. Perhaps the diets of brown trout and P. conspersa are so similar that fish simply replaced the caddis as top predator.