EVALUATION OF PREDATOR-INDUCED STRESS ON FIELD POPULATIONS OF STONEFLIES (PLECOPTERA)

The influence of predaceous fish in affecting components of fitness of immature and adult macroinvertebrates was examined using a predator enclosure/exclosure experiment in a southern Ontario stream. The stonefly Paragnetina media is a semivoltine species with a midsummer emergence period. After exposure of P. media to rainbow trout (Oncorhynchus mykiss) between May and August (repeated twice, 1987 and 1989), we observed significant reductions in nymphal density, size (head width), and condition (mass/head width). Reductions in density were attributable to nymphal emigration and predation. Selective predation on larger stoneflies (direct effects), together with changes in nymphal feeding rate, activity, and distribution (indirect effects), reduced size and condition of stoneflies exposed to trout. After August-to-October (I 986 and 1989) and January-to-April (1988) exposures, we observed no effect of trout on these parameters. Adult stoneflies (P. media and Alloperla mediana) cannot feed, and consequently any energy that they channel into reproductive efforts must be derived from the aquatic stage. Thus, we postulated that the impact of trout on immature P. media and A. mediana might manifest itself negatively on stonefly adults. Although size and emergence pattern of adult P. media and A. mediana did not differ between enclosure/exclosure sections of stream, in the presence of trout both species suffered significant reductions in condition, fecundity, and total number to emerge. These findings demonstrate that direct and indirect effects of a predator can cascade from immature to adult prey and from aquatic to terrestrial eco-systems.