THE EFFECTS OF TROUT ON EPIBENTHIC ODONATE NAIADS IN STREAM POOLS

1. In a southern Californian stream naiads of a lestid damselfly (Archilestes grandis Rambur) were much less abundant, moved less, exhibited fewer conspicuous behaviours, were more likely to occur in refuge areas, and had different diets in pools containing versus pools lacking rainbow trout (Oncorhynchus mykiss Richardson). To determine if trout were responsible for these patterns, we removed trout from some stream pools and added them to pools lacking trout, with unmanipulated trout and troutless pools acting as controls. 2. The abundance and emergence of A. grandis were drastically reduced, and the proportion of lestid populations in refuge areas greatly increased, when trout were added to pools; however, the removal of trout had less drastic effects on lestid abundance and distribution. Aeshna walkeri (Kennedy) was also more abundant in pools lacking than in pools containing trout. 3. Trout manipulations affected lestid behaviour, with swimming being observed only in troutless pools and movement tending to be greater in pools lacking rather than containing trout. 4. One week after manipulations started, the number of prey items per lestid gut was higher in troutless control than in trout addition pools. Ostracods and chironomids were more abundant, and mayflies were less abundant, in the diets of lestids from pools lacking versus containing trout. Comparisons of the environmental abundances of prey taxa and lestid diet composition indicated that lestid selectivities for Caenis were higher, and those for Paraleptophlebia, ostracods, and Eubrianax lower, in trout than in troutless pools. 5. Although similar at the beginning of manipulations, head widths of lestids in troutless control pools were greater than those in trout addition pools after 3 weeks.