THE EFFECTS OF TROUT ON EPIBENTHIC ODONATE NAIADS IN STREAM POOLS

1. In a southern Californian stream naiads of a lestid damselfly (Arch ilestes grandis Rambur) were much less abundant, moved less, exhibited fewer conspicuous behaviours, were more likely to occur in refuge are as, and had different diets in pools containing versus pools lacking r ainbow trout (Oncorhynchus mykiss Richardson). To determine if trout w ere 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 t o pools; however, the removal of trout had less drastic effects on les tid abundance and distribution. Aeshna walkeri (Kennedy) was also more abundant in pools lacking than in pools containing trout. 3. Trout ma nipulations affected lestid behaviour, with swimming being observed on ly in troutless pools and movement tending to be greater in pools lack ing rather than containing trout. 4. One week after manipulations star ted, the number of prey items per lestid gut was higher in troutless c ontrol than in trout addition pools. Ostracods and chironomids were mo re abundant, and mayflies were less abundant, in the diets of lestids from pools lacking versus containing trout. Comparisons of the environ mental abundances of prey taxa and lestid diet composition indicated t hat lestid selectivities for Caenis were higher, and those for Paralep tophlebia, ostracods, and Eubrianax lower, in trout than in troutless pools. 5. Although similar at the beginning of manipulations, head wid ths of lestids in troutless control pools were greater than those in t rout addition pools after 3 weeks.