THE EFFECTS OF TROUT ON WATER STRIDERS IN STREAM POOLS

Observations made in Rattlesnake Creek, Santa Barbara County, California, USA indicate that rainbow trout (S. gairdneri) affect the microdistribution of adult water striders (G. remigis). Gerrids occupied all areas of stream pools lacking trout but stayed in margins of pools containing trout. To test the hypothesis that the presence of trout controlled the distribution of water striders within pools, trout were removed from some pools and transferred to pools lacking trout. Undisturbed pools with and without trout served as controls for the trout transferrals. The results supported the hypothesis; within 1 wk, gerrids moved to the edge of pools where trout were added and to the middle of pools where trout were removed. Time budgets and gut analyses suggest that local patterns of gerrid distribution were maintained by harrassment of gerrids by trout. At times trout had significant effects on components of gerrid fitness.