DISTRIBUTIONS OF RAINBOW-TROUT, LARGEMOUTH BASS AND THREADFIN SHAD INLAKE CASITAS, CALIFORNIA, WITH ARTIFICIAL AERATION

Seasonal depth distributions of rainbow trout (Onchorhynchus mykiss), largemouth bass (Micropterus salmoides), and threadfin shad (Dorosoma petenense) were measured during August 1976 through January 1978 with vertical gill nets. Bass occupied shallow, warm water with mean depths mostly < 5 m during the summer, but migrated into deeper waters > 15 m during the winter when the lake destratified. Trout had the opposite pattern and were found in deep, cold water > 20 m during the summer, and in shallow water < 10 m during the winter. Shad depth distribution s did not have a distinctive pattern. Changes in the diffuser during 1 977 caused some minor changes in reservoir oxygen and temperature valu es, but had no apparent effect on fish depth distributions. Fish depth s were more readily explained by considering thermal preferences and p redation, with depth selection by bass apparently providing the main f orce effecting depth selection by shad throughout the year, and winter depth selection by trout. Trout and bass typically selected temperatu res and depths closest to their fundamental ecological niche temperatu res, but shad never did, apparently in response to predation pressures from bass and trout, especially bass.