SPATIAL AND ANNUAL VARIATION IN THE DIETS OF JUVENILE SMALLMOUTH BASS, MICROPTERUS-DOLOMIEU

We measured macroinvertebrate densities and abundance, size, and diet of juvenile smallmouth bass, Micropterus dolomieu, at five sites locat ed at varying distances from a metalimnetic release dam. We used these data to determine the influence of high prey abundances on stomach fu llness and age-0 year-class strength. Summer diets of juvenile smallmo uth bass (27.7-107.7 mm total length) were dominated by Ephemeroptera (primarily Baetidae) but diet composition was highly variable among ye ars and sites. A linear index of prey selection showed that Ephemeropt era were preferred and Trichoptera were avoided at all sites in all ye ars; but, selection indices were not consistent for Chironomidae or Am phipoda and Isopoda. In two of three years, stomach fullness (gut cont ent mass relative to predicted maximum) of juvenile smallmouth bass de creased with distance downstream of the dam, which reflected patterns observed in benthic macroinvertebrate densities. However, in 1989 when flooding and increased turbidity reduced abundances of juvenile small mouth bass, no differences in stomach fullness were found among sites. High stomach fullness of juvenile smallmouth bass was attributed to h igh prey abundances near the dam in years of low or normal streamflow. However, patterns in juvenile smallmouth bass abundances in mid-summe r could not be attributed to longitudinal variation in prey abundance.