Rapid expansion of irrigated agriculture in the western United States has prompted concerns for aquatic resources. Although the impacts of irrigation activities on quality and quantity of river water are well documented (e.g., high turbidity from soil erosion, eutrophication from nutrient runoff, pesticide contamination, reduced dis- charge), their effects on fish populations are still poorly understood. We studied the food, growth, and relative weight (a measure of body condition) of bluegills, Lepo- mis macrochirus, in relation to environmental factors in reaches of the San Joaquin and Merced rivers that have been affected to varying degrees by irrigation return flows. Fry of bluegills ate mostly cladocerans and copepods; fingerlings and larger fish ate immature aquatic insects, terrestrial insects, amphipods, and mollusks. Bluegill stomachs were fuller and contained a higher diversity of forage taxa in habitats with low turbidity and conductivity, weak buffering capacity, and low nutrient levels; bluegills also ate a more diverse diet where the potential forage supply (benthic macroinvertebrates) was most diverse. Bluegills attained mean total lengths of about 42 mm at age I, 86 mm at age 11, 116 mm at age III, 153 mm at age IV, and 166 mm at age V. Mean relative weight ranged from 96-111. Growth rate and relative weight were not significantly correlated with environmental or dietary variables. On the basis of our study, we concluded that environmental degradation from irrigation activities affected the diet of bluegills primarily by modifying the food supply, but growth rate and body condition were not affected.
