Five Virginia trout farms were selected for examination of effluent impacts on downstream water quality, periphyton production, and composition of macroinvertebrate and fish communities during fall 1994 and summer 1995. Annual trout production and feeding rates varied from 18.5 to 59.5 thousand kilograms and 15.6 to 876 thousand kilograms, respectively, and were correlated with discharge. Substrate embeddedness increased significantly (P < 0.07) downstream at two farms, but settleable solid concentrations in effluent were always less than 0.1 mL/L. Total ammonia nitrogen, un-ionized ammonia nitrogen, and nitrite nitrogen levels increased significantly (P < 0.001) downstream but were well below recommended thresholds for lethal exposure for aquatic organisms. Dissolved oxygen levels were significantly reduced (P < 0.001) downstream at postfeeding and predawn hours but were typically greater than 7.0 mg/L. Effluent water temperatures, pH, nitrate nitrogen, and total phosphorus concentrations did not differ from upstream levels. Settling ponds at two farms effectively reduced nutrient loadings downstream. Periphyton increased significantly (P < 0.001 ) downstream, but enrichment was localized to within 400 m. Macroinvertebrate richness and abundance of sensitive taxa (mayflies. stoneflies, and caddisflies) were reduced downstream, and pollution-tolerant noninsect taxa (isopods and gastropods) increased. Results of the U.S. Environmental Protection Agency's rapid bioassessment protocol (RBP). which integrates macroinvertebrate community metrics, reflected moderately impaired environmental conditions downstream at farms A and C and only slightly impaired or unimpaired water quality at the other farms. Results of the index of biotic integrity (IBI). which is based on fish community metrics, did not correspond with those of the RBP and reflected poor environmental conditions at farms C and D. Low fish species richness and abundance in these headwater streams limited the usefulness of the IBI for examining impacts of trout farm effluents.
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