Impacts of aquaculture effluent on water quality and biotic communities in Virginia headwater streams

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Virginia Tech


Of eleven commercial and state trout farms surveyed in Virginia, five farms were selected for intensive examination of effluent impacts during Fall 1994 and Summer 1995 to maximize effects of stream low flows and high temperature. Substrate embeddedness increased significantly downstream, but effluent settleable solids concentrations were always less than 0.1 ml/L. Total ammonia and nitrite levels increased downstream 0.1 - 0.7 mg/L and 0.003 - 0.01 mg/L respectively, but were well below recommended thresholds for lethal exposure. Dissolved oxygen levels were reduced downstream, but were typically > 6.5 mg/L. Effluent water temperatures, pH, nitrate, and total phosphorus concentrations did not differ from upstream levels. Downstream water quality was contingent on feed loading rates and the use of effluent settling ponds. Periphyton were enhanced up to tenfold (58 mg/cm² chlorophyll a) downstream of farms, but enrichment was localized to within 400 m. The richness and abundance of sensitive macroinvertebrate taxa (mayflies, stoneflies, and caddisflies) were reduced downstream and pollution-tolerant non-insect taxa (isopods and gastropods) increased. EPA Rapid Bioassessment Protocol (RBP) reflected moderately-impaired water quality downstream at farms A and C, and only slightly or unimpaired water quality at the other farms. The Index of Biotic Integrity (IBI), based on fish community metrics, did not correspond to the RBP, and reflected poor water quality at C and D. Low fish species richness and abundance in these headwater streams limited the usefulness of the IBI. The proposed general discharge permit for aquaculture in Virginia is discussed and trout farm management and design recommendations are presented.



aquaculture, effluent, impact, Water quality, macroinvertebrates, Virginia