Benthic Macroinvertebrates Response in the Haraz River to the Trout Farms Effluent

The effect of rainbow trout farms outflow in the Haraz river on benthic macroinvertebrates and the self-purification potential of river was investigated from each of ten selected stations along four farms. Benthic macroinvertebrates was investigated for different seasons a year by taking five samples. About 170000 macroinvertebrate specimens at 34 genera، 32 families and 12 orders were isolated and distinguished in this study. Maximum abundance at most stations was possessed by order Ephemeroptera، of which Baetis sp. followed by Acentrella sp. constituted 97. 9% of the Ephemeroptera at all stations. Indices of EPT richness، EPT percentage and EPT/CHIR (Ephemeroptera+Plecoptera+Trichoptera/Chironomidae) decreased at the immediate vicinity of the fish farms outflows، and increased upon distancing from the farms. On the basis of HFBI (Hilsenhoff Family Biotic Index) index، four grades of good، suitable، relatively poor and poor water qualities were distinguished at all the sampling stations. The combined results of biotic indices indicated that pollution was higher at the stations at the immediate vicinity of the fish farms outflows than the preceding ones، and the self-purification potential of water increased upon distancing from the farms، peaking to a significant level at a distance of 3. 5 km.