Study of qualitative effects of Rainbow Trout farms on Shahroud River using Hilsenhoff macrobenthic rapid bioassessment index

In this study pollution emissions of two Rainbow Trout farms situated on Khochireh and Hasanjoun branch of Shahroud River in Taleghan Dam basin was studied, using Hilsenhoff Rapid Bioassessment Family Biological Index (HFBI). Results of the Biological Index were comprised with Physical, Chemical and Contaminant results of affected rivers. Results showed that BOD5 in both branches were more than Shahroud River as a main branch but the farms didn’t have significant effect on organic matter increasing load (p<0.05). The effect of the farms on rivers pH were not significant (p<0.05). TSS in Hasanjoun River was more than Khochireh River (p<0.05) and Khochireh River had less phosphorus contaminant than Hasanjoun River (p<0.05). As a result, the load of organic contaminant in Hasanjoun was more than Khochireh branch (p<0.05). Changes in macrobenthos communities showed that the dominant families were Choronomidae (Diptera order) with 11.6 mean numbers in m2 and Elmidae (Coeloptera order) with 10.0 mean numbers in m2 in the study area. HFBI results based on macrobenthos diversity and density in sampling time showed that presence of Rainbow Trout farm in Khochire caused decreasing in river quality as little as one degree from “Moderate” in station1 to “Fairly poor” in station2. But this fish farm has not effect on Shahroud river main branch, based on no changing in HFBI results on stations3 and 4. The quality of Hasanjoun branch at fish farm upstream was “Bad” because of the presence of population centers in this area. Assimilative capacity of the river cause better condition in water quality at fish farm downstream in this branch as “Moderate” condition. Shahroud river at this area had “Very good” condition on station7 (upstream of Hasanjoun branch in Shahroud) and “Bad” condition on station8 (Downstream of Hasanjoun branch in Shahroud) that is shown the effects of organic pollutants entering from Hasanjoun branch to the Shahroud main branch. Results of physicochemical and biological comparison showed that in the sampling time, the fish farms had not impacts on Shahroud main river branch because of high water volume of this river.