Benthic macroinvertebrate susceptibility to trout farm effluents.

The direct effects of a Virginia trout farm on benthic macroinvertebrates were examined using multiple approaches. Static laboratory tests with the amphipod, Hyallela azteca, were conducted with exposures to water taken from a spring and from effluent both above and below a sedimentation basin. Onsite mesocosms were constructed to expose previously colonized artificial substrates to the same treatments as the laboratory tests. Flat-headed mayflies also were collected from a nearby stream and transported to the mesocosms for a 10-d exposure. There was no significant difference between treatments in the laboratory tests after 20 d, but after 28 d the control was significantly lower than the above-sedimentation basin treatment in one test. In the multispecies field tests, a clear decrease in total invertebrate and Ephemeroptera, Plecoptera, and Trichoptera (EPT) abundance was seen in the effluent treatments compared to spring water treatments. There was, however, a slight improvement in survival in treatment below the sedimentation basin. Only total invertebrate abundance after 21 d produced statistically significant differences. A significant difference was detected between the effluent and the spring treatments in the flat-headed mayfly field test. We suggest that in this study, the effluent alone does not explain the lack of taxa richness in the receiving stream. The main cause of mortality from trout effluents appears to be solids accumulating upon the organisms. Well operated and designed sedimentation basins are expected to, in part, reduce any effects on macroinvertebrates.