The use of benthic invertebrate community and water quality analyses to assess ecological consequences of fish farm effluents in rivers

The ecological consequences of freshwater land-based salmonid farms on rivers were studied by monitoring eight farms with substantial differences in annual trout production (29–368 t year−1) connected to rivers of contrasted hydrology (mean annual discharge ranging from 0.3 to 4.8 m3 s−1) in France. Samples of water and benthic invertebrates were collected in the river immediately upstream and at two locations downstream of fish farm outlets during low flow period in summer. Suspended solids, PO43− and NH4+ concentrations increased downstream of the outlets, suggesting that fish food-derived wastes led to a dilute mixture of suspended solids and dissolved nutrients. The total abundance of benthic invertebrates consistently increased immediately downstream of the effluent outlets, and a positive correlation was observed with the biomass of fish in the aquaculture facilities at the sampling date. The proportions of oligochaetes and chironomids increased downstream of the farms and, conversely, proportions of ephemeropterans, plecopterans and trichopterans decreased. Similarly, the relative abundances of shredders and scrapers decreased in the benthic invertebrate community downstream of the outlets, whereas deposit-feeders and filter feeders increased. This consistent pattern outlined a major change in the ecosystem functioning, with particulate organic matter from fish food-derived wastes becoming a central source of energy in river benthic food webs. Furthermore, the taxonomic-based ecological indicator usually used for monitoring biotic integrity in French Rivers overlooked these functional impairments in larger streams (mean annual discharge above 2 m3 s−1). This study outlines the interest of using benthic invertebrate functional feeding groups together with the results of water chemistry analyses for monitoring the ecological disturbances induced by land-based salmonid farm effluents in running waters.