Water discharged from fish farms — containing uneaten feed, faeces, chemicals, and pathogens — can significantly affect the welfare of wild fish and invertebrates in receiving waterbodies, creating tension between aquaculture and ecosystem health.
The welfare implications of aquaculture effluent extend beyond farmed animals to wild populations affected by farm discharges. Wild fish experiencing oxygen depletion, sea lice infestation, or altered prey communities due to farm pollution are experiencing welfare harms they cannot avoid if their habitat is affected. Sea lice on wild salmon and sea trout cause painful skin lesions and immune stress that are directly attributable to farm proximity in many documented cases. The regulatory challenge is that dilution and dispersal make causation difficult to prove for individual animals, even when population-level associations are clear. Precautionary management — including site selection away from wild migratory routes, sea lice monitoring and treatment obligations, and transition to land-based systems — reduces both environmental impact and wild fish welfare harm.