Benthic recovery and re-impact responses from salmon farm enrichment: implications for farm management

This paper describes a two-year study of spatial and temporal patterns and processes in the benthos in response to the removal of salmon cages from a sheltered coastal embayment, coupled with the simultaneous reintroduction of cages at an adjacent location. Significant recovery was evident at the fallowed site in the first six months; however, the macrofaunal assemblage remained impacted at the conclusion of the study. By comparison, the reintroduction of a fully operational farm overwhelmed the macrobenthic community within three months, with anoxic and near-azoic conditions developing. Both removal and reintroduction of the farms triggered alternating oscillations of geochemical and biological variables, which were attributed to effects on sediment chemistry from organic loading, 'boom and bust' cycles of opportunistic taxa in response to food supply, and the associated variations in metabolic potential. The study also revealed interesting spatial dynamics in the benthos and some useful indicators of different stages of recovery and re-impact. It is concluded that farm reintroductions should aim to gradually increase production; allowing time for the benthos to adapt to the additional organic flux, and be maintained at a level that avoids macrofaunal collapse. The sediment's ability to cope with organic inputs from fish farming, and hence the duration of the recovery period, is contingent on the organic load in each farming cycle and the extent to which the sediment community is allowed to recover. Understanding the influence of each of these on sediment processes is important for sustainable long-term management of farming operations.