Comparison of three potential methods for accelerating seabed recovery beneath salmon farms

Fish production from sea-cages is a globally significant and expanding industry, but farm production can be constrained due to localised but extreme seabed enrichment, which requires the farm to be rested for extended periods. This study compares the effectiveness of three potential techniques for accelerating seabed recovery in highly enriched sediments. Benthic changes induced by in-situ ‘harrowing’ (heavy raking of the seabed), ‘irrigation’ with oxygenated surface-water, and simulated sediment ‘removal’ are described in relation to passive recovery. Treatment effectiveness was assessed after four months based on physico-chemical and biological analyses of sediments, changes in benthic respiration in mesocosm experiments, and an assessment of the instantaneous water column effects induced during treatment. Results indicated significant sediment plumes associated with reduced dissolved oxygen levels, particularly during ‘removal’, but the magnitude and duration of the changes were negligible in an ecological effects context. Two treatments, ‘harrowing’ (HA) and ‘irrigation’ (IR), had little impact on seabed condition, particularly when compared with the natural recovery that occurred over the study period. Whereas, the ‘removal’ (RE) treatment (exposing the underlying sediment) significantly improved the physico-chemical and biological properties, and appeared to facilitate benthic recolonization. These findings suggest that, removal of degraded surface sediments has the potential to accelerate seabed recovery and can be a useful management strategy where trace metal concentrations (e.g. copper and zinc) have become unacceptably elevated. However, commercial-scale implementation would be contingent upon: i) further evaluation of water column effects associated with larger-scale treatments, and ii) the ability to safely dispose of the sediments.