Role of fish farm fouling in recolonisation of nearby soft-bottom habitats affected by coastal aquaculture

• Organic enrichment reduces species abundance and richness in soft-bottom communities.
• Amphipod colonisation near fish farms was researched using defaunated sediment.
• Colonisation in controls depended on species in sediment near the experimental trays.
• In farm areas, colonisation was strongly dependent on the input of fouling amphipods.
• This spillover can reduce the negative effect of fish farming on the sea bottom.

Organic loading from fish farming constitutes a significant disturbance to marine sediment, normally reducing species abundance and richness and creating disturbed patches in soft-bottom communities. In contrast, floating fish farms harbour a high abundance of invertebrates associated with fouling communities, particularly amphipods. Changes in macrofaunal recolonisation induced by fish farms were researched using amphipod assemblages as a useful representative group. The objectives of this experiment were: (1) to test the effect of fish farms on amphipod colonisation processes in defaunated sediments and (2) to define the influence of surrounding natural and artificial habitats as sources of initial colonisers. Experimental design included 36 plastic trays placed on the sea bottom (25–28 m depth) in fish-farming and control areas and retrieved one month after placement date. Significant differences were found in amphipod assemblage composition in control versus farm sites. While the recolonisation process in control areas depended on the species present in the sediments adjacent to the experimental trays, in fish-farming areas the recolonisation was strongly dependent on the input of amphipods from fouling communities. It is the first time that a spillover effect from fouling communities on floating habitats into the benthos is detected in the coastal areas. If the organic enrichment of aquaculture sediments is moderate, the ecological services of benthic habitats may be balanced, at least partly, by biomass exported from fouling communities on the same fish farms.