Variability of biofouling communities on fish cage nets: A 1-year field study at a Norwegian salmon farm

• Ninety species and multi-species categories belonging to 17 phyla were identified.
• The most frequent species were Jassa falcata, Mytilus edulis and Ectopleura larynx.
• Immersion period and sampling time had a strong effect on the biofouling community.
• Mesh size and variability between cages affected the biofouling community less.

The accumulation of biofouling on cage nets is a major problem and cost factor in finfish aquaculture worldwide. Norway is one of the main producers of Atlantic salmon, but biofouling on salmon farms has not been studied systematically. In a 1-year field study at a commercial salmon farm in Central Norway the effects of immersion period (1, 3, 6 and 12 months), sampling time, mesh size (13 and 25 mm half-mesh) and variability between three individual cages on the biomass, species richness and community composition of biofouling on net panels were investigated. Biomass accumulation ranged from 0.5 to 83.1 g per panel. A total number of 90 species and multi-species categories were identified with total species richness ranging from 1 to 52 species per sampling time. The five most frequent sessile macrofoulers were the amphipod Jassa falcata, the blue mussel Mytilus edulis, the hydroid Ectopleura larynx and the algae Polysiphonia stricta and Saccharina latissima. M. edulis and E. larynx, along with the mussel Hiatella arctica, contributed most to the biomass. Immersion period and sampling time had a strong effect on the biomass accumulation, the species richness and the community composition. The variability between cages and, to a lesser extent, the differences in mesh size only influenced the community composition for 1-, 3- and 6-month samples. After 12 months of immersion, the biofouling community had reached a climax state where neither mesh size nor variability between cages had a significant effect. The results of this study may contribute to the optimisation of current antifouling treatments of aquaculture nets and provide background knowledge for farm operation and management with regard to monitoring and cleaning procedures.