Salmon farm biofouling and potential health impacts to fish from stinging cnidarians

Biofouling growth on finfish aquaculture farms restricts water exchange, adds weight and drag to farm struc-tures, and may provide an onsite reservoir for pathogens. Biofouling assemblages often include an abundance of cnidarian taxa, which possess harpoon-like stinging cells and associated toxins. Understanding and managing biofouling risks to farm productivity is important, including the impact of frequent cleaning events on fish health. This study quantified spatio-temporal dynamics of biofouling recruitment to experimental net panels on farms in central New Zealand and associated stinging potential from commonly observed anemones. A total of 22 different taxa were recorded, including five cnidarians. Average biofouling biomass reached similar to 2 kg per m(2), and net occlusion peaked at similar to 46%, after eight weeks of immersion. The average length of nematocysts in three prevalent anemones (Diadumene neozelanica, Anthothoe albocincta, and Bunodeopsis sp.) was greater than the mean skin thickness of farmed salmon (Oncorhynchus tshawytscha). There was no minimum size of anemone that did not have nematocysts, so a higher frequency of net cleaning to prevent anemones from reaching a critical size threshold is unlikely to improve fish health outcomes. Our results can inform optimal cleaning strategies to manage inevitable biofouling management trade-offs and increase farmers' understanding of the risks to stock from drag, occlusion, stinging, and diseases associated with intact biofouling versus disturbance and exposure to cleaning debris.