“Net cleaning debris causes gill necrosis. The primary threat from polyp stages comes from debris generated by net cleaning activities. Cleaning is accomplished by high pressure water blasting, or by manual brushing or scraping of the nets. Debris consists of stingy and abrasive components, both of which lead to gill injuries, amoebic gill disease, necrosis, and mortality (Bloecher et al. 2018; Bosch-Belmar et al. 2017).”

“As bad luck would have it, hydroid regrowth is stimulated by the mechanical action of net cleaning (Guenther et al. 2010). Laboratory experiments have demonstrated that tiny bits of hydroids left on the nets after cleaning are sufficient to regrow entire colonies, and the more frequent they are cleaned, the faster they reproduce.”

“Likewise, the tiny fragments created by the cleaning process act like seeds, flowing downstream and settling out to become new vigorously growing colonies.”

“Hydroid seeding can be compared to the broom scene in the Disney film Fantasia, where each effort to destroy the brooms simply resulted in more brooms. Downstream hydroid seeding is a serious issue affecting farmed and native species alike. In the short term, it leads to a higher biomass of medusae stinging the salmon and native species, and in the long term, the extra biomass permanently alters the function of the ecosystem.”

“Hydroid seeding downstream beyond the farms is a serious biosecurity issue and environmental hazard for other industries and natural habitats, with knock-on effects back to the farms in terms of increased bloom impacts; the biosecurity plan must consider this.”

“Positive Feedback Loop Between Jellyfish & Salmon Farming” by Dr Lisa-ann Gershwin,
 Director, Australian Marine Stinger Advisory Services Pty Ltd

“Biofouling growth on finfish aquaculture farms restricts water exchange, adds weight and drag to farm structures, and may provide an onsite reservoir for pathogens. Biofouling assemblages often include an abundance of cnidarian taxa [which include jellyfish], 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.”

“Average biofouling biomass reached ∼2kg per m2, and net occlusion peaked at ∼46%, after eight weeks of immersion.”

“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.”

“Finfish farming is consistently challenged by biofouling because of its impacts on submerged infrastructure (Fitridge et al., 2012; Bannister et al., 2019; Bloecher and Floerl, 2021). Salmon farms have considerable floating and underwater structures, including pontoons, mooring systems, weights, farm barges, and large quantities of netting to retain farmed stock (pen nets) and, in some instances, to exclude predators (predator nets). For example, a typical Norwegian salmon farm has approximately 50,000m2 surface area of submerged structures (Bloecher et al., 2015). Biofouling of nets can have important consequences for fish health because net occlusion restricts water exchange, lowering oxygen levels and waste flushing to heighten stress on farmed fish (Bannister et al., 2019). Biofouling growth also increases weight load and drag on farm nets by up to 10–fold (Bi et al., 2018), resulting in poor hydrodynamic performance and pen deformation with associated reductions in pen volume and inadvertent periodic changes in stocking density (Klebert et al., 2013).”

“However, uncoated nets are susceptible to rapid biofouling accumulation and must be regularly cleaned or washed to prevent water flow problems (Bloecher and Floerl, 2021). This is generally carried out in situ using specialised underwater high-pressure cleaning equipment. Cleaning must be carried out more regularly than for copper-treated nets to avoid significant accumulations that can negatively impact stock. While there are clear advantages to avoiding biocidal coatings on nets, in situ cleaning without waste retention produces temporary plumes of cleaning waste that may affect fish health and adjacent marine communities and habitats (Floerl et al., 2016; Bannister et al., 2019; Bloecher and Floerl, 2021; Østevik et al., 2021).”

“Biofouling assemblages on salmon farms in New Zealand often contain anemones and hydroids in high abundance. […] When disturbed, cnidarians can release harpoon-like stinging cells (nematocysts) containing toxins or stinging strands (acontia) that have large numbers of nematocysts. […] In addition to direct contact, net cleaning waste may contain whole and fragmented organisms, including cnidarian species, that can release stinging structures with associated toxins (Carl et al., 2011). Over recent years in New Zealand, farmed Chinook salmon (Oncorhynchus tshawytscha) exposed to this material during net cleaning operations have subsequently developed skin health issues (Johnston et al., 2021), although the cause of the lesions remains unknown. Injured fish are generally more vulnerable to pathogens, thermal stress, and other factors that increase mortality (Svendsen and Bøgwald, 1997; Madetoja et al., 2000; Småge et al., 2017).”

“Biofouling biomass increased significantly with immersion duration—analogous to periods between net cleaning […].”

ScienceDirect