Antifoulants in Salmonid Aquaculture: Environmental Considerations for Marine Cage Farming

Antifoulant chemicals are commonly used in aquaculture to counteract biofouling, which may be a particularly significant problem to finfish culture operations. Biofouling reduces water flow and oxygen supply, and in turn leads to increased stress levels and disease-susceptibility in the cultured fish.

Copper-based net coatings, the main antifoulant used in marine salmonid operations, act as a biocide thereby preventing the attachment of fouling organisms to the cage. There are several environmental concerns associated with the use of copper antifoulants and their persistence in the marine environment, the most significant of which are i) toxic effects on local ecology and ecosystem processes, especially in sediments and ii) potential for bioaccumulation. Many marine invertebrates have been shown to be sensitive to metal toxicity, with juvenile (embryo and larval) stages often being particularly susceptible.

As a result where copper is released from aquaculture operations, vulnerable species can be eliminated changing the ecological balance. Other species may be more robust, with a greater capacity to regulate or sequester metals, and these species may appear unaffected or even thrive in response to the increased metal loads.

However, where an animal is able to survive through tolerance or accumulation of metals this can provide an avenue for bioaccumulation and biomagnification as a result of predation from higher trophic level species. Metals in antifoulants can become available to the broader environment either by dissolution from the active paint surface or as a result of ablation or physical damage of the painted surface. Sediments under fish farms using copper-based antifoulants typically show increased total metal loadings, however, it is important to note that for metals in the sediments to have any effect on local ecology or sediment processes they must be "bioavailable", which in tum is dependant on the form (speciation) of the metal. Metal speciation in sediments is complex and strongly related to the geochemical status of the sediments (i.e. redox, pH, oxic status, organic content), which in turn will be influenced by the extent of other processes such as bioturbation and resuspension. In anoxic sediments, metals are generally thought to be less bioavailable than in oxic sediments, as they are often tightly bound as insoluble sulphides. As metal speciation may be influenced by the level of organic enrichment, metal toxicity may fluctuate with variations in organic inputs associated with the farming cycle, and this should be taken into account in any monitoring or evaluation of impact. Until such time as there is a reliable replacement for copper-based antifouling products, sustainable management practices need to ensure that procedures minimise the environmental impact of currently used products and include the development of appropriate monitoring strategies.

In this chapter we review possible environmental impacts associated with copperbased antifoulant use, the mechanisms and processes that affect metal bioavailability, the ecological consequences that might be associated with particular metal loadings and monitoring and management strategies. Data from research conducted by the Tasmanian salmonid industry (Australia) are discussed.