Biofouling of salmon-cage netting : fouling quantification, removal and prevention

The quantification, removal and prevention of biofouling on salmon-cage netting were investigated in Tasmania, Australia. Trials were aimed at identifying environmentally-acceptable alternatives to traditional toxic antifoulants, and focussed on three areas, viz. (1) underwater cleaning, (2) a "foul-release" silicone coating, and (3) extruded polymers incorporating naturally-occurring compounds or a commercial shortlived biocide. Each of these strategies was found to be applicable and effective for the prevention and/or reduction of fouling on netting. Trials identified future directions to improve antifouling efficacy, and potential problems such as resistant organisms. The development of a quantification technique and the description of fouling in each trial also provided information on the composition and variability within fouling communities on netting. Image analysis of close-up underwater photographs of net fouling was used to quantify fouling removal and regrowth in terms of mesh occlusion. This enabled large numbers of non-destructive samples to be taken and analysed quickly, which compensated for the high degree of natural variability in the fouling communities. It also provided a permanent record of the fouling, allowed dominant species to be identified and counted, and permitted multiple sampling of the same area. Underwater cleaning of netting was found to be highly effective when there was good contact between the brushes and netting, and prevented significant fouling development over a 10 week period. However, the design of the trialed prototype cleaner and the displacement of netting away from the cleaner, reduced the severity of scrubbing, limiting fouling removal. This effectiveness was further compromised because residual structures of fouling organisms and the large quantity of debris led to rapid regrowth and recolonisation. The research identified areas for design improvement, the problems associated with residual fouling and regrowth, and the requirements for effective in situ net cleaning. The results demonstrate many difficulties that may limit the efficacy of in situ cleaning, but suggest that this technology can significantly extend the immersion time of nets during seasons of peak fouling. The application of a commercial silicone coating (Viridian 2000, International Paints) to netting proved effective for increasing the ease of cleaning and reducing the total mass of attached fouling. The silicone coating was particularly effective at preventing invertebrate fouling, and an absence solitary ascidians significantly reduced total fouling biomass. However, the total algal biomass on the silicone was comparable to uncoated netting, and some algal species were more abundant on the silicone (Ulva rigida and Polysiphonia brodiaei). This latter effect was considered to represent effects of surface colour, reduced competition, and differences in attachment strength. The application of current silicone coatings to fish-cage netting was considered to be problematic because their poor abrasion resistance and tear strength make them unsuitable for shore-based cleaning and handling procedures. Nonetheless, the flexibility of silicone coatings and the effective reduction of fouling adhesion make them ideal for fish cages that will be cleaned in situ, and may provide an effective non-toxic solution to reduce the cost of fouling removal. Extruded polymers (plastics) incorporating either naturally-occurring compounds or commercially-available biocides were found to prevent macrofouling development for up to 209 days in field trials. The efficacy of polymers that incorporate either an algal extract (Delisea pulchra or Laurencia rigida), an analog to a halogenated furanone (that occurs in D. pulchra), or one of four commercial biocides (Busan 11-M1, Irgarol 1051, Nopcocide N-96 and Sea-nine 211) was investigated. Fouling prevention was shown to be a function of the polymer type and initial loading of the antifouling compound. Sea-nine 211 and the natural product analog were highly effective when incorporated into ethylenevinyl acetate copolymers, and therefore this type of polymer may be suitable for mixtures of these antifouling compounds. A high-density polyethylene (Shell, HET 6100) and a copolymer of ethylene and acrylic acid (BASF, Lucalen® A) were also effective for Seanine 211. The current success with polymers that incorporate antifouling compounds, and the many options for increasing their efficacy, are encouraging for the development of netting which resists fouling