36 DECEMBER 2012 • WORLD AQUACULTURE • WWW.WAS.ORG seals and otters, and even birds-of-prey. It should be emphasized that frequent removal of dead and moribund fish to reduce the supply of easily available prey with a high biosecurity risk is important. If sea-farming of salmonids have production systems that differ from the standard, this may increase the risk of introducing infectious agents significantly. Use of wet feed or fresh catches of marine fish directly may introduce agents from a marine reservoir to the salmon. Mixing of year-classes extends the period for keeping stock at a site, as well as increasing the number of introductions of live material to a site. Joint operations involving use of the same staff and potentially the same boats and equipment at more than one site call for very strict hygienic procedures. Large net pens (e.g., 150-m circumference rings) may complicate surveillance of the fish population. Collection of dead fish and medical treatments may be more complicated, which may increase dissemination of infectious agents. Breeding Station — Production of Fertilized Eggs The production cycle at a breeding station covers the same stages of smolt and sea-site grow-out as the production of market-size fish. The production is more or less a closed circle, only rarely introducing new fish material. Thus, waterborne infections are rendered the main biosecurity focus. The consequences of the introduction of an infection into a breeding station is even greater than for the other farm categories because of the potential for spreading disease with fertilized eggs to many other farms. Therefore, the criteria for choosing a location for seawater supply must be very strict regarding distance to other aquaculture installations, particularly sea-farms, and direction of water currents in relation to this. The lowest possible biosecurity risk with respect to the water supply during the freshwater phase is also vital. To minimize the risk of spread of infections, all eggs must be disinfected in iodine after fertilization and kept under strict hygienic isolation until delivery. A second disinfection may be carried out in connection with transfer to a hatchery. Broodfish should be surveyed continuously for selected relevant pathogens, alternatively tested individually at spawning for pathogens that can be transmitted within the egg and thus protected by disinfectants. Biosecurity Matrices Potential sources and routes of infection into a fish farm, including levels of risk, may be summed up in a matrix, visualized in Figure 1. Here, the three main categories of infection routes are included: live fish material, water source, and vectors. For fish material, disinfected eggs are considered to constitute the lowest risk and there is increasing risk from fry to smolt, further to wild fish, both salmonids and marine fish, and sea-caged salmon has the highest risk level, with its high numbers and dense populations. The different water sources and water treatments may be ranked in a similar way, from well water and disinfected water, via waters inhabiting different fish categories and treated with FIGURE 1. Potential risks for the introduction of infection into a fish farm. The three main categories of infection routes (live material, water source, and vectors) are weighed against levels of risk. Lowest risk is to the left (green), increasing towards the right (red). FIGURE 2. Level of risks for the introduction of infectious pancreatic necrosis virus into an Atlantic salmon production site. Lowest risk is to the left (green), increasing towards the right (red). FIGURE 3. Level of risks for the introduction of pancreas disease into an Atlantic salmon production site. Lowest risk is to the left (green), increasing towards the right (red).
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