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Disease interaction between farmed and wild stocks
There are considerable disease interactions between wild and farmed stocks, especially among salmonid fish. Proper disease management in farmed stocks will reduce the impact on health among wild stocks.
Certain diseases can threaten the existence of wild fish stocks. One such disease is infections with the salmon parasite Gyrodactylus salaris. The parasite infects salmon in rivers. Another parasite is Lepeophtheirus salaries or salmon lice, which represent a threat to migrating salmon and trout smolt in salt water estuaries and fjord systems.
The fungal infection crayfish plague represents a serious threat to Astacus, a genus of crayfish in river systems. Furthermore, the parasitic infection bonamiosis has virtually eradicated stocks of flat oysters in large parts of Europe. Norway has in place a programme for monitoring bonamiosis and its status as a country free of the disease was accepted by the ESA/EU in 2005.
There is a close connection between the management of aquatic organisms and diseases that affect them. The planning and implementation of monitoring initiatives and measures designed to combat Gyrodactylus salaris, salmon lice and crayfish plague must therefore be implemented in close cooperation with the environmental authorities with sectoral responsibility for management of aquatic freshwater species.
The crayfish plague
In 2005, crayfish plague was discovered in southeast Norway in a waterway on the Norway’s border with Sweden. Crayfish plague has twice before threatened stocks of astacus in southeast Norway, but has been combated by introducing a ban on the catching of crayfish in order to prevent the fungal infection from spreading. The situation has been exacerbated, however, after signal crayfish were introduced illegally into watercourses on the Swedish side of the border. Signal crayfish is an alien species in Nordic fauna, and can pass on the disease to other species without becoming ill and dying. A plan for monitoring and initiating measures designed to combat crayfish plague must therefore be determined in the light of the new situation involving signal crayfish.
Gyrodactylus salaris
So far, one of the most commonly used pesticides against Gyrodactylus salaris has been rotenone. Due to its effects on the environment, however, there has been strong resistance within the society to the use of this pesticide. Therefore, in 2005, large-scale trials were carried out in the Lærdal river using an acidic aluminium solution, which is a more environmentally friendly method of combating the disease in combination with limited use of rotenone. The preliminary results of the trial are promising.
The use of aluminium as a means of combating Gyrodactylus salaris will therefore be subject to further evaluation, in order to reduce the use of rotenone. This will be done under the auspices of the National Steering Committee for Gyrodactylus, which was founded at the end of 2004 as a collaborative body managed by the Norwegian Food Safety Authority and the Directorate for Nature Management.
Salmon lice
Salmon lice pose a threat to both farmed salmon and wild salmon stocks. There is a documented link between the incidence of lice in farmed salmon and in lice found on wild salmon. The most important means of reducing the problem of lice in wild stocks is therefore to prevent, monitor and combat salmon lice in fish farms, as stipulated in the National action plan to combat salmon lice.
Counts of salmon lice in Sognefjord from 2000 until 2004 show a clear decline in the incidence of salmon lice. This clear decline proves that the work of reducing levels of salmon lice in general in fish farms has a positive effect on the incidence of salmon lice larvae in the fjord system. Annual salmon lice counts were introduced in the Hardangerfjord in 2004. “The Hardangerfjord project”, which is a cooperative effort between the Norwegian Institute of Marine Research, the Department of Ecology and Natural Resource Management (INA) at the Norwegian University of Life Sciences, the National Veterinary Institute, the Norwegian Institute of Fisheries and Aquaculture Research and the Hardanger fish health network have for two years in succession shown that there are low levels of lice on wild fish and low levels of lice on exposed test fish in netpens.
Furthermore, the monitoring of salmon lice among wild fish will also be part of an evaluation and monitoring programme in connection with the creation of national salmon watercourses and fjords. The Norwegian Food Safety Authority will be responsible for implementing this monitoring programme.
Francisellosis
The disease francisellosis, caused by the newly discovered bacterium Francisella piscicida is common in wild Atlantic cod outside southern Norway. It has caused severe problems in cod farming, particularly during warm periods. No vaccine is available yet and preventing outbreaks and transmission of the disease are the only possible actions. Limitation of transport of affected fish and changes in aquaculture facilities to allow the fish to go deeper is presently being evaluated.
From: http://www.fisheries.no/aquaculture/health_welfare/aquaculture_environment_diseases/