What do we really know about infection between farmed and wild fish?

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A project has attempted to map all available scientific and evidence-based knowledge about infection between farmed and wild fish. In order to be able to get through the extensive amount of scientific material in the report, we asked one of the scientists behind the report, Lill-Heidi Johansen from Nofima, to tell us what we actually know?

Olav Huslid (OH): This research project carried out by Nofima, the Institute of Marine Research and the Norwegian Veterinary Institute, with financial support from the Fishery and Aquaculture Industry Research Fund (FHF), appears comprehensive. I have gone through the report and searched for conclusions, but my initial impression is that the report first and foremost provides an overview of what we know about the various diseases and risks of infection, as well as which knowledge requirements and future research is necessary to combat the problem. Is this correct?

Lill-Heidi Johansen (LHJ): The objective of the project was to compile a review of the knowledge status in the field of infection between wild fish and farmed fish, and a description of future research requirements. Consequently, the report gives an overview of what we know (and don’t know) about the various diseases and risks of infection, as well as what future research is necessary in order to enhance the level of knowledge in the field.

Conclusions

OH: In light of this, I would like to ask what you believe are the main conclusions that may be drawn from this work?

LHJ: The main conclusion is that in general we have little documented evidence that pathogens are spreading or have spread from farmed fish to wild fish and vice versa. This does not necessarily mean that transmission does not occur. We know that many of the known pathogens that cause disease in farmed fish today existed in wild fish before aquaculture started. However we are aware of very few disease outbreaks in wild fish populations. The risk that the pathogens can multiply and cause the disease increase with the number of receptive hosts, short distance between hosts etc., as is the case with fish at fish farms. The report concludes that there are challenges associated with detecting pathogens in wild fish. Moreover, we do not know enough about the extensiveness of various pathogens in wild fish and about the risk of dissemination of pathogens, not only because it is challenging to study this but also because so few studies have been carried out, and virtually none on a large scale, in Norwegian waters.

OH: What advantages for research will come from mapping the salmon genome? How can you use it in your future work?

LHJ: The knowledge about the salmon genome will be an important tool to gain enhanced knowledge about, for instance, immune response. This could be used in the development of new vaccines to provide better protection for the fish against pathogens. This mapping can also be used in a breeding context to produce fish with greater resistance to diseases.

Health situation

OH: The report ascertains that the “overall health situation in Norwegian aquaculture is good”. What would you describe as the most important challenges for the aquaculture industry, and what is needed for these to be accomplished?

LHJ: The greatest challenges from a disease perspective are salmon lice and viral diseases. Salmon lice first and foremost because many link this with the environmental impact on wild salmonids and because there are challenges associated with the development of resistance against certain anti-parasitic drugs. But the industry has taken measures and is today working in a determined manner on several different fronts to reduce and combat the problems with salmon lice. With respect to viral diseases, vaccines currently only exist against a handful of these and the effect is not optimal. As far as we know, the viral diseases are first and foremost a threat to farmed fish. In order to gain better control, a significantly stronger effort is needed in the area of vaccine development. We’re partly talking about long-term fundamental research to develop new vaccine concepts that can provide better protection than today’s products. This will in turn contribute to reducing the potential for dissemination of disease. It is also important to mention that there is still much to be gained in the prevention of infection and disease outbreaks through production optimisation in both the juvenile phase and sea phase. Research and evidence-based knowledge already exists, but the fish farmers should make use of this to a higher degree. Breeding for enhanced disease resistance is another important contribution to reduce the problems concerning disease in aquaculture. In addition to preventative measures, there is a need for increased knowledge about reservoirs and transmission of various pathogens.

OH: I have some friends who are wild fish enthusiasts and who are usually in a rage with the industry. What would you say to them to “calm them down” with scientific facts?

LHJ: First and foremost, I would like to emphasise that there is not necessarily any antagonism between being a wild fish enthusiast and an aquaculture enthusiast, but as a scientist I have a neutral position in this debate. My role is to put forward the knowledge-based facts. But little evidence exists of the spread of pathogens from farmed fish to wild fish and very few instances, if any at all, of transmission resulting in disease outbreaks. More knowledge is required in this area, but there is little that indicates that infection from farmed fish, for example, has any affect on population regulation of wild fish. Even those some wild fish close to fish farms containing disease could theoretically be infected, the distance between fish in wild conditions will be so great that it is highly improbable that this could lead to any large-scale dissemination of disease with subsequent mass deaths. The risk of harmful effects on wild populations has proven to be considerably higher with the importation of so-called exotic diseases – pathogens that do not naturally occur (or occurred) in Norwegian nature. Examples of this include Gyrodactylus salaris and the furunculosis bacteria, both of which were introduced to Norway via imported fish from overseas.

Even though the critics are preoccupied with the dissemination of disease in general, it is maybe first and foremost salmon lice that is the subject of large-scale media coverage. There is a heated debate on this subject and not even the scientists can reach agreement about the effect of lice from farmed fish on wild fish, but some data that has emerged in recent times points to the fact that several factors exist which can influence the rate of survival and return migration of wild salmonids. However, larger studies are necessary in order to reveal the connection.

OH: Could you describe the development in the aquaculture industry and the work that has been done to combat disease and infection?

LHJ: The most important factors are the development of effective vaccines, especially against bacterial diseases. Knowledge about the specific pathogens, paths of infection, triggering factors etc., which has emerged after many years of research, has been made use of to prevent infection, disease development and diffusion of pathogens. This type of research will also be important for new diseases which occur in the future. Breeding for enhanced disease resistance has contributed to fish with greater resistance, and new methods now being used in breeding programmes could provide further improvements. There has also been a significant development of methods in fish health and the molecular biological analytical tools used today enable pathogens in farmed fish to be detected as early as possible and to make rapid and correct diagnoses. These methods are also used to study the affinity between pathogens in order to trace the sources of infection and paths of transmission. The collaboration between research and business is central for the positive development which has taken place within several aspects of the work involving fish health. However, a potential exists for better implementation of research-based knowledge within fish health in the industry.

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