A Scientific Review of the Potential Environmental Effects of Aquaculture in Aquatic Ecosystems - Volume 4

Cultured and Wild Fish Disease Interactions in the Canadian Marine Environment

A.H. McVicar
Garien Ltd, Aquaculture and Fish Health Consultants, 47 The Linton, Sauchen, Inverurie, Aberdeenshire, Scotland, AB51 7LG

G. Olivier
Fisheries and Oceans Canada, Gulf Fisheries Centre, 343 Université Avenue, PO Box 5030, Moncton, New Brunswick E1C 9B6

G.S Traxler
Fisheries and Oceans Canada, Science Branch, Pacific Region, Pacific Biological Station, Nanaimo, BC V9T 6N7

S. Jones
Fisheries and Oceans Canada, Science Branch, Pacific Region, Pacific Biological Station, Nanaimo, BC V9T 6N7

D. Kieser
Fisheries and Oceans Canada, Science Branch, Pacific Region, Pacific Biological Station, Nanaimo, BC V9T 6N7

A-M MacKinnon
Fisheries and Oceans Canada, Gulf Fisheries Centre, 343 Université Avenue, PO Box 5030, Moncton, New Brunswick E1C 9B6

Executive Summary

Intensive mariculture of fish is a recent industry in Canada relative to farming of land animals. This newness alone raises a unique set of questions and potential problems. Whenever there is a new use of the natural resources of an area, there is an inevitable alteration to that part of the environment being used. In the case of marine cage culture of fish, this is most obvious in the scenic sense and in the reduction of previously held rights to access by other users. However, other less apparent changes such as disease can also occur. In this context, the question frequently and justifiably asked is whether the changes due to the occurrence of infectious diseases in aquaculture introduce a significant or acceptable risk of detrimental effects to the environment and, in particular, to wild fish populations.

Over the last 20 years, several reviews have already comprehensively assessed the available scientific literature on the potential for disease interchange between wild and farmed fish (Hastein and Lindstad 1991; Brackett 1991; McVicar et al. 1993; McVicar 1997a, b; Hedrick 1998; Reno 1998; Amos et al. 2000; Amos and Thomas 2002; Olivier 2002). Notably, none of these reviews has found irrevocable evidence that fish farming has contributed to detectable adverse changes in wild fish populations, yet the topic remains one of the most controversial in the media and scientific community. The objective of this review is to focus on the main areas of potential risk using both the conclusions of individual authors who have reviewed the relevant literature and the outcomes of the different special workshops and conferences on the topic.

Disease in wild populations is rarely documented and therefore demonstrating changes in the patterns of disease in wild populations is challenging. As in any wild animal population, large numbers of different potential disease-causing agents can occur in any one species of fish. When epizootics do occur, clinically diseased specimens with high levels of infectious agents are usually easy to find, as was the case with the pilchard die-off in BC (Traxler et al. 1999). However, in comparison with farmed fish stocks, there are relatively few records of epizootics in wild fish. This cannot be interpreted as evidence of their absence or of a low level of risk of their occurrence. Highly pathogenic infectious agents that rapidly kill fish typically occur at low levels in non-epizootic situations. Furthermore, carriers of infection without evidence of clinical disease are difficult to detect due to the size and inaccessibility of the environment. Finally, sick animals are rapidly removed by selectively high predation (McVicar 1997b).

Much more is known regarding disease interactions among the host, pathogen and the environment of cultured fish than wild fish populations because cultured fish are more easily observed. A dependence on unreliable data on the relationship between diseases in farmed and wild fish populations has often led to widely different interpretations of the same information, which in turn has further fueled the considerable controversy in this area. It is well established in Canada and elsewhere that there is not a unidirectional transfer of infection from either farmed fish to wild or vice versa, but that interchange of infection between the different environments is normal. There is a tendency for those with interests in aquaculture and with wild fish to focus on the route of transmission from one direction only. A major constraint to reaching robust conclusions on possible changes and effects of disease is a widespread lack of adequate information on the disease status of wild stocks prior to the establishment of aquaculture. The inability to compare fish disease patterns before and after the establishment of fish farming is a problem not only within Canada, but also internationally. Information on the extent of variation in naturally occurring disease is required before an assessment can be made on any effects that may be superimposed by infections in aquaculture. It is also difficult to prove a negative effect on wild fish populations since fluctuations in fish populations are normal, but the causes are multifactorial and complex. Unfortunately, the background information on natural variations is usually sparse from areas where fish farming is now being conducted. Information from areas with no fish farms can provide a general perspective of the natural levels of infection that can occur but should be treated with caution in the absence of sequential information on temporal and spatial variations.

Reports have indicated that a variety of pathogens are present in numerous marine fish species, which may then act as reservoirs for pathogens of farmed fish (Kent et al. 1998). The finding of infectious hematopoietic necrosis (IHN) virus in migrating sockeye salmon in seawater raises the possibility of a marine source or reservoir of the virus (Traxler et al. 1997), although there have been no reports of losses in wild salmonids in the marine environment due to viruses (Bakke and Harris 1998). The low density of salmonids in the marine environment reduces the potential for disease to affect populations. An understanding of the dynamics of infection and of the persistence of disease is necessary before conclusions can be drawn on the extent of any new risk being posed by the occurrence of disease in farms to local wild fish populations. Different host species that are capable of becoming infected may show a natural range of susceptibility to the same infection. Under high stress conditions (e.g., elevated temperatures, spawning), even the same host may show higher susceptibility to infection (Bernoth et al. 1997). These complexities in the interaction between the occurrence of infection in fish and the development of disease and the insufficiency of basic research have contributed in a major way to the uncertainties in the evaluation of the level of impact of diseases in fish farms on wild fish populations. In this context, the role of sea lice on farms has been extensively debated internationally, but a conclusion has not been reached in any country (McVicar 2004). Similarly, the impact of IHN virus on wild stocks of fish is an area that is poorly understood and requires more research. As new aquaculture species are developed for culture in Canada, there will be a new opportunity to study disease interactions between wild and cultured species.

The introduction of new infectious agents into an area previously free of that infection could lead to serious outbreaks of disease (Olivier 2002; Kent 1994; Noakes et al. 2000). This can be due to the lack of an evolved resistance in the indigenous populations that may act as susceptible hosts. However, little can be done to prevent or restrict the natural spread of diseases associated with the normal movements of fish populations between areas or natural changes in distribution associated for example with climatic change. The focus must therefore be on human activities such as the transfer of live fish and eggs between aquaculture sites or where trade activities increase the risk of transferring infection significant to fish.

When wild fish are exposed to pathogens shed from farmed fish, neither infection nor disease is inevitable in the wild fish population. The following factors are critical: the occurrence and persistence of the infection in the source population; the availability of susceptible potential new hosts; the viability and concentration of the infectious organism in the environment; and the ability of the infection to affect the recipient population from individual fish infections (Olivier 2002). These complexities in the interaction between the occurrence of infection in fish and the development of disease and the insufficiency of basic research have contributed in a major way to the uncertainties in the evaluation of the level of impact of diseases in fish farms on wild fish populations.

The initial risk level of infection in wild fish associated with escaped farmed fish depends on the length of survival, behavior of the escaped fish after leaving the farm, and the reduced disease transmission opportunity in the lower fish densities outside the farm. Farmed fish in general are recognized to be maladapted to survival in the wild (Fleming et al. 2000) and the additional liability of fish carrying disease when they escape is likely to result in the early disappearance of the most seriously affected fish. The significance of the risk associated with diseased fish escaping from farms is therefore likely to be rapidly reduced towards levels equivalent to those for wild fish.

The introduction of new infectious agents into an area previously free of that infection can lead to serious outbreaks of disease (Kent 1994; Noakes et al. 2000; Olivier, 2002). Trade of live fish or eggs between areas carries risk of disease transfer as do other human activities such as processing where substantial concentrations of viable pathogens may be present. However, regulatory management steps taken by Canada has controlled this risk and the effectiveness of these actions is reflected in the fact that there are no recorded examples in Canada of any non-native fish diseases of concern being introduced either into farms or the natural environment.

Locally occurring diseases could get into farms principally through water, food or equipment. With the exception of treated or ground sources of seawater, fish farms are vulnerable to naturally occurring infections that are transmitted through water. It is more probable that the initiation of infections in marine fish farm is related to the level of infection in the surrounding environment, such as that in wild fish, and the proximity of wild fish to fish farms. Diseases may be transmitted in water typically for short distances or through the escape of infected animals or through direct contact with infection sources (infected animal or other contaminated material). However, as the level of risk will vary considerably with different infectious agents and because of the complexity of factors influencing the initiation of infection and thereafter the development of disease, the simplistic view of risk being directly related to the level of exposure is not tenable.

During the early stages of marine salmonid farming in Europe, disease outbreaks due to bacterial infections (e.g., Vibrio species) and parasites (e.g., Ichthyophonus) were directly attributed to the use of fresh fish as a main source of food. The processing of manufactured feeds, which is used exclusively in current Canadian mariculture, destroys known infections of concern and is no longer as a source of disease.

Although farm gear, including nets, graders, harvesting equipment, and even staff boots and clothing, can potentially transfer bacterial and viral infection between farms, the level of infection present on farms is usually sufficiently low that this is considered a relatively low-risk area compared with that associated with the transfer of live stocks. In epizootic disease situations particular caution has to be taken. Even in such circumstances, good farm management practices in relation to biosecurity measures can be effective in further reducing the level of risk and in helping to mitigate the possibility of future disease incidents.

The conditions such as crowding, which are typically found within a fish farm, are such that once infection is present there is risk of it spreading and causing a disease outbreak within the farm stocks. In this respect, fish farming is no different from intensive or semi-intensive farming on land. The development of effective vaccines in the fish farming industry has significantly reduced the problems associated with some of the serious diseases (Youngson et al. 1998). Where vaccines are not available, alternative disease management approaches have proved to be successful in reducing disease incidents on farms (McVicar 2004). Such pproaches include, removing all fish from a farm facility to break disease cycles, area or bay management, and use of single generations and targeted administration of chemotherapeutants at critical times in the disease development cycle (e.g., of lice).

Recommended Research

• Gaps in knowledge on potential disease hazards should be identified early and addressed through directed research based on the principles of risk assessment to provide options for their management in a structured manner.
• Baseline information on the disease status of wild stocks or a newly cultured species prior to the development of aquaculture in an area with emphasis on temporal and spatial variation would facilitate understanding of any effects that may be attributed to aquaculture.
• When a new species susceptible to a disease enzootic in native stocks is cultured, baseline information on pathogenicity and on wild-farmed fish disease interactions is desirable. An example would be nodaviruses in the family Betanoviridae.
• More information is needed on the cause–effect relationship in the transmission of disease between wild and cultured fish.
• Further studies are needed to objectively evaluate whether sea lice transfer between wild and cultured fish has any direct negative effect on the overall health of wild populations, particularly on the epizootiology of sea lice among Pacific salmon and its transmission to juveniles in inshore waters.
• Rapid diagnostic tests for IHN are needed. Studies on the factors involved in virus transmission and the susceptibility of other fish species should be conducted.
• Studies on waterborne disease agents or infective stages are needed to determine pathogen dispersion rates, since pathogens are frequently shed with feces.

References

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Bakke, T.A. and P.D. Harris. 1998. Diseases and parasites in wild Atlantic salmon (Salmo salar) populations. Can. J. Fish. Aquat. Sci. 55 (Suppl. 1): 247–266.

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