A Scientific Review of the Potential Environmental Effects of Aquaculture in Aquatic Ecosystems - Volume 5

Behavioural Interactions between Farm and Wild Salmon: Potential for Effects on Wild Populations

Laura K. Weir
Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada

Ian A. Fleming
Ocean Sciences Centre, Memorial University of Newfoundland, St. John's, Newfoundland, Canada

Executive Summary

Behavioural interactions between farm and wild fish occur at all three stages mentioned in this review. The ability of farm fish to migrate into rivers following escape from aquaculture in the ocean environment leads to interactions during breeding. As not much is known about salmon during the marine phase of their life cycle, it is difficult to assess how interactions between farm and wild fish will ultimately affect wild populations at this stage. However, farm fish show aberrant migratory patterns, most notably that they may disperse into many rivers and thus may affect more than one wild population. The effects of behavioural interactions between farm and wild fish are most evident during breeding. Farm females and mature male parr represent the most likely means of gene flow from farm to wild populations, which are enhanced by earlier ages at maturity of farm fish because of faster growth rate. However, behavioural interactions on the spawning grounds by large males and females, as well as mature male parr, can negatively influence the reproductive success of wild fish. Pure farm and hybrid offspring in the freshwater environment can effectively compete for food and space with wild individuals, and at this life stage the environmental effects of aquaculture rearing are diminished. Maternal effects heavily influence the success of farm offspring at early juvenile stages, and their survival is usually poor compared to wild fish (e.g. Fleming et al. 2000, McGinnity et al. 2003). In addition, farm juveniles are sometimes less successful at evading predators and are not usually dominant over wild fish in natural environments.

While overall trends suggest that farm and hybrid fish may not behave similarly to wild fish, and indeed have lower survival (e.g., McGinnity et al. 2003), variation among studies reflects the context-dependent nature of determining whether farm fish are successful in the wild. Their effect will depend upon a number of factors, including genetic origin, rearing conditions, the number, timing, magnitude and frequency of escapes, and the state of the wild population (Hutchings 1991). Thus, risk assessment will need to focus on those factors mostly likely to generate exposure to the hazard (e.g. escape), and to influence the risk of harm given an escape and the severity of that harm (e.g., Kapuscinski 2005). It will also need to recognize and incorporate various types of uncertainty. A key outcome of this process should be risk reduction planning and implementation.

Knowledge Gaps

Despite the growing number of studies on the subject, there remain many areas where little is known about the potential effect of farm fish on wild populations. Our report focuses mainly on the trends among studies investigating differences between farm and wild fish. However, significant variation exists among studies, emphasizing that the outcome of interactions between farm and wild fish is likely context-dependent. Some studies show that the outcome of interactions, or the magnitude of differences, between farm and wild fish depends upon the farm strain and wild population under comparison (e.g. Einum and Fleming 1997, Weir et al. 2004). This may be due to a lack of understanding of the interaction between the genetic and environmental effects of aquaculture on farm fish. Elucidating the effects of genetics and environment is important to assess how farm fish of different origin may affect specific wild populations. While some studies indicate that genetic changes may be occurring in some wild populations following farm escape (e.g. Crozier 1993, 2000), there is no documented indication that escaped farm fish are directly causing demographic changes in wild populations, although strong inference can be drawn from two whole-river release experiments that indicate this is likely the case (Fleming et al. 2000, McGinnity et al. 2003). From the population demographic perspective, survival and competition at sea of both wild and farm fish is not well known. In addition, our knowledge of the migratory and straying behaviour of escaped farm fish remains rudimentary despite the fact that aquaculture fish are most likely to escape from sea pens and their first interactions with wild individuals is in the adult migratory phase. While there are substantive data regarding interactions between farm and wild fish in artificial or semi-natural environments, field data documenting farm-wild behavioural interactions in rivers are also lacking, most notably for juveniles. Furthermore, although lifetime fitness over one or more generations has been studied (Fleming et al. 2000, McGinnity et al. 2003), the long-term demographic consequences of decreased farm fish fitness relative to wild in the natural environment have yet to be determined. While significant strides have been made in the state of knowledge regarding farm-wild interactions to allow risk assessment, knowledge gaps remain by which associated uncertainty could be reduced. A formal investigation of knowledge gaps, that includes sensitivity analyses of population dynamic/gene flow models, is needed to determine the types of studies to be undertaken to decrease existing uncertainty.

References

Crozier, W.W. 1993. Evidence of genetic interaction between escaped farmed salmon and wild salmon (Salmo salar L.) in a Northern Irish river. Aquaculture 113: 19–29.

Crozier, W.W. 2000. Escaped farmed salmon, Salmo salar L., in the Glenarm River, Northern Ireland: genetic status of the wild population 7 years on. Fish. Manage. Ecol. 7: 437–446.

Einum, S., and I.A. Fleming. 1997. Genetic divergence and interactions in the wild among native, farmed, and hybrid Atlantic salmon. J. Fish Biol. 50: 634–651.

Fleming, I.A., K. Hindar, I.B. Mjølnerød, B. Jonsson, T. Balstad, and A. Lamberg. 2000. Lifetime success and interactions of farmed salmon invading a native population. Proc. R. Soc. Lond., B 267: 1517–1523.

Hutchings, J.A. 1991. The threat of extinction to native populations experiencing spawning intrusions by cultured Atlantic salmon. Aquaculture 98: 119–132.

Kapuscinski, A. 2005. Current scientific understanding of the environmental biosafety of transgenic fish and shellfish. Rev. Sci. Tech. Off. Int. Epiz. 24: 309–322.

McGinnity, P., P. Prodöhl, A. Ferguson, R. Hynes, N.Ó. Maoilédigh, N. Baker, D. Cotter, B. O'Hea, D. Cooke, G. Rogan, J. Taggart, and T. Cross. 2003. Fitness reduction and potential extinction of wild populations of Atlantic salmon Salmo salar as a result of interactions with escaped farm salmon. Proc. R. Soc. Lond., B 270: 2443–2450.

Weir, L.K., J.A. Hutchings, I.A. Fleming, and S. Einum. 2004. Dominance relationships and behavioural correlates of spawning success in farmed and wild male Atlantic salmon, Salmo salar. J. Anim. Ecol. 73: 1069–1079.