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

Trophic Interactions between Finfish Aquaculture and Wild Marine Fish

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

EXECUTIVE SUMMARY

This review examines the literature on trophic interactions between marine finfish aquaculture and wild fish populations to determine the state of existing knowledge and identify research gaps for future study. Three questions are central to this discussion: What is the predation effect of caged finfish on wild fish and available prey? Do finfish farm sites attract wild fish and affect their productivity? What are the competition and predation effects of escaped finfish on local and regional wild fish populations?

Recent aquaculture reviews have highlighted ecosystem effects and provided general discussion on farm site attraction and the predation and competition effects of caged and escaped finfish. However, no comprehensive reviews exist. The majority of studies on these topics have focused on behavioural and genetic interactions between escaped finfish and wild fish, or on the attraction of wild fish to farm sites. Little direct empirical or experimental work has been conducted to address these questions and a knowledge gap exists in northern temperate marine systems, including Canada.

The published literature does indicate that cultured finfish can have measurable effects through predation of, attraction to, and competition with wild fish or available wild prey. These effects can occur at the level of local farm sites and ecosystems, and can occur potentially at regional scales. While these effects vary with respect to the three questions above, they are generally linked by the nature and productivity of the local ecosystem around farm sites (i.e. warm oligotrophic to eutrophic; cool oligotrophic to eutrophic), the number and species of cultured finfish, and the number and proximity of farm sites in relation to concentrations of wild fish. Available literature on these subjects is limited and therefore it is difficult to quantify the relevance and risk of effects resulting from trophic level interactions between farmed finfish and wild fish populations.

Only a few studies have examined the predation effects of caged finfish on wild fish, and those completed to date have been conducted in marine systems in British Columbia. Results indicate that caged salmon feed at low rates on wild fish and plankton prey and that this interaction is dependent on the salmon species cultured, season, and farm location. Farmed fish consumed low numbers of wild fish and plankton but showed trends in behaviour and prey selection similar to those of wild salmon. The studies were observational in nature and did not fully test the effects of caged salmon feeding on wild prey. Future Canadian studies should incorporate an experimental design to evaluate the predation effects of caged finfish on wild fish and prey, and determine the variation in potential effects among caged finfish species, season, and spatial proximity of farm sites to wild fish populations.

A few studies have been conducted in northern temperate marine systems to examine the attraction of wild fish to farm sites. Results indicate that wild fish use farm sites as artificial reefs or shelter, as well as enhanced sources of food from surplus pellet feed, farm and fish waste, and from the abundance of local macrofauna on or near these sites. Results also indicate that wild fish densities increase by 1- to 10-fold near cool northern temperate farm sites as a result of local attraction, but show little response in overall wild fish community biodiversity. Increases in density, population size and age structure, and overall community biodiversity near warmer nutrient-poor farm sites indicates higher levels of attraction response by wild fish. Further study is needed in Canada to examine the wild fish population and community level attraction to farm sites and determine what level of interaction exists. These studies should consider use of appropriate indicator species to determine levels of interactions.

Limited data exist on the number, local distribution, and feeding behaviour of escaped farmed fish and their potential interactions with wild fish. New study is needed to quantify potential interactions and effects of escaped farm fish on wild fish populations. The existing literature shows three patterns of interaction. High densities of escaped fish are negatively associated with the abundance and diversity of wild fish, and escaped fish are principally competitors and secondarily predators. Lower densities of locally escaped farmed fish have inconclusive effects on wild fish; although escaped fish do revert to wild-type feeding behaviour. Finally, exotic, as compared with endemic, farmed species show differences in feeding behaviour. There is a knowledge gap in available research linking escaped fish density, survival and feeding to wild fish populations in proximity to farm sites.

This state of knowledge should be integrated into an overall appraisal of the risks of farmed finfish on wild fish populations from other reviews to prioritize research and develop novel management approaches.

Gaps in Knowledge

1. New studies are required to determine whether caged marine finfish (salmon and other cultured fish species like sablefish, cod, halibut) feed on local wild populations of plankton and fish. These studies should examine local and regional scales of influence between farmed finfish and wild fish populations to determine the relevance of this interaction.
2. New studies are required to determine the level and type of attraction by wild fish to marine finfish farm sites in Canada.
3. Studies of farmed finfish escaped into natural environments are required to link cause and consequences for wild fish and their ecosystems at both local and regional scales.
4. Comprehensive monitoring and incident reporting are required to clearly establish the timing, quantities, species and condition of escaped farmed fish into the wild.
5. Most study results to date have incorporated little experimental design and have been empirical or observational in nature. Experimental or adaptive approaches should be considered to provide a range of risk exposure of finfish aquaculture to wild fish populations to reflect growing experience and understanding.
6. New study is required to compare and contrast the diet and consumption patterns of caged, escaped and wild salmon with the distribution of available food organisms and environmental conditions in marine environments.
7. The selection of research priorities for study of farmed fish and wild fish interactions should be based on the levels of perceived risk to wild fish populations and local ecosystems. The objective of this research should be to evaluate and quantify the risk.