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REVIEW AND SYNTHESIS OF THE ENVIRONMENTAL IMPACTS OF AQUACULTURE
CHAPTER SEVEN ENVIRONMENTAL LIMITATIONS ON THE SCALE OF THE SCOTTISH MARINE FISH FARMING INDUSTRY
7.1 In this report we have discussed the environmental effects of 5 main aspects of marine aquaculture in Scotland:
- The discharge of waste nutrients and their interaction in the wider marine environment
- Effects of other discharges from aquaculture, e.g. medicines and chemicals
- Disease impacts on wild and farmed stocks
- Escapes from fish farms and potential effects on wild populations
- Sustainability of feed supplies - including research on plant meal substitution
7.2 Within each topic the main areas of concern have been outlined and gaps in knowledge have been highlighted in order that future research might be focussed on projects that contribute to areas of most uncertainty. In each section, mention has been made the concept of carrying capacity. In this last section, a brief assessment is made of the relative importance of each of the key impacts and how this relates to the scale of the industry as a whole.
The discharge of waste nutrients
7.3 Contamination of sediment by wastes from finfish culture can be severe if the scale of the farm operation is not correctly matched to the local conditions of current speed and depth. However, fish farms only occupy a relatively small area of the Scottish coast and it is unlikely that effects of organic wastes on the seabed will be the environmental factor limiting increases in production.
7.4 Dissolved nutrients can be dispersed over a wide area but, on the basis of current understanding, it is concluded that nutrients from fish farms currently make only a small contribution to algal production and probably do not directly affect toxicity either by promoting toxic strains or increasing the toxicity of toxic strains. These conclusions are based to a large extent on the results of modelling studies, which need backed up by the collection of appropriate long-term data. Assuming they are confirmed, then it is unlikely that dissolved nutrients will become the factor limiting the scale of the industry, except in specific restricted systems with low flushing rates.
7.5 Shellfish farms produce much more limited local waste than finfish farms and the issue of carrying capacity revolves around establishing that there are sufficient planktonic organisms in the water to grow a given biomass without seriously depleting the resource. For many areas of Scotland this is unlikely to be a major problem even should there be a major expansion of the shellfish farming industry.
MEDICINES AND CHEMICALS
7.6 A variety of chemicals are used on fish farms. The most important in terms of potential impacts are thought to be sea lice treatment medicines and anti-foulants based on metals. Although these products are used under controlled conditions such as to protect the environment using the Ecological Quality Standards concept, there are still many important research gaps.
7.7 Even at present, many sites are to some extent restricted in the biomass that they can farm owing to the discharge limits for sea lice medicines. In the future, much depends on whether current strategies for minimising sea lice are successful. If they are then it is possible that sea lice medicines may not be an important limiting factor. However, if lice numbers continue to be a problem, and concerns for wild populations continue to grow generating a continued downward pressure on lice burdens, then it is likely that for many areas the use of these medicines will become a factor that limits the scale of the industry.
DISEASE IMPACTS
7.8 The most significant issue in terms of transfer of disease and parasites between farmed and wild populations is clearly sea lice. It is important that the relationships between the various factors that might influence the degree to which farmed lice can affect wild populations are determined. Current thinking indicates that smolts of both sea trout and salmon are more susceptible to infestation in fish farming areas. If protecting wild salmonid populations are agreed to be important then it is likely that lice transfer from farmed salmon will limit the scale of the industry, particularly in areas with important populations of wild fish. However, if, in the future, lice on farmed fish can be brought completely under control by some new chemical, vaccine or technology then the constraint from this aspect might be reduced.
ESCAPES
7.9 Escapes from salmon farms probably represent a serious threat to wild populations of salmonids. This is not the case for farmed rainbow trout, which are sterilised. The situation with other species is not yet well understood.
7.10 The magnitude of escapes varies over time but typically escaped salmon may be greater in number than the estimated adult population of wild fish in farming areas. The current level of escapes is probably unsustainable in terms of the health of wild populations. It is difficult to determine how this relates to the scale of the industry, as it is clearly the scale of escapes rather than the scale of the industry that is important. Were the industry to significantly improve containment and/or reduce the fertility of farmed fish then it is obvious that escapes might then limit the scale of production to a lesser extent.
SUSTAINABILITY OF FEED SUPPLIES
7.11 Fishmeal and oil will become limited in the future as more of the world's supply is used for aquaculture feeds. As this happens, the industry will become increasingly vulnerable to changes in supply caused by changes in the productivity or management of the relevant fisheries. Alternative feedstocks are being actively researched and, especially if fishmeal and oil become more expensive, new products are likely to be brought to market to fill the fishmeal/oil gap. In addition, managers of fisheries globally have a poor record of conserving fish stocks and sustaining harvests, so any unexpected reductions in fishmeal production caused by a decline in the fishery before alternatives have been fully developed could reduce the scale of the industry both in Scotland and worldwide.
SUMMARY
7.12 The supply of nutrients to the marine environment is unlikely to be the factor that limits the scale of fish farm production in the foreseeable future. More likely to limit production are the linked issues of medicine usage and sea lice transfer to wild populations. The rate of escapes of farmed salmon is probably unsustainable and represents a major threat to wild populations. Changes in fishmeal supply may affect the sustainability of the industry in the short-term but substitutes for fish meal/oil are actively being developed to fill the medium-term gap in supply.
ACKNOWLEDGEMENTS
7.13 The authors gratefully acknowledge the assistance of Professor Tom Cross who commented on an early draft of the section on escaped salmon.
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