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Review and Synthesis of the Environmental Impacts of Aquaculture




1. Particulate organic wastes from cage farms have a profound effect on the benthic environment and recovery, on cessation of farming, may take several years. Impact on the sea bed is the most obvious pollution effect from fish farms and measures of this effect are the main method of regulating and controlling the size of fish farms such that the local environment is not overwhelmed. However, severe effects are generally confined to the local area (a few hundred metres at most) and the total area of seabed used for this purpose is insignificant in terms of the total coastal resource.

2. Lack of long term monitoring programmes over the past 30 years has made it difficult to judge whether the perceived increase in Harmful Algal Blooms (HABs) is real and related to expansion in the fish farming industry. The sporadic data that do exist do not show conclusively that there has been a wide scale increase in the abundance of organisms responsible for harmful blooms in Scottish waters, although the number of reported incidents of toxicity (as opposed to abundance of toxic organisms) has increased. East coast Paralytic Shellfish Poisoning (PSP) incidents seem no worse than reported for 1968-1990 and the apparent spread of toxicity on the west coast and in the Northern Isles may be a result of wider monitoring or spread of toxic strains amongst existing populations. There is no evidence that the causative organism is becoming more abundant at new or traditional sites. Increased Amnesic Shellfish Poisoning (ASP) toxicity similarly might be attributed to increased levels of toxin monitoring.

3. Three main concerns are raised against the aquaculture industry: a) plant nutrients from fin-fish farms have led to an increased occurrence of algal blooms; b) plant nutrients from fin-fish farms have disturbed the natural ratios of nutrient elements in sea water so favouring the occurrence of toxic species over harmless algae and c) plant nutrients from fin-fish farms have made potentially toxic algae more poisonous. Lack of long term observational data preclude direct comparison of nutrient and phytoplankton levels from pre-fish farm development times and the present day. Modelling studies, however, have shown that only a few sea loch sites are strongly enriched with nutrients to such a level that they might exceed environmental quality standards but, in the main, enrichments are low. In addition, models have shown that the algal production attributable to fish farm nutrients in Scottish coastal areas is small relative to that generated by marine and terrestrial nutrient inputs.

4. It is concluded that the present level of fish farming is having a small effect on the amount and growth rate of Scottish coastal phytoplankton but that this effect should not be a cause for concern except in a few, heavily-loaded sealochs.

5. Despite many studies of algal growth in the laboratory, we are still a long way from understanding what controls the balance of organisms within the plankton but some broad aspects of the balance can be predicted. For those algae associated with eutrophication ( Gymnodinium mikimotoi, Phaeocystis pouchetii and toxic flagellates) substantial blooms do seem to be stimulated by nutrient enrichment and increases in the ratio of nitrogen and phosphorus to silicon. That the abundances of the toxic species of Alexandrium, Dinophysis and Pseudo-nitzschia are related to changes in nutrient ratio in the field remains speculative.

6. The perturbing effect of fish farm waste on nutrient element ratios in most Scottish cases can be shown to be small. Typical farm waste has a ratio of nitrogen to phosphorus which is close to natural ratios. However, there is a possibility that because of the absence of silicate in fish foods there may be a danger of exceeding the "safe" N:Si limit of 2.5 locally at heavily enriched sites in summer when background nutrient levels are low and silicate has been drawn down by the Spring Bloom. However, modelling studies suggest that broad area effects should be small. Similarly there is no convincing evidence to suggest that changes in nutrients as a result of fish farm inputs ratios are likely to stress potentially toxic species to cause them to increase their toxicity.

7. Our conclusion is that, except perhaps in a few enclosed waters, enrichment by fish farm nutrients is too little, relative to natural levels, to have the alleged effects. However, we cannot, as we would wish, always support this conclusion with data from series of measurements of nutrients, phytoplankton, algal blooms, and the presence and toxicity of harmful species, made at key sites over the several decades that span the development of the current fish farming industry.

8. We identify 3 key areas for future research:

a) New data collection focused at (i) sites for which significant data sets exists prior to fish farm expansion (pre-1984) and (ii) sites representing a range of nutrient loadings from fish farms. Such data might be analysed statistically and compared with predictions from mathematical models.

b) Studies to better understand the fundamental biological, physical and chemical processes in Scottish coastal waters. These include (i) investigations of variability in nutrient inputs from the Atlantic Ocean and Irish Sea sources; (ii) investigation of rates of loss of nutrients through processes such as denitrification; (iii) studies of physical exchange processes between sea lochs, voes and open coastal waters; (iv) improvement in the understanding of the role of pelagic protozoa on development of harmful blooms and (v) studies of the biology, toxicology and ecology of important Scottish harmful species and in particular Pseudo-nitzschia spp.

c) Continued development of simple, robust mathematical models that can predict ecosystem changes considered undesirable under the Water Framework Directive and OSPAR Comprehensive Procedures.

9. The cultivation of non-finfish species has few measured, negative environmental impacts, and those that have been recorded are restricted to the vicinity of the farm site. As this type of culture extracts nutrients from the marine system, carrying capacity considerations should be focussed on the extent to which the environment can supply these nutrients. It is likely that the cultivation of non-fish species can, to some extent, help reduce nutrient inputs from other activities including fish culture.

10. There is currently insufficient information available to determine the long term effects of medicine and antifoulants use. Further research is required into the effects of these products over the long term, particularly where multiple sources enter the same marine area. In the short term, the environmental risk is considered to be low if sea lice medicines and antifoulants are used according to regulatory guidelines but, in the case of antifoulants, more information is required relating to their use by the aquaculture industry.

11. Wild salmon and sea trout are at risk from infective larval sea lice that may be associated with marine salmon farms. Salmon are most at risk in long fjordic systems where they have to pass several farms during their migration to sea. The transfer of other parasites from farmed to wild fish is not thought to be a major problem at present. The introduction of the parasite Gyrodactylus salaris from Scandinavia would probably devastate the Scottish wild salmonid population although it is not thought that transfers relating to farming represent the only or greatest risk of introduction. The potential exists for transfer of infectious diseases such as Infectious Salmonid Anaemia (ISA) and Infectious Pancreatic Necrosis (IPN) from farmed to wild stocks but the real level of risk is not quantifiable given present knowledge.

12. Escapees from fish farms may interbreed with wild population resulting in losses of genetic variability, including loss of naturally selected adaptations, thus leading to reduced fitness and performance. Non-local genes have been introduced into wild salmonid populations for over a century, as a consequence of restocking programmes intended to increase population sizes. However, the effect of these programmes is probably insignificant compared with that caused by farm escapes simply owing to the large scale of escapes in comparison with the wild populations. Escapes from salmon farms, therefore, constitute a major threat to wild populations. Current methods to reduce fish farm escapes by reducing net damage from predators include the use of acoustic deterrents to exclude seals from the farm area. While these probably have no great consequence for seal populations they may exclude whales, dolphins and porpoises from a much larger area owing to their greater sensitivity to underwater acoustic noise.

13. The issues concerning the use of industrial fishmeal and fish oils in artificial pelleted diets in the Scottish salmon farming industry are wide-ranging and complex. Although aquaculture production is predicted to rise significantly over the next decades, catches from industrial fisheries are set to remain static in volume. Forecasts differ, but there are concerns over how the Scottish salmon growing industry may perform if fishmeal and/or fish oil supplies become limited. Firstly, the aquaculture industry in Scotland is relatively a very small component in the global aquaculture field and could be badly affected by global trends. Approximate estimates suggest that the proportion of the global fishmeal use attributable to the Scottish salmon industry is less than 0.8%. Secondly, the Scottish salmon industry is probably running at very low profit margins and is unlikely to sustain fish feed price rises as easily as sectors with higher margins of profit. Fish feed companies have been well aware of these two points for many years and research on fishmeal and oil alternatives is well advanced. However, because of the near-market nature of that research and development, there is little published literature on which to base a thorough assessment of the current status of alternative feed types. Therefore, current and forecasted future market forces have already created a situation where fish feed suppliers are actively developing alternatives to wild fishery sources of fishmeal and fish oil.

14. 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.