Case studies UK

Case studies elsewhere

There are both direct and indirect effects of various forms of marine aquaculture on kelp biotopes. These can be summarised as:

• the collection of non-kelp species as feed material or for stock
• nutrient input from uneaten food, faecal and urinary wastes from fin-fish cages, crustacean cages and around shellfish rafts and trestles
• poisoning from chemicals used principally to control disease, sea lice and micro-organisms (Beveridge et al., 1997)
• siltation from detrital waste

Gowen & Bradbury (1987) have reviewed the ecological impact of salmonid farming.

Material used to stock an aquaculture system or given to the farmed species as food and then not removed at harvest, is released as waste material. Food losses vary with species and system, food conversion ratios being a good indicator of loss to the environment. Direct measurements, using video recording techniques, suggest that, in the cages of intensive marine salmon farms, some 5-10% of food is typically not eaten In poorly run farms the figures may be much higher (Beveridge et al., 1997), due to currents rapidly dispersing the food beyond the reach of the fish. However, organic and chemical wastes are now being reduced. In salmon farming, for example, nitrogen and phosphorus wastes per unit production have been steadily falling thanks to improvements in feeding practices and feed formulation. Food conversion ratios (weight of food fed : biomass of fish produced) have dropped from 2.4 : 1 to 1.3 : 1 over 25 years, while dietary nitrogen and phosphorus levels have fallen from around 7.8% to 6.8% and from 1.7% to 0.8%, respectively, over the same period. Scope for further reductions is limited, however; as phosphorus inclusion levels in many commercial feeds already approximate the dietary requirements and nitrogen levels in diets are, if anything, increasing and so excess nutrients are released into the environment.

Although fish cages are usually positioned over sedimentary substrata, plumes of waste could stream into kelp forests, leading to anaerobiosis due to the oxygen demand of the decomposing material. The detrital rain from the cages could act in a similar way to terrigenous silt, reducing light penetration through the water column and smothering the algal surfaces. As a minimum impact the localised increase in nutrient levels might produce local eutrophication effects, particularly at slack tide.

A wide variety of chemicals are used by the mariculture industry, including compounds employed in and applied to construction materials (e.g. plasticisers and antifoulants), pesticides. (e.g. "Ivermectin" used to control sea lice on farmed salmon) and chemotherapeutants. Potentially the most serious effect of marine aquaculture could be the impact of anti-microbial compounds on benthic ecosystems, in view of the importance of detrital cycling, but as yet the microbial ecology of kelp biotopes is little known.

We believe there are unpublished environmental impact assessments for Scottish fish farms. Such information is regarded as commercially sensitive and is not generally available. There do not appear to be any impact reports on the effects of other forms of marine aquaculture.

None known.

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