Both forms of aquaculture, finfish and shellfish, take place in shallow inlets, bays and lagoons. In the UK, aquaculture has become most highly developed in Scotland, and in particular in the sea lochs and voes of mainland west coast, Western and Northern Isles. There are a variety of potential environmental impacts from aquaculture (NCC, 1989) however for the purposes of this review discussion below is limited to impacts on the benthos.

Finfish Culture - Atlantic salmon (Salmo salar) is the most commonly farmed species although there are farms for halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maxima). The farming of cod is currently being investigated. The overwhelming majority of farms consist of floating cages at sea although there are some land-based farms utilising pump-ashore technology.

To date, studies have shown that the most obvious benthic impacts of finfish culture relate to the depositition of organic material (faeces and uneaten food) and dispersion of nitrogenous wastes in solution. Benthic impact has been well documented and tends to be restricted to the immediate vicinity of the cage group, with the extent and severity of impact being most pronounced at low energy locations where water exchange and/or wave action is limited. Monitoring practices at farm sites are closely related to the extent of this "footprint" on the seabed. As well as impacting on the benthos, the release of hydrogen sulphide from anoxic sediments below cages has implications for the health of the farmed fish⁸² Over the past few years a trend has developed in the salmon industry away from the most sheltered sites to those with greater tidal exchange which helps to ameliorate direct impact on the benthos. Studies on the recovery of the benthos following organic enrichment from salmon farming indicate varying periods of recovery depending on prevailing hydrographic conditions, with the majority of sites studied showing some recovery within two years. Clearly, pump-ashore farms offer the potential for treatment of effluent prior to discharge.

A further potential impact on the benthos within shallow inlets and bays arises from the use of chemicals and medicines. A variety of compounds are employed ranging from anti-fouling treatments to antibiotics and treatments for sea lice infestation of salmon. Anti-biotics are of concern due, for example, to their potential to impact on microbial processes and through the development of drug resistance in fish pathogens.

There are concerns about the potential impacts on benthic communities in proximity to salmon farms from the discharge of medicines used to control sea lice infestations as active compounds are effectively being discharged directly into the environment. These medicines can be split into two broad categories: those delivered orally in medicated feed, and bath treatments which are added to the cages in solution. The most commonly used sea lice treatment has been the organophosphate dichlorvos, a bath treatment, although use has declined due to "Red List" status and the development of resistance in sea lice. Studies on environmental effects of dichlorvos demonstrated sublethal effects in intertidal invertebrate communities. Other bath treatments are currently in varying stages of development with cypermethrin and azamethiphos being recently authorised for use. In general, it is probably true to say that the greater dispersive characteristics of high energy sites are beneficial in ameliorating the impact of bath treatments. Sites with restricted exchange (lagoons) can be considered most vulnerable. In-feed treatments have a direct route to the benthos via any uneaten food. Recent studies of one such compound, ivermectin, demonstrated mortality in sediment dwelling worms with potential consequences for the recovery of the seabed ^82.

A major research study has recently been started to investigate the environmental impact of sea lice treatments but no results are yet available with which to inform this review.

Shellfish - A number of different methods of shellfish cultivation are used in UK waters with issues for consideration at the seed collection, on-growing and harvesting stages of the process⁶⁴. Depending on the species, molluscs may be suspended in lantern nets, laid in trays or poches (large meshed sacks) on the shore, attached to ropes suspended in midwater or relaid in more suitable areas for re-growing. Collection of seed mussels is not considered to have an environmental impact in the UK as it is not extensive and only licensed from unstable beds⁶⁴. In the Wadden Sea however, massive mortalities of eider ducks have been associated with greatly reduced mussel stocks as a consequence of harvesting spat for aquaculture⁸². Intertidal collection may result in some effects such as from trampling and disturbance of foraging birds.

There has also been concern about the inadvertent introduction of alien species (such as the seaweed Sargassum muticum) on shellfish which are imported as seed stock for cultivation.

The effects of on-growing depend on the habitat, type and scale of cultivation. Changes in sediment composition and benthic community structure have been observed under long-lived cultures of Mytilus edulis for example. A three year study showed that faecal matter and detached mussels increased sedimentation under the lines at a rate of 10 cm/yr. The effects on the sediment under the culture were reduced grain size, high organic content and a negative Redox potential. Benthic fauna were replaced by opportunistic polychaetes and only limited recovery was observed when the site was re-sampled 6 months after harvesting⁸⁹. In these respects the effects are similar to those beneath finfish cages.

Examination of the sediment structure and the infauna beneath Manila clam lays revealed no significant differences in particle size, organic content or photosynthetic pigment between control areas and the lays while the clams were growing²⁰. There were also no significant differences in the faunal diversity beneath the lays when compared to control sites, but there was a greater density of benthic species under the lays. The infauna were dominated by deposit feeding worms, Lanice conchilega, and the bivalve, Mysella bidentata, compared to the white ragworm, Nephtys hombergii, in the control area. In another study, species effects were seen in the first 6 months with the infauna dominated by opportunistic species⁹². The nets used to contain the clams and provide protection from predation, increased sedimentation and settlement of green macroalgae and are likely to have had a major influence on some of the infauna⁹². Effects on benthic communities of small scale culture may be limited and localised. If the area covered is large there is potential for conflict with bird feeding or roosting sites⁶⁴.

The harvesting stage of cultivation has also raised various concerns relating to physical disturbance. Harvesting of clams by hand raking has been reported as causing a 50 % reduction in diversity and abundance of infauna⁹⁷. Suction dredging may be another method which is used. In one study this caused an 80-90 % reduction in non-target fauna and left a trench 10 cm deep²⁰. A sediment plume was created but reduced to background levels within 40 days. Regeneration of species diversity and abundance, after harvesting in the winter, was completed by the summer - a period of 7 months. Natural sedimentation had nearly restored the sediment structure to pre-harvesting conditions after 4 months suggesting that there may be minimal long term effects if sites are left to recover. In Scotland Manila clam has only been trialed; no commercial production has taken place. Restricting harvesting to early winter could ameliorate site restoration if the main mechanisms for recolonisation is by larval settlement.

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