There are concerns about the environmental impact of aquaculture to the seabed close to fish farm cages, including potential effects from persistent organic pollutants and trace metals. Samples of sediments from aquaculture areas in the Western Isles (3 sea lochs), the north-west coast of Scotland (3 sea lochs) and from the Shetland Isles (3 sea lochs or voes) were collected in 2005 and 2006 and concentrations of persistent organic pollutants (polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs)) and trace metals (copper and zinc) were determined.

The results indicate that any impact on the environment from organic pollutants or trace metals is of a local nature. Concentrations of PCBs are below the EAC (environmental assessment criteria) for all bar one congener in one sample, and below the BAC (background assessment concentration) for most. Although slightly elevated Cu and Zn concentrations were measured in sediments from <330 m from some farms, concentrations in sediments outwith the 25 m AZE (allowable zone of effect) were below those that cause benthic effects. Only the farms in Loch Shell, Loch Greshornish and Loch Kishorn had concentrations for Cu and Zn which might cause adverse affects in the environment, and all of these samples were from within 25 m of the cages.

Of the individual lochs, sediments from Loch Shell, Loch Kishorn and Swinister Voe had Total Organic Carbon (TOC) inversely proportional to particle size (% <63 µm fraction) with the TOC highest at the cage edge, suggesting a measure of organic enrichment below these cages. TOC was proportional to particle size (% <63 µm fraction) for the other fish farms in the study, suggesting that there was no organic enrichment at these fish farms. BACs were exceeded for two thirds of the sites, though not for all congeners.

BACs were not exceeded for any congeners in any sites in Collafirth Voe. Concentrations were below the EAC for all except one site, at the head of Loch Ewe, where CB52 was above the EAC. Therefore, the PCB concentrations in these sea lochs would be considered to be unlikely to give rise to unacceptable biological effects.

There was no correlation between the PBDE concentrations and distance from the fish farm cage. The PBDE concentrations (sum of 9 congeners, excluding BDE209) were comparable to those of a New Brunswick, Canada, fish farm. There are as yet no assessment criteria for PBDEs therefore the significance of the concentrations could not be assessed. Only trace amounts of PBDEs were found in Collafirth Voe, Sandsound Voe, Swinister Voe and Loch Ewe (north) and for all other sea lochs in the study the concentrations are comparable with those found at remote sites around Scotland and comparable with the lower end of concentrations found in the more industrialised Firth of Clyde. Concentrations are significantly less than the concentrations from the former sewage sludge dump site at Garroch Head.

Concentrations of Cu and Zn were highest closest to fish farms and decreased with increasing distance from the fish farm. Concentrations between the lower and upper sediment quality criteria (SQC) for sites within the allowable zone of effect (AZE) indicate benthic effects which are potentially problematic. Only samples from within the AZE of Loch Greshornish and of Loch Shell had concentrations within the potentially problematic region for Zn and in Loch Shell, Loch Kishorn and Swinister Voe for Cu.