Long-term Monitoring of Polycyclic Aromatic Hydrocarbons in Mussels (Mytilus edulis) Following the Braer Oil Spill†

Abstract

On January 5, 1993, 84700 t of Norwegian Gullfaks crude oil was released into the coastal region of south Shetland when the tanker MV Braer grounded at Garths Ness. A Fisheries Exclusion Zone was designated under the Food and Environment Protection Act 1985 (FEPA), prohibiting the taking or harvesting of fish or shellfish within the Zone so as to prevent contaminated products reaching the marketplace. The criteria set for lifting of the Order were that the particular species must be free from any petrogenic taint and the concentration of polycyclic aromatic hydrocarbons (PAHs) must be within the range for reference samples. Between April 1993 and February 1995 the Order was progressively lifted for wild fish, salmon, crustacea, excluding Nephrops norvegicus (Norway lobster), and molluscs, with the exception of mussels. As part of the monitoring exercise, mussels from a reference site were transplanted in June 1995 to three sites within the Zone, where they were suspended in plastic mesh boxes from rafts to a depth of 5 m. Samples were collected at regular intervals over the following 12 months and the concentration and composition of PAHs were determined by gas chromatography with mass spectrometric detection. The total measured PAH concentration at the control site increased from 13.7 to 66.1 ng g–1 wet mass of tissue between June 1995 and February 1996. This trend was reversed by July 1996 when the PAH concentration was 12.8 ng g–1. The mean across the year for the control site was 24.0 ng g–1 (SE = 8.9 ng g–1, n = 6). A similar seasonal trend in PAH concentration over the year was observed at all sites within the Zone, but the PAH concentration was consistently greater at these sites, reaching a maximum concentration of 316 ng g–1 in February 1996. Although no taint was detected in any of the mussels, these results meant that it was not possible to lift the Prohibition Order for mussels. Further monitoring at three sites outwith the Zone and three sites within the Zone is under way together with investigations into the specific source of the PAHs.