INCIDENCE OF ABNORMAL SHELL THICKENING IN THE PACIFIC OYSTER CRASSOSTREA-GIGAS IN POOLE HARBOR (UK), SUBSEQUENT TO THE 1987 TBT RESTRICTIONS

Juvenile Crassostrea gigas stock were experimentally deployed in 1989 at three locations within Poole Harbour (Dorset, UK); a mariculture and boating centre characterized until 1987 by high TBT levels, and abnormal shell thickening in C. gigas, preventing culture. After six months, the stock grown at sites adjacent to shellfish beds and featuring low boat densities, showed normal growth, whereas that grown within an embayment featuring high boat densities, showed the onset of abnormal thickening. Grossly thickened senescent C. gigas recovered from a commercial shellfish bed showed recent reversion to normal growth. These results suggest that since the 1987 UK restrictions on TBT use, levels within much of the Harbour have declined sufficiently to allow a return to C. gigas mariculture, but that levels remain too high within areas of high boat density. The study also compares the effectiveness of three established methods for assessing the degree of shell thickening.