Population dynamics of naturalised Manila clams Ruditapes philippinarum in British coastal waters

The Manila clam Ruditapes philippinarum was introduced to Poole Harbour (lat 50 degrees N) on the south coast of England in 1988 as a novel species for aquaculture. Contrary to expectations, this species naturalised. We report on individual growth patterns, recruitment, mortality and production within this population. On the intertidal mudflats the abundance of clams (> 5 mm in length) varied seasonally between 18 and 56 individuals m(2). There appear to be two recruitment events per year and there were 6 year classes in the population. A mid-summer decline in abundance was partly due to increased mortality but probably also a result of down-shore migration in response to high water temperatures and the development of anoxic conditions. A winter fishery removes c 75% of clams of fishable size (maximum shell length >= 40 mm) and c 20% of the annual production. The fishery depresses the maximum age and size attained by the clams but appears to be sustainable. Clam mortality due to factors other than fishing is highest in late-winter to early spring. The growth of the clams is intermediate in comparison with many published studies but remarkably good given their intertidal position. As on the coasts of the Adriatic Sea, where the clam is also non-native, the Manila clam has thrived in a shallow, eutrophic, lagoon-like system on the English coast. While the Poole Harbour population is currently Europe's most northerly reported self-sustaining, naturalised population, given forecasts of increasing air and sea temperatures it might be expected that this species will eventually spread to more sites around the coasts of Northern Europe with associated economic and ecological consequences.