On the 11th November 2006 whilst walking the shoreline of the Fleet at low tide, I noticed a large number of bivalves (Venus verrucosa, Tapes decussatus and Paphia aurea scattered along the shore between the bridging camp and Pirate’s Cove. You may well think that this is nothing unusual but all the material was freshly dead with valves united – some individuals were live or moribund. All these species do occur in Portland Harbour and can be found washed up along the shore of the Fleet near Ferrybridge, never in large numbers, and very rarely in this condition.
I decided to collect up a size range of each to add to my collection. At this point I should add that none of the specimens were fully adult. On getting home and having a closer inspection of the shells, some of which I had thought were Tapes decussatus were rather different and had the features of the manilla clam Ruditapes philippinarum.
The Fleet has a high conservation status with designation at local, national and international level, and, therefore, the possible introduction (accidental or otherwise) of this species was of great concern. I took a selection of the shells to Dan Moxon, Fleet warden at the Chesil Beach & Fleet Centre, Ferrybridge and explained the significance of the find, although neither of us were sure of the identification or the shell’s provenance. Subsequent examination by Dennis Seaward at the Fleet Study Group meeting proved inconclusive. Further discussion on the matter with David Harfield at Charmouth pointed me towards Terry Wimbleton’s article in Mollusc World 2. Terry kindly agreed to look at some of the Fleet specimens and confirmed that they were indeed sub-adult R. philippinarum.
We cannot be certain how the manilla clam has got to the Fleet. The consensus of opinion is that they have come from Poole Harbour where there is a commercial fishery for the species. It is now spreading out from the fishery and can be found around the harbour in fairly large numbers. We think it probable that the manilla clams were brought from Poole to the Fleet by rod and line anglers to use as bait - the unused live individuals being discarded at the end of the day.
The matter is now back in the hands of those responsible for the Fleet Nature Reserve for whatever action they deem appropriate. We cannot be certain whether R. philippinarum could reproduce and survive in the Fleet, as the habitat at that locality is probably unsuitable. However, it is quite possible that they will eventually arrive in Portland Harbour and the tidal reaches of the Fleet where habitat and conditions would be much more favourable.
The point of this article is that it brings home the fact of how easy it is for man to spread an alien species into a habitat of high conservation value and the impact it could have on those interests. I would hope that publicity through the local media and angling clubs, plus national angling press might stop this happening again in the Fleet or elsewhere.