A study of the naturalisation and dispersal of a non-native bivalve, the Manila clam, Ruditapes philippinarum (Adams and Reeve 1850) in estuaries along the south coast of England

The Manila clam, Ruditapes philippinarum, was introduced into the British Isles in the 1980s for the purpose of aquaculture in order to take advantage of the rapid growth rate and high profitability of this non-native species. The decision to import the Manila clam, was based on the findings of a report commissioned by the Ministry of Agriculture Fisheries and Food which determined that the Manila clam would be able to be grown to marketable size faster than the local species Ruditapes decussatus, but would not be able to reproduce due to the inclement seawater temperature regime around the British Isles. Within four years of its introduction into Poole Harbour in Dorset, the Manila clam had successfully reproduced and by 2004 it had naturalised in the Harbour. This thesis sets out to determine the factors that influence reproductive and recruitment success of the Poole Harbour population of Manila clams and to determine what factors could influence the further dispersal of the Manila clam along the South and South Eastern coastline of England. The population of Manila clams at Holton Mere in Poole Harbour was quantitatively sampled on a monthly basis, with some omissions, from June 2009 until August 2012. For each sample month, the density and population dynamics of the population were estimated. From February 2010 until August 2012 the condition index of the population was estimated on a monthly basis. The population dynamics of the Holton Mere population were influenced by fishing pressure with the removal of the majority of clams over the minimum landing size of 35mm. Condition index followed a seasonal pattern with high condition in summer and low condition in winter. Recruitment success in the Holton Mere population was inconsistent, with successful recruitment events coinciding with higher levels of condition index. A study to correlate environmental parameters with clam condition index, using long term monitoring with a multi-parameter sonde were undertaken in Poole Harbour between July 2011 and July 2012. Seawater temperature and food availability was found to positively correlate with condition index. Experiments determined that predation by the European shore crab, Carcinus maenas has the potential to influence the success of recruitment events due to high levels of predation on newly settled and juvenile clams. Reduced salinities and low temperatures influence the rate of predation of juvenile Manila clams by crabs. Salinity influences behaviour in both adult and larval Manila clams, with burial by the adults and swimming in the larvae ceasing at salinities below 18psu. Manila clam larvae actively swim through haloclines into areas of reduced salinity. Manila clam larvae are able to tolerate salinities as low as 10psu for 24 hours with low levels of mortality. The Manila clams’ tolerance to reduced salinity allows it to colonise areas of marginal habitat where competition is low. Predicted increasing seawater temperatures will allow the Manila clam to extend its range northwards by causing cold water species to vacate ecological niches for the Manila clam to occupy and to improve the consistency of reproductive success. The Manila clam is now established along the Southern coastline of England and is unlikely to disappear. As such it should be classed as a naturalised species and managed in the same way as native species. The Manila clam is likely to spread northwards in the future and will provide both economic and ecological benefits in the form of new fisheries and also prey for local species including wading birds. The high growth rate and versatility of the Manila clam would allow it to be used in polyculture systems and be grown in habitats that were previously deemed unsuitable for bivalve culture.