The Commercial Exploitation of the Chinese Mitten Crab Eriocheir sinensis in the River Thames, London: Damned if We Don’t and Damned if We Do

Abstract

The invasive Chinese mitten crab Eriocheir sinensis is now well established in the River Thames, London. This crab is considered to be a delicacy in many SE Asian countries and their expatriates when its gonads are ripening and fully developed prior to mating. Sexual maturity is attained during the autumn months as mitten crabs migrate in large numbers from freshwater streams to the higher salinity brackish waters of estuaries where mating occurs. Up to US$ 40 can be paid for a single mature female mitten crab in the right condition at high-class restaurants in China, Japan and Singapore. Consequently, commercial exploitation of mitten crabs may be a viable method of reducing the population size in the Thames catchment. A recent pilot study concluded that Thames mitten crabs were fit for human consumption and that the population is large enough to be exploited. There are, however, still major issues that need to be discussed, including whether we really want to start a mitten crab fishery in the Thames. Basically, we are damned if we do and damned if we don’t.

Mitten crabs appear to have few natural enemies in the River Thames that are capable of considerably reducing their numbers. If exploitation of mitten crabs does not go ahead therefore, evidence suggests that the population will continue to increase in numbers and expand its distribution westwards in the Thames catchment. The density of mitten crab burrows already recorded in unprotected river banks will probably proliferate, causing more erosion. Furthermore, larval densities will continue to rise in the estuarine plankton, increasing the risk of uptake into ballast water for subsequent risk of dispersed into new watersheds nationally by local coastal shipping and to other continents via trans-oceanic vessels.

If commercial exploitation is regarded as a suitable method by which to reduce numbers of Thames mitten crabs, then another set of potential environmental impacts of this activity needs to be addressed. For example, the European Commission has initiated an Eel Recovery Plan (Council Regulation 2007b, No 1100/2007) to try to return the European eel stock to more sustainable levels of adult abundance and glass eel recruitment. However, during a pilot fisheries study using fyke nets, the eel proved to be a significant part of the Thames mitten crab by-catch. Consequently, commercial exploitation of mitten crabs could be to the detriment of the Thames eel population as fisherman are unlikely to return such a valuable catch back to the river. Moreover, if the Thames mitten crab fishery were to be a financial success, there is a real concern that this contentious immigrant could be intentionally dispersed throughout other UK watersheds, as has been the case, for example, of the invasive freshwater crayfish, Pacifastacus leniusculus. Furthermore, a mitten crab fishery would require strict controls with respect to fishing equipment, fishermen, traders and restaurants including the possible restriction of live imports of Eriocheir sinensis into the UK.