Monitoring downstream migrations of Eriocheir sinensis H. Milne Edwards, 1853 (Crustacea: Brachyura: Grapsoidea: Varunidae) in the River Thames using capture data from a water abstraction intake

The International Union for Conservation of Nature has cited Eriocheir sinensis as one of the world’s worst 100 invasive species. Outsidetheir native range, this alien species has had adverse impacts, both ecologically and economically, on river catchments. Understanding thelife cycle of the Chinese mitten crab, especially details of the migration period, is important for the potential control of this exotic species.The mitten crab has been reported from the River Thames, London, England and in this watershed the population continues to increase innumbers, disperse in a westerly direction and reports of a downstream migration date back to 1996. Recently, regular collections from arubbish screen at a River Thames water abstraction point were used to monitor the migration of adult crabs over three years (2008–2010).Details of size, sex and condition of the crabs were recorded as were data on the abstracted flow. The main migration period runs fromAugust to early November with peak numbers of crabs recorded in September/early October. In all years the sex ratio of captured crabs washeavily skewed towards males, which were significantly larger than females. Furthermore there is some evidence that female crabs movelater in the migration period than males and that peaks in numbers of both male and female crabs are associated with full moon periods; peaknumbers demonstrating significant lunar periodicity. In addition there is a weak association between crab numbers and abstraction flow rate.The key findings are used to recommend the timing of any future control measures that might be designed to reduce the population of thisinvasive species in the River Thames. These recommendations could also be applied to other river catchments where the species isproblematic