Distribution patterns of the Chinese mitten crab (Eriocheir sinensis H. Milne Edwards, 1853) in Lake Vanern, Sweden

The catadromous Chinese mitten crab (CMC), Eriocheir sinensis is well known for its extensive invasion routes across the world. However, little is known about both adult and juvenile behaviour after they arrive to a new region. Particularly if the CMC has utilised freight ship ballast tanks as its invasion vector to new freshwater areas like coastal connected larger lakes. The Swedish Lake Vanern, Europe's third largest freshwater lake, offers a suitable study area since only a handful of CMC had been reported between its first record in 1954 and 2004. Hence, the increased catch of of CMC in the mid 2000s was unexpected and provided a rare opportunity to study the initial phase of a biological invasion. Fortunately local fishermen have traditionally, since the mid 1970s, utilised large stationary fish trap nets, evenly distributed from the inlet to the harbour of Lidkoping outward into the main part of the lake. During the peak occurrence in 2005 the traps captured CMC frequently for 90 days starting on August 10. Daily catch increased from September 19th to October 17th. Thereafter the number decreased until November 7th when the last crab was captured. Only one crab out of the 21 caught in the two traps furthest away from the harbour inlet was caught before September 19th. The number of caught CMC differed significantly between the trap nets. Almost half (48.4 %) of all CMC were caught in the two traps closest to the harbour inlet and 41.9% in the second trap-line, consisting of two traps 6 km from the harbour inlet. The remaining crabs were caught in the traps furthest away. Catch pattern from this unique invasion event is discussed in relation to CMC dispersal/migration, invading sample size, behavioural traits and catch efficiency of traps.