Limnetica 37

The Chinese mitten crab (Eriocheir sinensis, Milne Edwards, 1853) is a highly invasive catadromous species established in the Tagus river. Since there is insufficient data about the species in this invaded area, the aim of this study was to characterize its population structure and reproductive dynamics. This population is at a much lower latitude than most other well studied European E. sinensis populations, therefore strong differences should be expected, possibly affecting the choice of management strategies. E. sinensis bycatch was obtained in the upper Estuary using fyke nets on an approximately monthly basis from September 2013 to October 2014. Population structure, female gonad development, fertility, condition and growth were analyzed. An overall sex ratio of 1.54 (M/F) was found, but it oscillated significantly along the year. Ovigerous females were found from March to May, mostly during a period of low flow and high salinity (3-7 PSU). The average number of eggs per female was 36 350, which is low when compared with other areas where the species is established. Loss of appendages was observed in 44.4% of the crabs, potentially affecting its commercial use. In conclusion, this well-established short lived population, presents an unusually low fertility which may explain why there have been no remarkable population explosions. To decrease the risk of larvae transport by ballast water from the Tagus to other nearby estuaries seasonal control measures could be applied from April to July.