Green crab control: Reduction of an invasive population using a Before-After, Control-Impact designed removal

While many efforts to eradicate large and connected populations of nonindigenous species (NIS) in terrestrial systems have been successful, equivalent efforts have not been mounted in marine systems. This project tests the effects of locally removing the European green crab (Carcinus maenas), which was first introduced to Bodega Harbor, CA in 1994. Since June 2006, we have removed more than 10,000 C. maenas through an intensive sampling and removal effort using a Before-After, Control-Impact design. We compared Catch per Unit Effort (CPUE) of C. maenas in Bodega Harbor with control bays, Elkhorn Slough, Tomales Bay, and Bolinas Lagoon before and three times in the year after the start of removal. CPUE decreased more in Bodega Harbor than in control bays. Furthermore, CPUE in Bodega Harbor has steadily decreased and we have removed over 67% of crabs marked in an initial mark-recapture study. Continued removal has also affected the demography of trapped crabs: their median size decreased substantially and their sex ratio shifted from a male to a female bias. This project is also tracking how NIS management affects native biota. We tracked native shore crab, Hemigrapsus oregonensis, demographics and predation on tethered shore crabs before and after C. maenas removal. Higher levels of predation on native shore crabs initially were correlated with high C. maenas catch. However, after removal, predation was no longer correlated with C. maenas abundance. Thus, our trapping efforts appear to be controlling the C. maenas population in Bodega Harbor and positively affecting native biota.