Citizen science program detects range expansion of the globally invasive European green crab in Washington State (USA)

The European green crab, Carcinus maenas, has been established on the west coast of North America since at least 1989, yet populations were limited to coastal embayments for more than two decades. In 2012, the first population was identified within the Strait of Juan de Fuca, which provided the impetus to develop a citizen science program to monitor for invasive green crab within the inland marine waters of Washington State (USA). In 2016, 116 volunteers monitored 26 sites using baited traps and visual surveys. On August 30, 2016, a single live male crab (74 mm carapace width) was captured in Westcott Bay on San Juan Island, Washington, by volunteers - the first detected range expansion in these inland waters. In September 2016, an additional crab was collected in Padilla Bay, Washington. The citizen science program and subsequent rapid assessment efforts by multiple partners found one green crab molt carapace in Westcott Bay, and three additional live crabs in Padilla Bay. Based on our results, the current extent of the invasion might be spatially and numerically restricted, but the occurrence of green crab in the San Juan Islands and Padilla Bay could portend future establishment of the species in the inland waters of Washington State and elsewhere in the Salish Sea. The citizen science program and rapid assessment efforts serve the dual purpose of providing ongoing monitoring and limited control in habitats vulnerable to invasion.