Lifting Barriers to Range Expansion: the European Green Crab Carcinus maenas (Linnaeus, 1758) Enters the Salish Sea

Although the European green crab Carcinus maenas has persisted in Oregon and Washington coastal estuaries, and thrived in the inlets of the west coast of British Columbia since 1998, populations of this species had not established themselves in the inner Salish Sea, between southern Vancouver Island, the mainland, and Puget Sound. It has been hypothesized that the Strait of Juan de Fuca acts as a semipermeable barrier preventing C. maenas larvae from entering this inland sea. Most years, the water is too cold (< 10 degrees C) for larvae to develop and the predominately estuarine surface outflow flushes larvae out to sea. In late August and September of 2016, a total of five live Carcinus maenas were discovered in Westcott Bay and Padilla Bay, Washington, suggesting that this species recently entered the inland sea as larvae. Unusually warm surface water in the northeast Pacific (> 2.5 degrees C above average) from the fall of 2013 to 2015 would have allowed larvae to survive alongshore transport off the coasts of Oregon, Washington, and southern BritishColumbia. Reversals of the estuarine current in the Strait of Juan de Fuca forced by strong southerly winds associated with major storms along the outer coast in late October 2014 and 2015 could have transported the warm ocean water and larvae inward through the strait and, with the aid of local winds and tidal currents, into the inner Salish Sea. Preferential inward transport would have been with the Olympic Peninsula Countercurrent that forms along the U.S. side of the channel.