OOS 28-5 - Ocean warming favors the range expansion and abundance of non-native species

Upper Ocean Heat Waves and strong El Niño events in the Pacific Northwest are linked to range expansion and increase in abundance of the European green crab (Carcinus maenas). When abundant, this successful invader has been implicated in devastating shellfish beds and degrading valuable eelgrass habitat around the world. Since their introduction into San Francisco Bay during the 1980s, green crabs now thrive in estuaries and inlets in Northern California, Oregon, Washington, British Columbia and, most recently, in Southeast Alaska. The mechanism for this rapid range expansion was larval transport by the north-flowing Davidson Current and the Vancouver Island Coastal Current. Range expansion was not gradual, but was most pronounced in years when ocean conditions were unusually warm. The 1997-1998 El Niño saw rapid range expansion to Oregon, and Washington coastal estuaries as well as to inlets on the west coast of Vancouver Island. The 2014 Upper Ocean Heat Wave, combined with the 2015-2016 El Niño, was followed by the discovery of green crabs in the inland Salish Sea, between Vancouver Island and the mainland. It was only during the winters of 2014-2015 and 2015-2016 that the Strait of Juan de Fuca was warm enough (greater than 10 degrees C) for green crab larvae to survive and be carried from the open ocean into the Salish Sea by the Olympic Peninsula Counter Current during winter storms. These two major range expansions were followed by more gradual ones during non-El Niño years. It is expected that other non-native species with planktonic larvae would follow similar patterns of range expansion.