Temperature-induced range expansion of a subtropical crab along the California coast

We describe the range expansion and first occurrence of the subtropical crab Portunus xantusii (Stimpson, 1860) in northern California during 2016 and link the range expansion to the regional extreme water temperature event during this time. We collected P. xantusii occurrence data from crab trapping surveys conducted along the California coast as well as incidental observations by fishermen and SCUBA divers. We then analyzed 10 years of regional offshore temperature patterns using National Data Buoy Center data around the trapping region. We also examined evidence of northern California warm water refugia using sensors monitoring Tomales Bay, Elkhorn Slough, and San Francisco Bay. We found that P. xantusii was present in every major estuary north of Monterey Bay and as far north as Tomales Bay and that the documented range expansion was likely due to the unusual oceanographic event that occurred northern California during this time period. Mean offshore temperatures and mean nearshore temperatures during 2014-2016 were about 2 degrees C (one standard deviation) higher than the 2006-2013 mean, with extreme temperatures reaching three standard deviations above the 2006-2013 mean. We suggest that this unusual warm water event permitted survival of dispersing larvae of P. xantusii larvae northward via coastal currents, and that the extended warm water period allowed P. xantusii to complete its development. Long-term crab trapping programs in place since 1994 within this region provide robust support for the absence of P. xantusii prior to 2016. Temperature data indicate that the estuaries in which adult P. xantusii was found could allow persistence of adult P. xantusii in northern California.