Not all marine biogeographic boundaries track temperature in an eastern boundary current

Marine biogeographic boundaries are often treated as thermal limits that should shift poleward with warming. We tested this along the Pacific coast of California and Baja California with seasonal plankton surveys (CalCOFI, 1951–1969) and hydrography measured at the same stations and, for 1997–2012, by the IMECOCAL program. Three larval fish assemblage boundaries behaved differently. The Point Conception boundary lay at 32°N in winter and 35°N in summer and autumn, moving with the 14.75°C isotherm (unresolved in spring); temperature at the boundary varied by 0.7°C among seasons, against 2.2–3.0°C for a fixed boundary. The Punta Eugenia boundary stayed at 29°N in every season; the salinity edge near it shifted by several degrees between years, but not consistently with regional temperature. The southern Baja California boundary sat on the strongest front of the coast and barely moved. Boundaries on one coast differ in mechanism and should respond differently to warming.