Reconstructing habitat use by juvenile salmon sharks links upwelling to strandings in the California Current Marine Ecology Progress Series

The use of nursery areas by elasmobranchs is an important life history strategy that is thought to reduce juvenile mortality and increase population growth rates. The endothermic salmon shark Lamna ditropis uses the California Current System (CCS) as a nursery area, though little is known about how juveniles use this ecosystem. Juvenile salmon sharks consistently strand along the west coast of North America. Strandings in the southern CCS occurred throughout the year, while those in the northern CCS were limited to summer and autumn, when mean sea surface temperatures were warmest. Strandings primarily occurred when water temperature was between 12 and 16 degrees C, suggesting that juveniles occupy a relatively narrow thermal niche. Stable isotope analysis (SIA) indicated that juveniles primarily feed on offshore meso-and epipelagic prey from the outer shelf, slope, and oceanic habitats as opposed to inshore and coastal habitats, although sharks appeared to move closer to shore prior to stranding. Generalized additive models indicate that the probability of stranding was greatest when mean water temperatures were relatively high (similar to 14 degrees C) and sharks were exposed to acute cold-water events (similar to 9 degrees C) during coastal upwelling. This suggests that juveniles are thermally limited and stressed by upwelling events, resulting in bacterial infections that are the proximate cause of the strandings.