ABSTRACT
Previous studies have shown that adult White Sharks (Carcharodon carcharias) migrate from aggregation sites near the coast to pelagic habitats situated between North America and Hawaii. Studies using satellite-linked radio transmitting (SLRT) tags have allowed for a better spatial description of this region while also delineating sex-speci¢c offshore-habitat partitioning. Although females roam widely in the pelagic habitat, males occupy a more de¢ned region called the Shared Offshore Foraging Area (SOFA). Here we report the ¢rst direct observations of the SOFA, made from a ship in June 2009, and compare these observations with oceanographic remote sensing data to characterize the SOFA habitat. White sharks tagged with SLRT tags provided real-time tracking positions to guide the research vessel toward offshore regions used by White Sharks. The timing of the expedition coincided with a seasonal contraction of the SOFA. Ship observations in the region where the sharks were detected revealed an absence of epipelagic ¢sh and small cetaceans, but Sperm Whales (Physeter macrocephalus) and three species of spawning squid (Architeuthis and two species of Ommastrephid squids) were present. Oceanographic remote sensing data showed the region to be an epipelagic cold spot, broadly characterized by downwelling conditions, reduced
horizontal motions or other dynamical processes, no major temperature fronts, and very low phytoplankton biomass. However, the presence of apex predators like White Sharks, Sperm Whales, and squid suggests that the SOFA ecosystem supports a considerable mesopelagic biomass. Oceanic hot spots are currently described primarily by epipelagic and surface observations; this study demonstrates the need to incorporate mesopelagic observations into the characterization of hot and cold spots.
