Examining Shortfin Mako and Blue Shark Movements in Relation to the Southern California Bight Oxygen Minimum Zone
In the summer months shortfin makos (Isurus oxyrinchus), blue sharks (Prionace glauca) and North Pacific swordfish (Xiphias gladius) inhabit the highly productive waters of the Southern California Bight. The result is the unintentional take of shortfin mako and blue sharks primarily due to their spatial overlap with swordfish near the surface at night. Although these species overlap spatially their movements are driven by both physical and ecological processes. These behavioral responses, if they are predictable, can be used to separate species to reduce bycatch, inform stock assessments, and predict shifts cause by human induced climate change. This paper will focus on understanding the oxygen limits for three mako and three blue sharks double tagged with Pop-off Satellite archival tags and Satellite linked radio transmitter tags in the SCB California Cooperative Oceanic Fisheries Investigation survey grid. The sharks were linked to CalCOFI stations to better understand differences in oxygen preferences. Neither species spent a large amount of time in low oxygen waters. The results suggest that blue sharks make more dives into low oxygen environments and remain there for longer periods of time than shortfin mako. Neither species exploited the deeper low oxygen waters to the extent that swordfish do. Fishing at deeper depths, in particular during daylight hours, may be a way to lessen bycatch.