California Sea Grant College Program

Survey of the Mexican SCB Sector Artisanal and Commercial Shark Fisheries Hypotheses: a) Common thresher sharks represent a substantial portion of the catch of artisanal and commercial shark fisheries in the Mexican SCB sector. b) Exploitation of common threshers and other elasmobranchs is important to the economy of northern Baja California and, by extension, is directly linked to U.S. fishery management. Mexican SCB Longlining Survey Hypotheses: a) Thresher shark nursery grounds extend south throughout the Mexican sector of the SCB. b) Juvenile threshers in Mexican SCB waters will exhibit seasonal patterns of abundance similar to those observed in the California SCB (i.e. higher abundance in early Spring and Summer when pupping is thought to occur). Tracking and Archival Tagging Hypotheses: a) Juvenile thresher sharks utilize distinct inshore areas as nursery grounds and have a strong preference for continental shelf waters. b) Within these nursery grounds, juveniles have specific habitat preferences (e.g., substrate composition, algal coverage, topography, etc.). c) Juvenile threshers display the same diel movement patterns noted for adults, remaining in the warmer mixed layer by night, and making vertical forays by day. However, we expect that juveniles will inhabit shallower depths and higher temperatures than adults, and also swim at slower speeds. d) Juveniles undergo relatively large-scale movements that take them throughout the SCB and further south.