Ontogenetic migration of a female scalloped hammerhead shark Sphyrna lewini in the Gulf of California

Background: Little information exists on the vertical and horizontal movements of juvenile scalloped hammerhead sharks (Sphyrna lewini). Measurements of the sizes of juveniles caught in nets close to shore and those swimming in schools at seamounts and islands in the southwestern Gulf of California, Mexico indicate that at least part of the population undergoes a migration during their life cycle at a length of approximately 115 cm total length (TL) from coastal to offshore waters. Three juvenile hammerhead sharks were outfitted with archival tags in Mazatlan and La Paz Bay (LPB), Mexico during February 2006 and January 2007, respectively. Results: A single juvenile female measuring 95 cm TL tagged in LPB was recaptured. It traveled a distance of approximately 3,350 km during a 10.5-month period from LPB northward to the central Gulf of California and returning to LPB, 11.7 km north of the location it was tagged. During this migration, the hammerhead increasingly moved from shallow coastal waters to deeper waters to possibly maximize its foraging success. After moving offshore, the shark remained in shallow and warm waters of less than 30 m depth and as high as 32°C during the day, and made repeated dives to deeper and colder waters of 250 m depth and 11°C, at night. Conclusions: Our study documents a female scalloped hammerhead shark changing life history phases from a nursery-inhabiting juvenile inshore to a migratory sub-adult offshore. We also infer that this shark swam within a school of conspecifics at an offshore island or seamount during the day, and migrated away at night, diving to greater depths to feed on mesopelagic prey. We show that this female shark carried out her complete biological cycle in both coastal and offshore areas of the central and southwestern Gulf of California, suggesting maximization of foraging opportunities and continued growth. In this study, we provide evidence for the first time of an ontogenetic migration of a juvenile scalloped hammerhead shark in the Gulf of California, which could be of great significance for the regional management and conservation of sharks in the Gulf of California.