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

Abstract

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.

Extensive use of mesopelagic waters by a Scalloped hammerhead shark (Sphyrna lewini) in the Red Sea

“…Overall, there was no significant difference between the mean depths of the shark throughout the diel cycle. This result contrasts with those of previous studies where deep diving in S. lewini was reported to occur almost exclusively during nighttime and/or evening twilight [21,22,25,30]. Changes in diving behavior of other shark species have previously been attributed to stress reactions in response to the capture and handling process [e.g., 36].…”

“…Dietary studies of S. lewini reported high proportions of mesopelagic prey items, such as teleosts, cephalopods, and crustaceans in gut content analyses, further pointing toward mesopelagic feeding behavior in this species [32,[52][53][54]. Potential daytime feeding of the focal shark at mesopelagic depths contrasts the findings of a number of previous studies suggesting that feeding in S. lewini occurs exclusively at night, while daytime serves as a resting phase, which is spent refuging in shallow waters close to seamounts or islands [9,22,32,[51][52][53][54][55]. While data from a single individual cannot be extrapolated to the species level, the repetitive nature of continuous vertical movements performed by this individual over the entire 182-day tracking period suggests that foraging may be continuous throughout day and night.…”

“…The tagged animal conducted a cyclical migration to offshore habitats, similar to what has previously been reported for a conspecific in the Gulf of California [22]. Possible functions underlying such cyclical migratory behavior could be related to a number of factors, including social (e.g., sexual segregation and schooling), reproductive, or feeding activities [32,33].…”

“…At the same time, S. lewini populations in the western Indian Ocean appear fragmented with limited dispersal between the Arabian Seas region and other Indian Ocean regions [18]; yet, stock assessments and species-specific studies are missing in the area [19,20]. Gathering ecological information of movement data is a critical first step toward appropriate management strategies of shark species, and several studies have examined movements of S. lewini in different ocean systems (e.g., [21][22][23][24][25][26][27]). This species is known to conduct offshore migrations and to inhabit a highly expanded vertical niche in the open ocean, tolerating large fluctuations in depth, temperature, and extremely low levels of dissolved oxygen [22,23,28].…”

“…Gathering ecological information of movement data is a critical first step toward appropriate management strategies of shark species, and several studies have examined movements of S. lewini in different ocean systems (e.g., [21][22][23][24][25][26][27]). This species is known to conduct offshore migrations and to inhabit a highly expanded vertical niche in the open ocean, tolerating large fluctuations in depth, temperature, and extremely low levels of dissolved oxygen [22,23,28]. Although several studies have investigated vertical distributions of S. lewini, the tracking duration in these studies was comparatively short and data resolution coarse [e.g., 21,28].…”