Scalloped hammerhead shark (Sphyrna lewini) tracking records in the Red Sea

Results: The tag was physically recovered after a deployment period of 6 months, thus providing the complete archived dataset of more than one million depth and temperature records. Based on a reconstructed, most probable track, the shark travelled a circular distance of approximately 1000 km from the central Saudi Arabian Red Sea south- eastward into Sudanese waters, returning to the tagging location toward the end of the tracking period.

Mesopelagic excursions to depths between 650 and 971 m occurred on 174 of the 182 days of the tracking period. Intervals between such excursions were characterized by constant oscillatory diving in the upper 100 m of the water column. Conclusions: This study provides evidence that mesopelagic habitats might be more commonly used by S. lewini than previously suggested.

We identified deep diving behavior throughout the 24-h cycle over the entire 6-month tracking period. In addition to expected nightly vertical habitat use, the shark exhibited frequent mesopelagic excursions during daytime. Deep diving throughout the diel cycle has not been reported before and, while dive functional- ity remains unconfirmed, our study suggests that mesopelagic excursions may represent foraging events within and below deep scattering layers.

Additional research aimed at resolving potential ecological, physiological and behavioral mechanisms underpinning vertical movement patterns of S. lewini will help to determine if the single individual reported here is representative of S. lewini populations in the Red Sea.