A dawn peak in the occurrence of 'knifing behaviour' in blue sharks

Background: Knifing is a behaviour whereby a shark swims directly at the surface with its dorsal fin out of the water. While this behaviour has been reported in a number of species, information on the frequency and timing of such behaviour could provide insights on how sharks use the ocean-atmosphere interface. Findings: Our analysis of the timing of the reception of satellite (Argos) messages from SPOT-tagged blue sharks has revealed important insights on knifing behaviour in one of the ocean's most abundant large predators. We found that knifing behaviour was common in all tagged sharks and occurred during 54-76% of days tracked, with a mean (and SD) of 4.7±0.4 knifing events per day when observed. The frequency of knifing behaviour increased during the dawn period in all sharks and was supported by analysis of high-resolution depth data from a recovered archival tag. One shark also had a pronounced peak in knifing activity at dusk. Conclusions: We suggest that blue sharks may be using surface waters during twilight periods to maximise foraging opportunities. Light conditions at dawn are consistent with surface-dwelling prey being both more dispersed and silhouetted by ambient light conditions, making individual prey more visible. The application of this analysis to other species of sharks may provide further insights on knifing behaviour.