Sharing the menu: sharks take shifts

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Large coastal sharks engage in ‘shift work’ to share their resources, according to a new study from Murdoch University’s Harry Butler Institute.

The study, published in Paroceedings of the Royal Society B, tracked six large coastal shark species living in the balmy waters of the Gulf of Mexico, and found that, in areas they share with one another, the members of each species time their hunting periods to avoid overlap.

‘What we found was there’s this group of species of sharks that live in the same place at the same time, and they eat pretty much the same thing,’ says Karissa Lear, a co-author of the study.

The team caught a total of 172 animals, tagging their dorsal fins with acceleration data loggers – similar to a Fitbit or the health app on a smartphone.

‘That technology that counts your steps all day long, it’s that same technology but on a shark,’ says Lear. ‘We can see each tail-beat the shark makes and changes in posture, and we basically get a metric of overall movement that the fish makes, so you can see when they’re foraging.’

Despite sharing similar prey and overlapping in their seasonal haunts, the sharks were able to essentially ‘share’ the fishy banquet by taking shifts to avoid one another’s primary hunting times. For example, the team found bull sharks took the early shifts, tiger sharks hunted under the midday sun, blacktip sharks were most active during the evening hours, and hammerhead sharks hunted in the dead of night.

According to the authors, this behaviour is a form of ‘niche partitioning’, a well-documented mechanism in the animal kingdom that allows competitors to coexist by dividing resources. Niche partitioning can commonly take many different forms, including specialising in different food or prey, using different areas to forage or hunt, or temporal partitioning (as in the case of these sharks), in which species rotate their peak foraging times to minimise overlap.

The paper is the first time this type of temporal-partitioning behaviour has been described among marine predators.

Lear says that both climate change and commercial fishing put many shark species in danger, so understanding their temporal variability may be key to their protection.

‘A lot of research has shown that healthy marine ecosystems often have a lot of predators, and so understanding how all of these species coexist is important to understanding how we can promote those circumstances,’ she says.

‘It’s really important for us to understand what might happen to an ecosystem if, for example, a species of shark was removed, or if an important prey species was removed because it’s been overfished.’