Study Reveals Sharks Take Turns to Hunt
Different species of shark hunt at different times of the day. A study of six species of sharks in the Gulf of Mexico sharing the same stretch of ocean revealed that each species chose a distinct time period in which to forage for food.
Not surprisingly the tiger shark appears to have the pick, choosing midday when its preferred method of stalking its prey silhouetted against the surface is at its most effective. Bull sharks preferred mid-morning. Afternoons were for sandbar sharks and the evenings were left to the smaller blacktips. The giant hammerheads and the scalloped hammerheads shared the night.
Researcher Karissa Lear of Murdoch University in Perth, Australia, was mapping the behaviour of various shark species found in the Gulf of Mexico when much to her surprise she realised that different species were far more active at set times of the day.
It is unusual in nature for predators sharing an area to work in shifts either on land or in the sea. It had also been assumed that sharks tended to hunt from dawn to dusk as the opportunity arose.
'Very few instances of time partitioning on a daily scale have been observed,' said Lear. 'However, this could be more common than we think in marine ecosystems, which haven’t been widely studied in this way because tracking and observing underwater animals can be more difficult.'
The researchers tagged 172 individual sharks with acceleration data loggers and studied 3,766 hours of their behaviour. In a paper published in the Biological Science Journal of the Royal Society, Lear and her colleagues suggest that the distinct time periods for foraging could be driven by a combination of physiological constraints and hierarchical pressure within the group of competing sharks.
The tiger shark is the largest and most dominant of the species and it seems the others fit in around them. There was a limited amount of overlap between hunting activity amongst the six species but very clear demarcations were evident. The only two species with a substantial overlap were the hammerheads that hunted at night.
The hunting periods reflected the physical advantages and disadvantages of the various species. For example, hammerheads have superior binocular vision compared with the other species giving them an advantage at night foraging. The smaller blacktip reef sharks seem to avoid interaction with the larger species which can feed on them.
All the sharks were predating on a wide range of cephalopods, fish, rays and other elasmobranchs. Only the tiger sharks also included marine reptiles and seabirds in their diets.
The team believe it is crucial to understand the mechanisms that allow the coexistence of predators. As the natural systems of the oceans become increasingly pressured by global warming, over-fishing and pollution, the sharks are going to find it harder and harder to survive.
The report concludes: 'Understanding the mechanisms that allow marine predators to coexist will help to preserve and restore healthy, predator-rich marine systems'.