New research suggests grey reef sharks behave differently depending on lunar cycle. Sharks remain in deep water during full moon, moving to shallow water with new moon.
In a new study, scientists have revealed the diving behaviour of sharks appears to be influenced by the moon, water temperature and time of day. The study was conducted with grey reef sharks, a species commonly found on coral reefs in northern Australia and the Indo-Pacific. Tagged near Palau, about 40 grey reef sharks consisting mostly of adult of females were followed by scientists from UWA and the Australian Institute of Marine Science, who recorded their movement and diving patterns over a two-year period.
Findings published in the journal PLOS ONE revealed the sharks descended to both a greater and a wider range of depths around the time of the full moon. Diving was also affected by seasonal changes, as the group was recorded diving to an average depth of 35m in winter and 60m in spring.
"To our knowledge, this is the first time such patterns have been observed in detail for reef sharks," says lead researcher Gabriel Vianna of the University of Western Australia (UWA) in Perth.
Remaining closer to the surface in winter, the sharks moved to a range of depths during summer. Being cold blooded, they may prefer warmer water to conserve their energy with warm water providing optimal conditions for foraging for food.
The findings also suggest that the time of day could influence how deeply sharks dive.
"We were surprised to see sharks going progressively deeper during the morning and the exact inverse pattern in the afternoon, gradually rising towards the surface," said Vianna. The behaviour may relate to how much light is reflected on the reef at different times during the day. Better knowledge of shark behaviour could help reduce the risk of sharks coming into contact with locals and tourists fishing.
"In places such as Palau, which relies heavily on marine tourism and where sharks are a major tourist attraction, the fishing of a few dozen sharks from popular dive sites could have a very negative impact on the national economy," Vianna added.
To our knowledge, this is the first time such patterns have been observed in detail for reef sharks.
—Gabriel Vianna, University of Western Australia-