A study of Grey reef sharks (Carcharhinus amblyrhynchos) published in the journal PLoS ONE has revealed that the Moon, water temperature and even time of day affect the diving behavior of sharks.
A team of marine scientists led by Dr Gabriel Vianna of the University of Western Australia tagged about 40 Grey reef sharks near Palau, east of the Philippines, and followed them for 2 years using acoustic telemetry.
They found that the sharks stayed in deep waters on nights of a full Moon but rose to the night-time shallows with the new Moon.
“The changes were ultimately related to feeding and possibly predator avoidance,” the scientists said.
“We also found that the diving behavior of grey reef sharks was related to water temperature,” Dr Vianna explained.
The sharks, mostly adult females, were recorded diving to an average depth of 100 feet (35 m) in winter and 197 feet (60 m) in spring.
In winter the sharks remained closer to the surface where the water was warmer and temperature more constant. In summer – when the warmer layer of surface water expanded – the sharks tended to move in a broader range of depths.
The researchers suggested that because sharks are cold-blooded, they may prefer warmer waters to conserve energy. They also found that the time of day could also affect 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,” Dr Vianna said.
“This matches how light changes on the reef during the day. To our knowledge, this is the first time such patterns have been observed in detail for reef sharks.”
“The research had conservation implications because sharks tended to congregate around reefs in many places across the Indo-Pacific and their diving behavior might make them susceptible to being inadvertently caught by people fishing at different times of the day,” Dr Vianna said.
“Better knowledge of shark behavior might help reduce this chance.”
Bibliographic information: Gabriel M. S. Vianna et al. 2013. Environmental Influences on Patterns of Vertical Movement and Site Fidelity of Grey Reef Sharks (Carcharhinus amblyrhynchos) at Aggregation Sites. PLoS ONE 8 (4): e60331; doi:10.1371/journal.pone.0060331