The moon and water temperature affect the diving behaviour of sharks, researchers reported on April 16, in a discovery that could help prevent fishermen from catching the marine predators inadvertently.
A team from the University of Western Australia's Oceans Institute and the government-run Australian Institute of Marine Science spent nearly three years monitoring grey reef sharks off Palau in the Pacific. They tagged 39 sharks - common on coral reefs throughout the Indo-Pacific region - and used acoustic telemetry to follow them, finding they stayed in deep water on full moon nights but rose to the shallows with the new moon.
Similar patterns have previously been recorded in species such as swordfish, yellowfin and big eye tuna, suggesting the reef shark behaviour was related to feeding. The study also said it may be an anti-predator response where reef sharks seek to avoid increased light nearer the surface that may aid the hunting abilities of larger sharks. "We also found that the diving behaviour of grey reef sharks was related to water temperature," said lead researcher Gabriel Vianna. The sharks, mostly adult females, dived to an average depth of 35 metres (114 feet) in winter when deeper water was colder and 60 metres in spring when temperatures warmed up. In summer, when the warmer layer of surface water expanded, the sharks tended to move in a broader range of depths. The authors said that because sharks were cold-blooded, they may prefer warmer waters to conserve energy. The research, published in the science journal PLOS ONE, also found that the time of day could 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," 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."
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