Grey reef sharks are being forced to desert their natural habitats the coral reefs as high ocean temperatures cause them to bleach, according to new UK-led research.
Abandoning reef residency at times of environmental stress is being reflected in more widespread and frequent migrations to different areas, and increasingly long periods of absence.
Such effects were seen to persist for as long as 16 months during the extreme El Niño event of 2015-2016, which caused substantial bleaching in the study region – the remote Chagos archipelago in the Indian Ocean. The scientists have expressed concern about what will happen when bleaching becomes an annual event, which is predicted to occur from as soon as 2043.
The international research team was led by marine scientists at Lancaster University and ZSL (Zoological Society of London) and funded by the Bertarelli Foundation. Between 2013 and 2020 they monitored shark movements by attaching acoustic satellite trackers to more than 120 sharks and installing acoustic receivers around Chagos coral atolls.
More than 714,000 acoustic detections were recorded and, in collaboration with Earth Observation scientists at King’s College London, these were combined with satellite data recording reef environmental stress.
Trade-off for sharks
“These results provide some of the first evidence of how reef-change in response to environmental stress, something that is becoming both more extreme and more frequent, is affecting the movement of sharks,” said principal investigator Dr David Jacoby of Lancaster University.
“Grey reef sharks are a common, resident predator to the reefs of the Indo-Pacific, venturing away from the reef to feed, but many are having to decide whether to escape the stressed reefs.
“Faced with a trade-off, sharks must decide whether to leave the relative safety of the reef and expend greater energy to remain cool, or stay on a reef in suboptimal conditions but conserve energy.
“We think many are choosing to move into offshore, deeper and cooler waters, which is concerning. Many reefs around the globe have already seen significant declines in sharks due to exploitation and this finding has the potential to exacerbate these trends.”
Complex balance
“As large predators, grey reef sharks play a very important role in coral-reef ecosystems,” said lead author Dr Michael Williamson from ZSL’s Institute of Zoology. “They maintain a delicately balanced food-web on the reef and they also cycle nutrients onto coral reefs from deeper waters where they often feed.
“A loss of sharks and the nutrients they bring could affect the resilience of reefs during periods of high environmental stress.”
“As climate change brings increasing uncertainty and more and more frequent extreme stress events, the important ecological role these predators play on coral reefs is likely to change, as they spend more time away from the reefs they are attached to,” said Dr Jacoby.
“The implications of this are not yet fully understood, but given the complex balance of species and trophic interactions that occur on coral reefs, there will certainly be substantial changes.”
Nutrient flows
One positive was that acoustic receivers at certain locations marked an increase in shark residency – one explanation for which could be guano.
Recent research in the Chagos Archipelago has indicated that some reefs have greater nutrient flows because of their resident seabirds, resulting in enhanced fish biomass and more resilience to multiple stress factors.
“Some of our receivers that were seeing a greater number of sharks residing were also near islands with seabird populations,” noted Dr Wiliamson. The study has just been published in the journal Communications Biology.
Also on Divernet: Reef sharks in greater danger than expected, Divers catch grey reef sharks napping, Chagos coral hard-hit – and first thresher spotted, 90m down: Coral bleaching is cutting deep
