Reviewed by Danielle Ellis, B.Sc.Sep 10 2024
In a study that was published in Communications Biology, researchers from Lancaster University found that sharks are leaving coral reefs during stressful environmental conditions, like high temperatures that can cause coral bleaching events. They did this by combining satellite remote sensing with a network of underwater acoustic receivers.
Grey reef shark. Image Credit: Bertarelli Foundation / George Duffield
According to the study, grey reef sharks are being forced to leave the coral reefs that they call home due to ocean warming.
The repercussions on these sharks, which are generally firmly linked to specific parts of shallow reef ecosystems, include decreased residence, more extensive and frequent movements to other places, and extended periods of absence altogether.
The consequences of the 2015-2016 El Niño event, which produced significant bleaching in the study region, lasted up to 16 months, causing concern.
According to the scientists, this behavioral shift is "concerning" because climate change is expected to trigger bleaching episodes on a yearly basis by 2043.
Between 2013 and 2020, an international research team led by marine scientists at Lancaster University and ZSL, with funding from the Bertarelli Foundation, attached acoustic trackers to over 120 sharks and installed receivers around coral atolls to monitor shark movements at Indian Ocean reefs.
They collected over 714,000 acoustic detections and, in partnership with Earth Observation experts at King's College London, merged them with satellite data measuring various aspects of reef environmental stress.
According to the scientists who conducted the study, this has significant implications for both sharks and reefs.
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. 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.
Dr David Jacoby, Principal Investigator, Lancaster University
Jacoby said, “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.”
He stated, “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.”
Sea surface temperatures are frequently directly associated with stress on coral reefs, even though the study did not investigate the specific processes relating reef stress to shark movement and residency.
Sharks are ectotherms cold-blooded animals whose body temperature is regulated by their external environment. Reef sharks in other regions exhibit behavioural thermoregulation to avoid physiological damage from adverse water temperatures, and this is one of the potential drivers of the findings in this study.
Dr Michael Williamson, Study Lead Author and Postdoctoral Research Assistant, Institute of Zoology, Zoological Society of London
Significantly, the delicate equilibrium in reef ecosystems could be impacted by sharks moving away.
Dr Williamson added, “As large predators, grey reef sharks play a very important role in coral reef ecosystems. 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.”
Dr Jacoby stated: “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. 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.”
Nonetheless, the study's conclusions also give cause for hope.
Not all monitored areas witnessed a decrease in habitat utilization; in fact, certain acoustic receivers in specific places saw shark residency increase. These findings suggest that localized variables can impact shark decisions and that some reefs are more resistant to stress.
“We now need to find out what exactly is driving decision-making in these animals during periods of stressful conditions,” further added Dr Jacoby.
Although these characteristics were not included in the study, experts believe that various reefs respond differently to stress.
Dr Wiliamson concluded, “Recent research in the Chagos Archipelago, where we conducted our study, has shown that those reefs that have greater nutrient flows from seabirds have significantly enhanced fish biomass and therefore a higher likelihood to be resilient to multiple stressors. Some of our receivers that were seeing a greater number of sharks residing were also near islands with seabird populations.”
Co-authors include Michael Williamson and David Curnick of ZSL; Emma Tebbs of King’s College London; Francesco Ferretti of Virginia Tech; Aaron Carlisle of the University of Delaware; Taylor Chapple of Oregon State University; Robert Schallert and Barbara Block of Standford University; and David Jacoby of Lancaster University.
Journal Reference:
Williamson, M. J., et al. (2024) Environmental stress reduces shark residency to coral reefs. Communications Biology. doi.org/10.1038/s42003-024-06707-3.