Grey reef sharks are beginning to leave their coral reef habitats due to rising ocean temperatures, according to new research findings. Scientists employed satellite remote sensing and a series of underwater acoustic receivers to determine that these sharks are relocating away from coral reefs during environmentally stressful times, like periods of high temperatures that may lead to coral bleaching.
Grey reef sharks are beginning to leave their coral reef homes due to warming oceans, according to recent studies.
Researchers found that during times of environmental distress, such as elevated temperatures that could cause coral bleaching, sharks are abandoning their usual shallow reef habitats. The impact on these fish, which typically remain tied to specific reef areas, includes a decrease in their residency rates, more frequent relocations, and longer absences.
Alarmingly, these effects were noted to last for extended durations, reaching up to 16 months after severe stress events like the 2015-2016 El Niño, which resulted in major coral bleaching in the region studied.
With projections indicating that climate change could cause annual bleaching events by 2043, scientists are expressing concern over these behavioral shifts.
A multinational research team, led by marine scientists from Lancaster University and ZSL, with funding from the Bertarelli Foundation, tracked over 120 sharks by attaching acoustic devices and installing receivers around coral atolls to observe their movements in the Indian Ocean from 2013 to 2020.
They recorded more than 714,000 signals and collaborated with Earth Observation scientists at King’s College London, integrating these signals with satellite data representing various measures of reef environmental stress.
The researchers indicate that their findings could have significant implications for both the sharks and the coral reefs.
“These results offer some of the initial evidence on how reef changes due to environmental stress, which is increasing both in intensity and frequency, impact shark movements,” stated Dr. David Jacoby from Lancaster University, the leading investigator of the research. “Grey reef sharks are common predators in the Indo-Pacific reefs, typically leaving the reef for feeding. However, many are now faced with the choice of fleeing from stressed reefs.”
“Confronted with this dilemma, sharks must choose whether to leave the comparative safety of the reef and expend more energy to stay cool or remain on the reef in less-than-ideal conditions and conserve energy.”
“We believe many are opting to migrate to deeper and cooler offshore waters, which is concerning. Numerous reefs have already seen significant shark population declines due to overfishing, and this data could worsen those trends.”
While the study did not delve into the exact reasons linking reef stress to shark behavior, it is known that stress on coral reefs is commonly associated with sea surface temperature increases.
“Sharks are ectothermic–cold-blooded creatures whose body temperature relies on their surroundings,” noted Dr. Michael Williamson from ZSL’s Institute of Zoology and the paper’s lead author. “Reef sharks in other areas display behavioral thermoregulation to prevent bodily harm from unfavorable water temperatures, which may explain some of the results in this study.”
Importantly, the migrations of sharks could disrupt the delicate balance within reef ecosystems.
“As apex predators, grey reef sharks are crucial to the health of coral reef ecosystems,” explained Dr. Williamson. “They help maintain a finely tuned food web in the reef while also bringing nutrients from deeper waters onto the coral. The reduction of sharks, as well as the nutrients they contribute, could reduce the resilience of reefs during periods of high environmental strain.”
Dr. Jacoby added: “As climate change leads to more unpredictable and frequent extreme stress events, the significant ecological role these predators play in coral reefs is likely to evolve as they spend increased time away from their accustomed habitats. The full implications of this are still unclear, but given the intricate species and trophic interactions present on coral reefs, we can expect substantial transformations.”
However, there are also some positive observations in the study’s outcomes.
Not every monitored site recorded a decrease in habitat utilization; in fact, some specific locations showed an increase in shark residency. These results suggest there may be localized factors affecting shark behavior, indicating that some reefs possess greater resilience against stress. “We now need to understand what influences the decision-making of these animals during challenging conditions,” Dr. Jacoby mentioned.
Even though these elements were not analyzed in this study, researchers propose that different reefs may react uniquely to stress.
“Recent research in the Chagos Archipelago, where this study took place, has indicated that reefs benefiting from greater nutrient flow from seabird populations experience significantly higher fish biomass, which may enhance their resilience to multiple stressors,” Dr. Williamson explained. “Some of the receivers recording more sharks were near islands with seabird numbers.”
This study is detailed in the paper titled ‘Environmental stress reduced shark residency to coral reefs,’ published in the journal Communications Biology.
The paper’s authors include: Michael Williamson and David Curnick from ZSL; Emma Tebbs from King’s College London; Francesco Ferretti from Virginia Tech; Aaron Carlisle from the University of Delaware; Taylor Chapple from Oregon State University; Robert Schallert and Barbara Block from Stanford University; and David Jacoby from Lancaster University.
