Little Cayman reefs devastated by marine heatwave

(CNS): A survey of reefs around Little Cayman has revealed a shocking decline in coral cover from 26% at the start of 2023 down to less than ten percent by this summer. The devastating loss of coral is down largely to the global bleaching event fuelled by last year’s marine heatwave. The survey by the the Central Caribbean Marine Institute (CCMI) and is the first study documenting the serious negative impact ocean warming is having on Cayman’s precious marine resources.

The results from this recent study show the greatest reduction in coral cover recorded through the 26-year period that the CCMI has conducted these monitoring surveys far worse that the previous all-time low in 2005 when cover fell to 15%. The decrease in coral cover and health following this mass bleaching event is the most severe since CCMI’s records began and the reefs currently have just 9.8% cover.

The researchers said the “extreme decline” is attributed mostly to ‘weedy’ species, that are less thermally tolerant. These are fast growing corals by they don’t provide as much of the reef structure as larger, slower-growing boulder corals. Meanwhile, the boulder corals, that are the key reef-building species, although bleached showed higher rates of recovery and less mortality giving some hope for the longer term future of the reefs around Little Cayman.

But the key problem with the loss of weedy species means that overall, the reefs will lose habitat complexity and shelter for marine species and a reduction in reef-building species will result in flatter reefs that provide less coastal protection.

These species and other similar fast-growing corals can recover faster than boulder-like species so scientists remain hopeful that the reefs will recover as these species spring back up. However, given that 2024 is likely once again to be the hottest year on record around the world and indications that the ocean has been as hot this year in our region as it was last, if not hotter, the question remains how much can the reefs sustain before recovery becomes impossible.

The CCMI said there was no denying that Little Cayman’s reefs are changing. There is hope the resilience can persist but concerns remain that the reefs are in a phase shift, whereby the ecosystem changes from a coral-dominated to macro-algal dominated habitat. At that point, it is unlikely that the reef would be able to provide the same services, such as habitat provision for fish and coastal protection from storms.

But the thriving fish populations in Little Cayman, as a result of the marine protections that Cayman has put in place over the years and adhered to in Little Cayman, will help to maintain the reef in a coral-dominated state.

The main hope for the future for Little Cayman is the extremely resilient fish populations despite the coral loss. Fish populations continue to thrive there, showing consistent increases since 2016 in density and biomass even now after this terrible bleaching event. Large increases were recorded in herbivorous fish, particularly the critically important parrotfish, that are crucial for the recovery of the reef ecosystem. The presence of these herbivorous fish support coral growth and reef recovery through grazing. These fish manage the levels of macro-algae on reefs, which, if left unchecked, can outcompete corals for space on the reef and hinder coral growth.

This bodes well for the future but there are still wider threats to the marine environment from climate change.

The CCMI is carrying on with its research into the resilience or not of coral reefs and the factors that help or hinder but the project has also taken a hit. According to the release during the bleaching event last year CCMI’s coral nursery suffered severely loosing over 90 corals. But using genetic information on every individual coral the scientists have been able to identify the specific genetic makeup, or genotype, of fragments that survived the extreme heatwave which is important research that will assist in the institutes work to help prevent global reef loss as the planet continues to warm.

“Going forward, these findings of genetic resilience will be incorporated into CCMI’s coral restoration programme to increase the chances of success as the impacts of climate change intensity,” the institute’s researchers said in the release.

In addition to this resilience-based coral restoration programme, CCMI has also conducted some cutting-edge research when it explored and mapped the offshore seamount, Pickle Bank. Around 45 miles offshore this coral reef habitat is relatively undisturbed by human activity and the diverse, healthy reefs found by CCMI researchers provide hope for the future of reefs in Cayman.

See the Reef Lecture in full on the CCMI’s Youtube Channel here outlining the research work and survey results.