Severe coral loss leaves reefs with larger fish but low energy turnover

By comparing reef surveys from 2003-2004 and 2018, an international team of researchers led by James Cook University, found severe coral loss, of up to 83% in some areas, was associated with increases in fish biomass, productivity and consumed biomass, meaning the reef now has more energy stored in the form of fish weight, is able produce more fish weight and these fish are being consumed by predators.

Renato Morais, lead author of the study, said "It's as if the herbivorous fish community has been scaled up, with larger fish growing and providing more food for predators when they die. However, this does not come without a cost."

Superficially, the increased biomass may seem positive from a human perspective, with the presence of bigger fish after a coral reef collapse suggesting a stable population. However, the researchers warn that reduced turnover, or recycling of biomass, in the reef could mean that this trend benefiting large fish might not last long.

"The fish have not multiplied. Instead, there are more bigger fish and less smaller ones." Said Mr Morais "This suggests that many of these long-living herbivorous fishes, such as surgeon fish which can live up to 40 years, could have been there before the corals died, only growing bigger. Eventually, these older fish will die and, if not replaced by young ones, productivity could collapse."

The increased growth of large fish like surgeonfish, parrotfish and rabbitfish is likely to have been made possible by the accessibility and quality of algal turf, the preferred food of these herbivores. These algal 'lawns' grow abundantly over the skeletons of dead coral. A recovery of the coral could result in a reduction in this food source and collapse of these herbivores.

Erosion of dead coral structures and subsequent loss of refuges for fish could also cause a population crash. Although the researchers did observe a decline in coral structure in the reef area studied, it is possible that it had not reached a level where fish biomass would start to decline.

Between 2014 and 2017 the reefs around lizard island, where the research took place, were subjected to two back-to-back mass bleaching events and two severe cyclones that decimated the coral populations. Combined, these events led to an 80% decline in coral cover throughout the islands.

Previous research has mainly taken a static look at the impacts of coral loss on fish. In this study the researchers looked at the cumulative effects of coral mortality over time using metrics often absent from coral reef studies: fish growth, mortality and energy turnover.

To record the data, the researchers carried out fish and benthic (sea floor) surveys at Lizard Island, on Australia's Great Barrier Reef in 2003 to 2004, and 14 years later in 2018. The benthic surveys quantified live coral cover and algal turf cover. Fish surveys recorded 12 common types of reef fish families.

Because the study comprises of two snapshot assessments 15 years apart the researchers were limited to looking at long term trends. "If there were changes to the energetic balance of the fish assemblages at that reef that happened between surveys but did not have a lasting effect, they would have gone unnoticed." Explained Mr Morais.

Mr Morais also cautions that the findings apply to the one reef the researchers surveyed and other reefs could behave differently, although a number of features suggest similar changes may have taken place elsewhere. For instance, increases in herbivore populations are common in post-coral reefs. Collecting the same data on other reefs will help to establish if this is the case.

The researchers are looking to follow this reef to see if new energetic shifts occur. "Any further shifts will depend on what happens to the reef," said Mr Morais, "Will there be a recovery of corals? Or will this degraded state be maintained? Then, will these large and old herbivorous fishes be replaced by younger ones? There are many aspects in this story to be investigated."