Fish that thrive after severe coral loss may collapse

According to a new study published by the British Ecological Society, severe coral loss on the Great Barrier Reef has been linked to the appearance of large herbivorous fish with long lives. The reduced turnover, or lack of recycled biomass, may ultimately cause the ecosystem to collapse.

An international team of researchers led by James Cook University compared reef surveys from 2003 and 2018. They found that up to 83 percent of corals had been lost in the hardest hit areas. Severe coral losses were associated with increases in fish biomass, productivity, and consumed biomass.

“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,” said study lead author Renato Morais. “However, this does not come without a cost.”

At a glance, it would seem that the appearance of bigger fish after severe coral loss is a good sign which suggests the population is stable. But the researchers warn that decreased recycling of biomass could mean that the bigger fish may not be supported for long. 

“The fish have not multiplied. Instead, there are more bigger fish and less smaller ones,” said 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.”

Algal turf, which grows abundantly over dead coral skeletons, is the preferred food of herbivorous fish like surgeonfish and parrotfish. This may explain why the large herbivores seem to thrive after severe coral loss. 

If the corals recover, the fish will lose their main food source and their populations will crash. 

The research was focused on reefs around Lizard Island, where the corals were devastated by two back-to-back bleaching events and two severe cyclones between 2014 and 2017. The collective impacts of these events wiped out 80 percent of the corals around the island. 

Previous studies have primarily looked at the loss of corals. This study examined the cumulative effects of coral mortality over time based on fish growth, mortality, and energy turnover.

The researchers analyzed two snapshot assessments that were 15 years apart, which means they were limited to assessing the 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 Morais.

He noted that the findings apply to one specific reef and other reefs could behave differently, although a number of factors indicate that similar changes have taken place elsewhere. For example, herbivore populations commonly increase in post-coral reefs. 

The researchers plan to follow up on the Lizard Island reef to see what type of energetic shifts may occur. 

“Any further shifts will depend on what happens to the reef, said 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.”

The study is published in the journal Functional Ecology.

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By Chrissy Sexton, Earth.com Staff Writer