For coral reefs, more is less

The story of coral reef bleaching ramifications continues to unfold with an international study revealing the long-term impacts on fish populations and associated ecosystems.

Scientists found considerable increases in large, long-living herbivorous fish such as surgeonfish (Acanthuridae), parrotfish (Labridae-Scarini) and rabbitfish (Siganidae) around lizard island in Australia’s Great Barrier Reef, which has suffered extensive coral loss.

But this has come at a cost.

“The fish have not multiplied,” says lead researcher Renato Morais from James Cook University, Australia. “Instead, there are more bigger fish and less smaller ones.”

This could mean the big fish, which are resilient and can live up to 40 years, have been around since before the corals died, he explains. “Eventually, these older fish will die and, if not replaced by younger ones, productivity could collapse.”

It’s likely the large fish have thrived on algal turf that covered the skeletons of dead coral, while the reef degradation has extensively reduced other populations that rely on coral such as butterflyfish (chaetodontidae) and damselfishes (Pomacentridae). 

These findings are consistent with other research, but the new study throws a different light on altered fish abundance and biomass by showing the impact of coral death on energy flow, such as the production or recycling of fish matter.

To investigate this, the researchers surveyed the fish in 2003-04 and 2018, after two severe cyclones and consecutive mass bleaching events that destroyed coral populations.

They applied simple principles to estimate biomass (total fish weight) consumed by predators and fish productivity (how much new fish weight is produced), relying on predictions of their expected growth and chance of dying. 

The recycling rates, or turnover, are calculated by summing productivity and consumer biomass and dividing them by fish biomass, a standard fisheries procedure rarely applied to underwater surveys of coral reef fish.

Results surprisingly showed that after extensive coral loss of up to 86%, the fish biomass, productivity and consumed biomass all increased over the 15-year period, by 71%, 41% and 37%, respectively, mainly due to the increase in large herbivores.

This means the reef now has more energy stored in the form of fish biomass, can produce more fish weight, and that these fish are being consumed by predators.

It might seem heartening for fisheries, but Araujo says the overall picture is worrying.

“Superficially, the enhanced productivity of these fishes following coral mortality might seem like good news,” he says. “However, increased productivity did not translate into increased biomass recycling.”

In fact, it declined dramatically – older fish were being replaced 19% less in 2018 than in 2004, telling a different, more complex story altogether.

“This new perspective is important,” Araujo says, “because fish productivity sustains many coral reef fisheries with critical social and economic importance.”

The study is published in the journal Functional Ecology.

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