Coral bleaching found to change fish communities

The repeated bleaching of coral reefs due to rising sea temperatures has been found to have long-term effects on fish communities in the Seychelles.

Coral bleaching in the Seychelles in 1998 has led large predator fish like snappers and small fish like damselfish to dramatically drop in number, with them largely being replaced by seaweed-eating fish, found the study led by Lancaster University.

This stayed the case even though 18 years passed before another mass bleaching event as some reefs were unable to recover, researchers have said.

Researchers say the study, published in Global Change Biology, shows that coral bleaching leads to permanent changes in biodiversity and the range of fish species living in coral reefs, a situation which will have a knock-on impact on fishing communities.

Dr James Robinson, of Lancaster University, the lead author of the study, said: ‘Although the 18-year period between major mass bleaching events allowed corals to recover on some reefs, we found evidence that fish populations were not able to return to their pre-bleaching levels, and they were substantially altered on the reefs that become dominated by seaweeds.

‘The Seychelles case study suggests under current levels of ocean warming – where the average frequency of bleaching events is less than 10 years – permanent changes to reef fishes are likely on most coral reefs globally.’

The Lancaster University-led research looked at reef recovery in the Seychelles for 16 years after they were bleached in 1998, before another major coral bleaching event hit the reefs in 2016.

The research found that while some coral reefs were able to recover in the 16 years after 1998, other reefs were not able to recover, transforming into fields of seaweeds and collapsing the fish’s existing habitat structure.

While seaweed-dominated reefs saw the biggest changes, all the reefs that the researchers surveyed saw herbivore fish that feed on seaweed – like parrotfish and rabbit fish – become the dominant community instead of predators and small fish.

Even on the reefs where the coral did recover between bleaching events, the number of fish species recovered but did not return to its original composition.

This has led researchers to conclude that the same changes could be the ‘new normal’ for similarly damaged coral reefs around the world.

Professor Nick Graham, of Lancaster University, said: ‘The new normal for coral reefs will be reef fish communities which have fewer species and are dominated by herbivores and invertebrate feeding fish.

‘This will alter the way coral reefs function, and the fishery opportunities for coastal communities adjacent to coral reefs.’

This research follows on from another recent Lancaster University study, which found that chronic exposure to poor quality water is affecting the recovery rate of corals in the Great Barrier Reef, slowly putting it under threat.

The study warned that while improving water quality will help coral reefs to recover once they have suffered bleaching events, the emphasis must be on slowing climate change to allow existing coral cover to be maintained as much as possible.


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