Australian scientists have found evidence that climate change may play havoc with fish populations.

Researchers from the ARC Centre of Excellence for Coral Reef Studies (CoECRS) have tracked a dramatic decline in the numbers of coral-feeding fish, after a reef was destroyed by bleaching caused by high sea temperatures.

Dr Morgan Pratchett, a senior research fellow at CoECRS, and colleagues spent five years charting a collapse in numbers of coral-feeding butterflyfish on reefs affected by severe coral bleaching.

“The evidence suggests that climate change is already hitting certain fish populations – and this could have an adverse effect on industries like tourism,” he warns. “People come to the reef expecting to see colourful fishes.”

“The impact of coral bleaching is most obvious in fish which feed specifically on corals, as do most butterflyfishes. It is not yet clear what knock-on effects this may have on populations of mixed feeders or predatory fish like coral trout – and hence on the fishing industry or recreational angling.”

Their research results indicate that the impacts of coral bleaching can be more far-reaching and last longer than previously thought.

Dr Pratchett said the researchers had also noted a steady decline in body condition of the coral-dependent butterflyfish at Trunk reef on the Central Great Barrier Reef following bleaching events in 2000-2002.

This suggested that the fish were gradually starving to death and the decline in numbers indicated they had also failed to breed in the months and years following the destruction of their reef.

“It looks as if they didn’t move elsewhere, but stayed where they were and starved. Fish can be very territorial and it may be hard for refugee fish which have lost their reef to relocate elsewhere, because the locals will try to keep them out.”

Coral bleaching is caused by high water temperatures which cause the corals to shed their symbiotic bacteria and die. Bleaching events have been noted worldwide and appear to be on the increase as the earth warms, leading to accelerated degradation of reefs.

There have been several major bleaching events affecting corals along Australia’s Great Barrier Reef in recent years, the worst being in 2002, Dr Pratchett said.

More than 30% of coral reefs throughout the world are already severely degraded and corals on 60% may be lost by 2030 due significantly to bleaching as the climate warms, research by CoECRS scientists indicates.

“Ours and other studies indicate that when coral bleaching occurs affecting up to 10 per cent of the reef, it affects the abundance of nearly two thirds of the fish species on that reef.

“As the damage rises to 20 per cent and above, there is a marked decline in the richness of fish species on the reef – and the losses can last for years.

“However we are also confident that, if the corals recover, the coral-feeding fishes will come back,” Dr Pratchett says. “That’s the encouraging news for managers, tourism operators and visitors alike.”

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