Starving fish are killing sections of the Great Barrier Reef already weakened by climate change, an Australian scientist says.
And some fish species also face extinction - with potentially serious consequences for commercial fisheries.
Morgan Pratchett, of the James Cook University-based ARC Centre of Excellence for Coral Reef Studies in Townsville, has spent the last two months at Britain's University of Newcastle using chemical markers, such as carbon 13, to study the feeding interaction between corals and fish.
The technology of using stable isotopes like carbon 13 has previously been used to study the food chain in large marine food fisheries, but never before to analyse fish behaviour and impacts on coral reefs.
"There's a whole suite of coral-feeding fish and during normal conditions, they don't kill the reef," Dr Pratchett told AAP.
"However, during periods of stress on the reef, they do contribute to excess mortality among corals.
"The typical example is when you have coral bleaching and the coral is already experiencing reduced energy reserves and then as the fish continue to feed on them it imposes a further energetic cost."
Dr Pratchett said that butterfly fish, which are the main group of coral-feeding fish, had all but disappeared from some sections of the central Great Barrier Reef.
"We are already looking closely at one particular species, which is a really specialised butterfly fish which could face a really high extinction risk as a consequence of ongoing bleaching," Dr Pratchett said.
"It's already happened in some locations, that's for sure.
"Even in the central Great Barrier Reef during the bleaching that happened in 2001 and 2002, this one species, which is the chevron butterfly fish, disappeared entirely."
Their recovery had been so slow that only a few chevron butterfly fish had been seen in the region in the last year, he said.
It was just another indication of the effects of global warming, he said.
"It's getting more and more serious and just another one of the stories that show we need to respond to climate change.
"If this translates to other fish being affected, like commercially important fish, then there's a significant economic cost associated with it."
Dr Pratchett is in the first year of a five-year study which will examine other species.
"One of the things I'm hoping to use the stable isotopes for is to study the effect on coral trout and if they feed on butterfly fish, will they be affected as well."
"Previously, studying what coral trout required catching the trout and looking inside their guts, but the stable isotopes allow me just to take samples of fish that have been caught for commercial purposes or whatever and just analyse it that way."
