Coral reef fish are more likely to engage in risky behaviour and be unable to identify predators if they swim in waters contaminated with petroleum-based oil, researchers said.

Concentrations equivalent to only "a couple of drops in a swimming pool" could be enough to impair their judgement, scientists said in the study published Tuesday in the journal Nature Ecology & Evolution.

"The fish were unable to identify friend from foe and they stopped travelling in groups," study co-author Jodie Rummer from Australia's James Cook University's ARC Centre of Excellence for Coral Reef Studies said.

"The fishes also had trouble selecting suitable habitats, swam toward open waters, and could not swim away quickly from danger."

The research, which the scientists described as the first of its kind, focused on larvae -- the juvenile stage when fish are especially vulnerable -- and on six fish species from Australia's Great Barrier Reef.

The oil concentrations used in the study reflected "many polluted coastlines in industrialised regions worldwide", the scientists from Australia, Norway and the United States said.

The scientists said the oil exposure appears to "impair higher-order cognitive processing and behaviours necessary for the successful settlement and survival of larval fishes".

"This emphasizes the risks associated with industrial activities within at-risk ecosystems," they added.

"Each year, over six million metric tons of petroleum products are estimated to enter global oceans from anthropogenic sources such as industrial discharge, urban run-off and shipping operations."

The scientists said when the fish were exposed to increased oil concentrations in the lab tests, there were higher death rates and changed behaviour.

With ecosystems such as the Barrier Reef already under pressure from coral bleaching and development, they added that limiting pollution -- particularly oil -- near reefs was key to preserving such biodiverse sites.

"If an oil spill were to occur, this study suggests there could be major consequences for reef fish, coral reefs, and the people working in fisheries and tourism," lead author Jacob Johansen from the University of Texas said.

"Over the past 35 years, many of the world's coral reefs have declined. Still, many governments continue to allow industrial activities, including oil drilling and exploration, in sensitive reef habitats."

UNESCO's World Heritage Committee earlier this month said it had decided not to place the Barrier Reef on its list of sites "in danger" despite concern over two straight years of mass coral bleaching -- the result of warming sea temperatures linked to climate change.

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