Small amounts of oil in marine environments cause reef fish to make risky decisions, a team of international researchers has found.
They say the study, published today in Nature Ecology & Evolution, emphasises the risks of increasing industrial activity in areas like the Great Barrier Reef.
"Our findings were quite alarming," said study co-author Dr Jodie Rummer from James Cook University.
"In such early life stages, if these reef fishes are exposed to oil, they're experiencing some really dangerous cognitive difficulties."
For the study, researchers from the US, Norway and Australia introduced oil into simulated environments and tracked its impact on six species of reef fish from the families Pomacentridae and Lethrinidae, all of which are found in the Great Barrier Reef.
They found when fish in their early life stages were exposed to oil, at concentrations akin to a few drops in a swimming pool, they were worse at escaping from predators, chose poorer habitats to live in, reduced their travel in groups and swam more often towards dangerous open waters.
Dr Rummer said oil exposure during the critical early weeks of a fish's life could damage its cognitive ability and chances of survival.
"They're unable to make proper decisions and that could lead to their ultimate demise," she said.
The oil could be affecting the function of neurotransmitters in the brains of the fish, Dr Rummer said, in turn causing poor decision-making and slower responses.
Previous studies, undertaken in the aftermath of the Deepwater Horizon oil spill, established that oil exposure can cause heart deformities in some fish, which could affect swimming performance and survival.
But the researchers said that until now, no study had looked at what oil did to fish's brains and the decisions they made.
Vital part of ecosystem
The species looked at in the study were damselfish and emperors — which typically live in tropical coral reefs, grow up to 30 centimetres in length, and are usually brightly coloured.
Dr Rummer said the fish played an important role in keeping a reef healthy by helping remove algae that could otherwise suppress coral growth.
They also form an important part of the ecosystem's food chain, as prey for larger fish species.
In one part of the study, fish could choose between four habitat types — three reefs of varying complexity and a plain seafloor that provided no protection from a predatory fish.
Most fish would usually choose a highly complex reef system, which offered the best protection from predators. The researchers found after exposure to oil, they were more likely to choose the sandy seafloor or a simple reef.
The team also simulated a predator attack, and found oil-exposed fish were less likely to group together in a shoal, less likely to stay put once they had found a good shelter to hide in, and less likely to avoid open areas: all valuable behaviours for boosting a reef fish's chances of survival.
'We're taking too many risks'
According to Dr Rummer and her colleagues, several areas around the world already have oil present at levels that match — or exceed — the concentrations used in the study. This includes parts of the Great Barrier Reef, the Red Sea, and the Caribbean.
"That's what's particularly alarming, we already have areas that are polluted to this extent and haven't really started thinking about what that might mean for reef fish populations," Dr Rummer said.
The Great Barrier Reef has already undergone two back-to-back mass bleaching events, caused by warmer ocean temperatures.
Scientists estimated two-thirds of coral coverage in a 700-kilometre stretch of reef in far north Queensland died in 2016.
There are concerns any additional stresses placed on the reef by the proposed Adani Carmichael coal mine, and increased industrial activity at the Abbot Point port, could further compromise reef ecosystems.
"If we're adding one more stressor by increasing shipping or allowing more potential sources of pollution, spills or accidents, to be within our Great Barrier Reef marine park boundaries, we're taking too many risks I think," Dr Rummer said.