Suicidal Reef Fish

Increasing ocean acidity may impact marine fishes' ability to replenish their populations

July 7, 2010 - Humanity’s rising CO2 emissions could have a significant impact on the world’s fish populations according to groundbreaking new research carried out in Australia.

Larval fish may become easy pickings for predators as the world’s oceans become more acidic due to CO2 fallout from human activity, an international team of researchers has discovered.

In a series of experiments reported in the latest issue of the Proceedings of the National Academy of Science (PNAS), the team found that as carbon levels rise and ocean water acidifies, the behaviour of tiny reef fishes changes dramatically – in ways that decrease their chances of survival by 50 to 80 per cent.

“As CO2 increases in the atmosphere and dissolves into the oceans, the water becomes slightly more acidic. Eventually this reaches a point where it significantly changes the sense of smell and behaviour of larval fish,” says team leader Professor Philip Munday, left, of the Australian Research Council’s Centre of Excellence for Coral Reef Studies (CoECRS) at James Cook University.

“In our experiments we created the kind of sea water we will have in the latter part of this century if we do nothing to reduce emissions. We exposed baby fish to it, in an aquarium and then returned some to the sea to see how they behaved.

“When we released them on the reef, we found that they swam further away from shelter and their mortality rates were five to eight times higher than those of normal larval fish of the same species,” Professor Munday says.

“Instead of avoiding predators, they become attracted to them. They appear to lose their natural caution and start taking big risks, such as swimming out in the open - with lethal consequences.”

Dr. Mark Meekan from the Australian Institute of Marine Science, a co-author on the paper, says the change in fish behaviour could have serious implications for the sustainability of fish populations—including important food fish species—because fewer young fish will survive to replenish adult populations.

"It's a very scary world out there for a little fish when it first arrives on the reef and lots and lots of mouths out there are ready to eat them. Normally little fish are very, very shy. They're not very bold. They hang inside the coral and they're very wary.

"Now, once we subjected these little fish to excess carbon dioxide, basically they lost that caution, that natural caution that they should have had. They started wandering out from the coral, became much bolder and as a result, much more vulnerable to predators. It's because higher levels of carbon dioxide affect the clownfish's sense of smell." In lab experiments, confused larvae in more acidic water tended to swim toward, rather than away from, chemical scents emitted by their known predators.

Ocean pH Trends

“Every time we start a car or turn on the light part of the resulting CO2 is absorbed by the oceans, turning them slightly more acidic. Ocean pH has already declined by 0.1 unit and could fall a further 0.3-0.4 of a unit if we continue to emit CO2 at our present increasing rate," says Meekan.

“We already know this will have an adverse effect on corals, shellfish, plankton and other organisms with calcified skeletons. Now we are starting to find it could affect other marine life, such as fish.”

Earlier research by Professor Munday and colleagues found that larval Percula Clownfish, right,  were unable to find their way back to their home reef under more acidic conditions. The latest experiments cover a wider range of fish species, including Pomacentrus wardi, and show that acidified sea water produces ominous changes in fish behaviour.

“If humanity keeps on burning coal and oil at current rates, atmospheric CO2 levels will be 750-1000 parts per million by the end of the century. This will acidify the seas much faster than has happened at any stage in the last 650,000 years.

He adds it should be clearly understood that this impact is likely to happen independent of global warming, and is a direct consequence of human carbon emissions.

The research team concludes “Our results demonstrate that additional CO2 absorbed into the ocean will reduce recruitment success and have far-reaching consequences for the sustainability of fish populations.”

Professor Munday adds “In its 2008 report on the state of the world’s fisheries the UN Food and Agriculture Organization said “the maximum wild capture fisheries potential from the world’s oceans has probably been reached.”

"If you add the impact of ocean acidification and other climate change impacts to this, it means there are grounds for serious concern about the future state of world fish stocks and the amount of food we will be able to obtain from the sea.”

The article “Replenishment of fish populations is threatened by ocean acidification” by Philip L. Munday, Danielle L. Dixson, Mark I. McCormick, Mark Meekan, Maud
C.O. Ferrari and Douglas P. Chivers, appears in the latest issue of the Proceedings of the National Academy of Science (PNAS).

From materials released by James Cook University and the ARC Centre for Excellence Coral Reef Studies. Images courtesy James Cook University.