Changing ocean chemistry could turn some fish species into easy meals, with senses of smell so scrambled they’re actually attracted to their predators.
Researchers discovered the potentially deadly problem through a series of experiments on common reef-dwelling fish that were raised in seawater with acidity levels resembling what’s expected by the century’s middle and end.
“Instead of avoiding the odor of a predator, they’re attracted to it,” said biologist Douglas Chivers of the University of Saskatchewan. “When you take them into the wild, their behavior has changed. We ended up with huge mortality.”
When carbon dioxide dissolves in seawater, the concentration of hydrogen ions increases, making it more acidic. Global oceanic pH — the scale used to measure acids and bases — has changed by 0.1 in the last century. The number looks small, but in geological terms it’s a massive change, and Earth’s oceans are more acidic now than at any time in the last 650,000 years. Scientists say marine pH could change by another 0.3 by the year 2100.
Concerns about the effects of changing ocean acidity on animals has focused on weakening shells in corals, crustaceans and shellfish, but fish may also be affected. Chivers’ findings, published July 5 in the Proceedings of the National Academy of Sciences, build on earlier work showing acidified waters make it hard for clownfish to find home, a feat they accomplish by recognizing subtle olfactory cues in water.
In the latest study, the researchers raised clownfish and damselfish in the sort of water conditions expected by 2050 under current CO2 pollution rates, and those that could prevail by the century’s end if those rates don’t change. A control group was raised in current water acidity levels.
In an aquarium, clownfish from the control group instinctively fled from the scents of their natural predators. So did those in the mid-century group. But half the fish raised in end-of-century concentrations swam straight towards the scents. Had predators rather than scientists been waiting, they would have been eaten.
Damselfish were then raised in a similar set of conditions, and relocated to coral reefs in the wild. Once again, fish from the low- and mid-level acidity groups behaved normally, but those raised in higher levels were disoriented. The latter were between five and nine times more likely to die than the others.
In the future, the researchers plan to study ocean acidification’s effects on other species. They also want to know what happens to whole populations over multiple generations. Will species be wiped out? Or can they adapt, with acidity-resistant fish breeding fast enough to replace those lost to olfactory disorientation?
“That’s the million-dollar question,” said Chivers. “We don’t know yet. It’s probably going to depend on how fast acidification happens.”
Photo: Joshua Nguyen/FlickrCitation: “Replenishment of fish populations is threatened by ocean acidification,” by Philip Munday, Danielle Dixson, Mark McCormick, Mark Meekan, Maud Ferrari, and Douglas Chivers. Proceedings of the National Academy of Sciences, Vol. 107 No. 28, July 5, 2010.