High carbon dioxide concentrations in the ocean may turn fish into reckless daredevils, according to a study published in PNAS this week. When scientists exposed two different species of fish larvae to elevated carbon dioxide levels, the fish began to ignore the smell of predators, multiplying their mortality rate up to nine times the current level. The oceans are predicted to have high enough concentrations to completely impair the fishes' predator detection as early as the end of this century.

To test how fish reacted to elevated carbon dioxide levels, researchers reared sets of clownfish and damselfish larvae in three different carbon dioxide concentrations. The highest was 850 parts per million, a reasonable estimate for what the concentration is on track to reach by the year 2100.

At 700 parts per million, the fish were noticeably more reckless, venturing farther away from shelter and not responding to threats as actively as fish in control tanks. At this level, mortality was not noticeably higher.

However, at 850 parts per million, the fish were almost entirely ignorant of the scent of predators. These fish put themselves in a position to die much more often, and researchers found that as a result their mortality rate was between five and nine times higher than controls.

The scientists who conducted the study are unsure whether increased carbon dioxide levels simply obscure the scent of predators, or if breathing the higher concentrations damages the predator detection area of the fishes' brains. Either way, if the symptoms translate to or are worse in other species of fish, it will be very difficult for fish populations to recover from current levels even if we cut back on fishing.

If the 700-parts-per-million-or-less threshold persists long enough, the authors note natural selection should work its magic, weeding out the fish who can't smell predators around carbon dioxide. But if the affected species can't accomplish this transition before carbon dioxide concentrations make them completely stupid, they will be likely to become extinct.

PNAS, 2010. DOI: /10.1073/pnas.1004519107 (About DOIs).