Ocean Acidification Could Make Fish Lose Their Fear Of Predators, Study Finds


Add “losing fear of predators” to the long list of impacts acidifying oceans could have on fish and other marine life.

A new study published in Nature Climate Change has found that elevated CO2 levels in marine waters make reef fish attracted to the smell of their predators, rather than being repelled. Researchers looked at multiple species of reef fish living near natural volcanic CO2 seeps in Papua New Guinea, an environment the study says is acidified to levels comparable to projections of what the entire ocean’s acid content will be in the next 100 years. They compared the behavior of the fish living in the acidified environment to fish in nearby, less acidified reefs, and found that, while fish in the nearby control reefs avoided water streams that contained predator odor, fish from the acidified reef spent 90 percent of their time in water streams that smelled of their predators.

On top of that, fish that lived under high-CO2 conditions were bolder than other fish — meaning that they emerged more quickly from their hiding places after a disturbance and ventured farther from their hiding places than other fish — and couldn’t differentiate the smells of different habitats. Fish from the control reef spent more than 80 percent of their time in hiding, while two species of damselfish from the acidified reef spent less than 12 percent of their time in shelter, and two other fish species studied spent no time in shelter, preferring to swim in more exposed, open water

Danielle Dixson, assistant professor at Georgia Tech and co-author of the study, told ThinkProgress that the results of the study were surprising because scientists long believed that fish would be able to deal with ocean acidification due to their natural mechanism for coping with increased levels of CO2. When fish are exposed to high acid environments, they absorb the acid into their bodies, and to compensate for the increased acid, they increase the amount of bicarbonate — a base — their bodies produce.

“The thing that people didn’t really think about was that when they up-regulate all this bicarbonate, it interacts with neurological pathways,” Dixson said.

Dixson said that when there’s too much bicarbonate in the fish’s system, their GABA receptor stops working properly, causing the cognitive issues the researchers recorded. These effects have major implications for the future of the ocean ecosystem. The balance of the acidified reef ecosystem in the study did not suffer as a result of the cognitive problems of the fish, mainly because there were fewer predators in the environment and because, when young fish were killed by predators in the acidified environment, other young fish would migrate from nearby, less acidified reefs to replace them. But when all the oceans are at the level of the CO2 seep ecosystem, this replacement from fish in less acidified waters won’t be an option.

“It is hopeful that there are still fish that live [in the CO2 seep sites] and that they’re metabolically the same as the fish that live in the non-CO2 seep site, but the degree of aid that the control sites are providing the CO2 sites is unknown,” Dixson said. “As the world acidifies — in 100 years when the ocean is expected to be the equivalent of a CO2 seep reef — there won’t be these safe havens that can help.”

The study isn’t the first to document ocean acidification’s wide-reaching effects on fish and other marine species. Another study from August also found that fish could become confused and hyperactive as acid levels increase in the ocean, and also found evidence that the metabolism of fish could change. That study also found that when atmospheric carbon levels reached 500 to 650 parts per million — levels that are predicted by 2100 — corals, echinoderms (such as starfish), mollusks and fish were negatively impacted. Researchers have also predicted that ocean acidification could lead to a decline in shellfish, and that coral will struggle to build its skeleton as acidity rises. The effects of acidification could be so harmful to coral that Oceana predicts some species of coral could become functionally extinct within 20 years.