Acid test points to coming fish troubles

Young fish can suffer severe damage from the ocean acidification expected within this century

The increasing acidification of ocean water may seriously jeopardize survival in young fish, two new studies find. Until now, studies of acidification’s effects on fish focused on adults — and found little evidence of life-threatening harm.

Seawater absorbs carbon dioxide from the air; the more CO2 absorbed, the more the pH drops, tipping the oceans’ waters toward the acidic end of the scale (SN: 3/15/2008, p. 170). So, forecasted increases in atmospheric CO2 concentration will hasten ocean acidification.

In both new studies, survival of young fish diminished as seawater’s CO2 concentration rose to levels scientists expect to see between 2050 and 2100. Already, however, fish in certain regions periodically encounter such acidification.

Both studies appear online December 11 in Nature Climate Change.

The studies looked at Atlantic cod and a small estuarine fish known as silversides. If acidification affects other species similarly to these, “it would be certainly important,” concludes Atsushi Ishimatsu of Nagasaki University in Japan, who was not involved in either study. Moreover, he notes, these data represent only part of the threat because “ocean acidification will not occur in isolation but together with other environmental changes — in particular, warming.”

In just-fertilized silverside eggs, hatchling survival fell steadily from about 50 percent at around current pH values, or 410 parts per million CO2, to about 10 percent at 1,000 ppm, a concentration scientists predict may occur around 2100. Hatchling length also fell — and rates of severe body malformations rose — with elevated CO2, reports Hannes Baumann from Stony Brook University in New York, who led one of the studies.

Lower concentrations had an effect, too: Eggs incubated in moderately acidified water — 600 ppm CO2 — were far less likely to survive than were eggs in water at current pH levels. But 10 days in water with 600 ppm CO2 posed little survival risk to hatchlings that as eggs had incubated in normal pH water. “This indicates that most of the risk to survival occurs during the egg stage,” Baumann says.

CO2 values around 600 ppm could occur within 40 years, he notes — and even if CO2 emissions stabilized tomorrow, ocean acidification would climb for another 50 years.

In the second study, researchers in Europe raised larval cod for seven weeks in big outdoor tanks provided with water from an adjacent fjord. Each tank’s CO2 concentration was held at either the current global average or at higher concentrations of 1,800 or 4,200 ppm. CO2 concentrations in the Kiel Fjord, close to where Baltic cod spawn, can reach roughly 2,300 ppm, the authors note.

Fish in the acidified waters grew faster but matured later and died at progressively higher rates. Cod exposed to elevated CO2 concentrations also exhibited higher rates of severe damage — including tissue death and malformations — in a host of organs including the liver, pancreas, kidney and gut.

Those with severe damage probably died. “We have to assume that,” says Andrea Frommel of the Leibniz-Institute of Marine Sciences in Kiel, Germany, “because the damage we had seen was pretty much nonreversible.” Among fish that survived beyond a month and began breathing through their gills, the researchers found little CO2-linked damage, emphasizing the selective impact on younger fish.

“These studies are the first to demonstrate direct effects of ocean acidification on tissue development and survival of larval fishes,” notes Philip Munday of James Cook University in Townsville, Australia. “We don’t yet know if fish will be able to compensate for these problems over a number of generations.” It’s something the authors of both new studies plan to investigate.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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