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Study: Ocean Acidification Has No Ill Effects on Reef Fish Behavior

A new comprehensive study three years in the making shows the damaging effects of ocean acidification will not inhibit the behavior in coral reef fish as previously believed.

(CN) – A new comprehensive study three years in the making shows the damaging effects of ocean acidification will not inhibit the behavior in coral reef fish as previously believed.

Conducted by an international coalition led by scientists from Australia and Norway, this study observed the effects of ocean acidification and now challenges previous reports that a more acidic ocean will damage coral reef fish behavior. Their research showed that when coral reef fish were exposed to the same levels of carbon dioxide expected by the end of the century, it did not change their activity levels or ability to avoid predators.

"Contrary to previous studies, we have demonstrated that end-of-century CO2 levels have a negligible impact on the behavior and sensory systems of coral reef fish," said Timothy Clark, the lead author of the study and an associate professor at Deakin University in Australia.

The authors stress that while this is good news on its own, the issue of ocean acidification and global warming still present a very real threat to coral reefs. Rising levels of acidity will remain a problem for sea creatures that rely on calcium carbonate to make shells and skeletons, such as coral reef organisms. The corals themselves become harmed when ocean temperatures increase, undergoing bleaching events and even death.

One of the ocean’s many functions that benefit humankind is acting as a sponge to soak up a percentage of carbon dioxide, which is released into the atmosphere from the burning of fossil fuels. With CO2 levels skyrocketing, however, the increased absorption leads to ocean acidification.

A previous study in March 2019, led by the U.S. National Oceanic and Atmospheric Administration (NOAA), showed that the world's oceans absorbed 34 billion metric tons of carbon from the burning of fossil fuels from 1994 to 2007 – about 31% of all carbon dioxide released into the atmosphere.

If this trend continues, researchers believe that by the end of the century the oceans could absorb such large quantities of CO2 that levels will be higher than what most marine species have experienced in the past 30 million years.

Because fish have regulatory systems that help them to cope with changing water acidity, most fish physiologists believed the sea creatures would be able to handle the increased acidity. That was until a half dozen highly publicized reports showed that many fish, especially coral reef fish, were dramatically affected by the increased CO2 levels. The studies showed fish so disoriented they would actually swim towards predators, rather than away from them.

"The reports described effects across a range of life stages, including altered smell, hearing, vision, activity levels, boldness, anxiety and susceptibility to predation," said Fredrik Jutfelt, an associate professor at the Norwegian University of Science and Technology and one of the authors of Wednesday’s study, published in the journal Nature.

Despite the evidence from these reports showing negative effects of CO2 on reef fish, there were some substantial inconsistencies, even though the studies relied on similar methods to test for results. Due to this, Clark, Jutfelt and their colleagues decided to test if they could replicate the results by conducting their own carefully documented, extensive research.

What they found were exactly opposite results, observing completely normal behavior in the fish species they studied.

"Unexpected scientific results always spark interest from other scientists, but before too much trust is placed in the findings, the effects need to be repeated by other research teams. This independent replication is an important part of science," Jutfelt said.

Jutfelt and the research team wanted to respond internationally to the need to study issues of such global importance in a way that invites other researchers to replicate the work for more well-rounded results. They designed their three-year long study to match the species, life stages and location and seasons of the previous studies that showed such catastrophic effects, Jutfelt said.

The researchers also made it a point to very carefully document their experiments with videos and made their raw data and analysis available so that other researchers could see exactly what they had done, said Josefin Sundin, the last author on the paper and a researcher at the Swedish University of Agricultural Sciences.

"As far as possible, we used automated tracking software to analyze the videos from our experiments, to minimize observer bias, and we also made our raw data and analysis code available to other researchers,” Sundin said.

Their findings were consistent, clear, and irrefutable: Coral reef fish behavior wasn't changed by ocean acidification. Although these findings offer a small glimmer of hope for coral reef fish species, climate change continues to present an enormous and serious problem, the researchers said.

"While our new work suggests ocean acidification may not cause population declines because of behavioral disturbances in coral reef fishes, climate change is currently destroying the reef habitat through coral bleaching during heat waves," Clark said. "So, despite our new results, coral reefs and their fish communities remain in grave danger because of increasing atmospheric CO2."

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