Warmer seas could be making fish forgetful.

Most of the global ocean has been exceptionally hot this summer. Now, a new study suggests for the first time that high water temperatures can cause memory loss in reef fish, and even render them unable to learn at all.

That could have deadly results. Tropical fish live in complex environments and they’re constantly faced with decisions: What to eat, whom to fight, where to go. Surrounded by predators, every move could mean life or death.

“Cognitive ability in fishes is very important for their survival,” said Ana Carolina Luchiari, a fish biologist at the Universidade Federal do Rio Grande do Norte in Natal, Brazil, and an author of the study, which was published this month in the journal PeerJ.

A lot has been done to understand how heat can kill fish, Dr. Luchiari said. But there’s relatively little research on how heat affects learning and memory in tropical fish that manage to survive high temperatures.

“Decision-making in the natural environment is very important, it’s something you have to do every day,” she said. “If you decide wrongly, you are exposed to risk.”

Many environmental changes, including acidification and low oxygen, can stress fish. Because warmer water holds less dissolved gas, hotter temperatures can cause oxygen levels in the water to drop. Like humans, fish get a little loopy without enough oxygen.

“When it gets too hot, brain function starts to fail and people do things that are considered odd and strange because their ability to think is breaking down,” said Simon Morley, a fish ecophysiologist at the British Antarctic Survey who was not involved in the study. The same is true for fish. “This study brings to the fore that behavior could be altered by the impacts of climate change.”

To investigate how warmer waters affect memory in tropical fish, the researchers studied damselfish, which often dwell in reefs. They are highly territorial and aggressive. Despite their small size, they are known to chase off much larger intruders, including snorkeling scientists, Dr. Luchiari noted. Their aggression comes in part from the need to defend the algae they feed on.

Damselfish “need to know a lot to live well in this environment,” Dr. Luchiari said.

The high stakes of memory loss for damselfish made them a compelling subject for the study.

The researchers designed a maze with a reward in one hallway. For about two weeks before maze training began, three groups of fish were gradually exposed to different temperatures: 28to 28.5 degrees Celsius for the control group, 30to 30.5 Celsius for the second, and 31.5 to32 Celsius for the third.

The researchers chose 32 degrees Celsius, or 89.6 Fahrenheit, as the limit because, in an earlier experiment, fish exposed to water at 34 degrees Celsius began experiencing life-threatening physical effects.

The researchers spent five days training the fish to navigate the maze and to associate a blue tag with their reward. Five days after training ended, they tested the fish to see which groups could remember how to find the tag, and their reward, in the maze.

The control group did well, quickly remembering how to reach the reward in the maze. But fish in even the moderately hot group didn’t fare as well. Although they learned to navigate the maze quickly during training, five days later, all evidence of their experience had vanished. In earlier experiments, Dr. Luchiari found that damselfish could remember experiences for at least 15 days, so an inability to remember the maze after only five was striking.

Fish in the hottest group failed to learn the maze at all, taking roughly the same amount of time to navigate it throughout the whole experiment. “They are not behaving well in higher temperatures,” Dr. Luchiari said. “In a natural environment, they aren’t going to find shelter or recognize their neighbors or find food easily.”

The study’s findings highlight a need for more research into how heat affects fish cognition at all latitudes and in different species, predator and prey alike, with the potential for cascading ecosystem changes.

“It’s a really nice first step to alert the community that temperature might be having these effects on memory and learning,” said Alastair Harborne, a tropical fish ecologist at Florida International University who was not involved in the study. “Even a couple of degrees of change can have a big effect.”

How temperature changes of different durations and magnitude affect fish memory remains an important open question. But what’s clear for tropical fish, Dr. Luchiari said, is that “we are putting them in a very dangerous situation.”