Reef fish communities are collapsing under climate stress

Coral reefs are full of life, with fish swimming through corals and algae covering the seafloor.

But climate change is starting to damage these underwater ecosystems. As oceans warm and become more acidic, reefs are losing the structure that many marine species depend on.

A new study published in the Journal of Animal Ecology found that fish may still behave normally under climate stress.

However, when reefs lose shelter and complexity, the social groups that help fish survive begin to break apart.

Reefs as climate windows

Marine ecologists from Adelaide University carried out the study near the coast of Japan. The region offered a rare opportunity to observe reefs already living under conditions that resemble the future ocean.

One reef reflected present day ocean conditions and acted as a control site. Another reef experienced water temperatures roughly 1 degree Celsius warmer because of the Kuroshio Current.

A third reef sat near volcanic carbon dioxide seeps, where bubbling CO2 lowered ocean pH and recreated the warmer, more acidic seas expected later this century.

“The reefs we work at in Japan are unusual in that they are near volcanic CO2 seeps on the seafloor, which create climatic conditions analogous to projected future ocean conditions,” explained Professor Ivan Nagelkerken, the project leader at Adelaide University.

“Some reefs sit under present-day seawater chemistry, others are warmer, and some experience both elevated temperature and acidity together. These natural climate analogues allowed us to ask real ecological questions in a natural setting.”

Fish depend on groups

The team focused on a small electric blue species called Pomacentrus coelestis, also known as the neon damselfish. These fish live in shoals, feeding and moving together while keeping watch for predators.

“Watch a reef long enough and you realise that fish are almost never alone. They move in groups, feed in groups, and react to danger as a group,” said study lead author Dr. Angus Mitchell.

Shoaling gives small reef fish several advantages. More fish mean more eyes scanning for threats and a lower chance that one fish becomes a predator’s target.

“For small reef fish, being part of a shoal is a survival strategy,” Mitchell said. “More eyes spot predators sooner, more bodies mean any one fish is less likely to be the unlucky one.”

Tracking fish behavior across reefs

Between 2021 and 2024, divers used GoPro cameras to observe fish behavior across all three reef systems.

The study included periods of normal conditions and intense marine heatwaves, including the major 2023 heatwave.

Researchers measured feeding rates, activity levels, hiding behavior, and how close fish stayed to shelter. They also tested flight initiation distance, which measures how close a threat can approach before a fish flees.

The team also counted fish populations, sampled plankton, and measured reef vegetation height to understand habitat complexity.

Larger shoals made fish bolder

One pattern appeared clearly across every reef. Fish living in larger shoals behaved more boldly.

They spent more time feeding and swimming in the open. They hid less often and allowed threats to approach more closely before retreating.

“Fish in bigger groups tend to be bolder,” Mitchell notes. “They forage more efficiently, stay out in the open more, and spend less time hiding.”

These social benefits remained strong even during heatwaves and under altered ocean chemistry.

Heatwaves had little impact

The researchers expected warming and acidification to directly disrupt fish behavior. Instead, the fish remained surprisingly stable.

Marine heatwaves did not significantly change feeding activity, refuge use, or escape behavior. Even during the intense 2023 heatwave, the damselfish continued behaving much as they normally would.

“The direct effects of warming, acidification, and heatwave stress on individual fish behaviour were mostly minimal,” said Nagelkerken.

“Across all reef types, even during a heatwave, the fish behaved in much the same way. They kept feeding. They did not suddenly become more active.”

The authors suggest this may be because the study took place near the cooler edge of the species’ range, where moderate warming is less damaging.

Acidified reefs lost fish communities

Although individual behavior stayed stable, the reef exposed to both warming and acidification showed another problem.

Shoals there were dramatically smaller, with some groups up to 79 percent smaller than those at healthier reefs. Population density also dropped sharply.

The fish still behaved normally, but there were not enough of them gathering together to create large shoals.

Habitat was the problem

The researchers checked whether food shortages explained the decline in fish numbers. They found the opposite.

Zooplankton levels at the acidified reef were actually higher than at the warming reef. The real issue was habitat structure.

Corals were nearly absent at the acidified reef. Tall canopy-forming algae had shrunk dramatically.

Instead of a layered and complex environment, the reef had become a flattened carpet of turf algae.

Without shelter and structure, fewer fish settled there and fewer survived.

Reefs need social life

The study suggests climate change may alter marine behavior indirectly. Instead of warming immediately changing how fish act, climate change first reshapes the habitat.

Habitat loss then changes fish populations and group size.

Shoaling depends on healthy reef communities and enough fish living together in the same space.

“In the real world, fish do not experience climate change in isolation,” Mitchell said. “They experience it as members of communities, shaped by the habitat around them and the other individuals they live alongside.”

“Our results suggest that even when individual fish seem to be coping fine behaviourally under climate stress, the social structures supporting their behavioural expression can quietly fall apart.”

The findings show that a reef can still contain fish while losing the crowded social networks that once defined it.

When reefs lose their structure, fish do not just lose shelter. They lose the communities that help them survive.

The study is published in the Journal of Animal Ecology.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–