Environment

Stench From Dead Coral Reefs Stops Fish From Learning to Avoid Predators

Yellow damselfish swim within a coral reef. Researchers at the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) at James Cook University have found that fish release a chemical distress signal when caught by predators, dramatically boosting their chances of survival. Dying coral reefs threaten their ability to develop this instinct.
(Oona Lönnstedt )

Amid the ongoing threat of coral bleaching and deaths, researchers decided to study large changes in the fish population that coincided with this phenomenon. They discovered that this degradation can directly affect how fish learn to avoid predators.

In a study recently published in the journal Proceedings of the Royal Society B, a team of scientists determined that the smell caused by dead coral affects the ability of fish to sense the presence of a threat.

One of the strongest El Niños on record has warmed waters in the ocean, causing once vibrant reefs to die almost completely. Last month it was revealed that 93 percent of the Great Barrier Reef has been devastated by coral bleaching, which occurs when coral is stressed by changes in their living conditions such as temperature, exposure to light extreme low tides, or a tweak in ocean nutrients. They purge the symbiotic algae living in their tissue, causing them to turn white.

The team of scientists crafted an enclosed coral reef that simulated the community they were studying, according to BBC. They split their faux reef into dead coral and living coral and used young damselfish to study how the coral affected their ability to smell predators.

“Baby fish use chemical alarm signals released from the skin of attacked individuals to learn the identity of new predators. By pairing the alarm cue from their wounded buddy with the smell of sight of the responsible predator, fish are able to learn which individuals are dangerous and should be avoided in the future,” James Cook University professor Mark McCormick said in a release.

When corals die and become covered in algae, the sense of smell in the reef seems to change, which affects how fish learn this instinct that’s crucial to their survival, Phys.org reports.

(MORE: Coral Deaths Threaten Coasts With Erosion, Flooding)

“When we were doing the experiments we were bringing in some of this dead coral, and we started to get complaints from our friends in the laboratory because the stuff was so smelly,” McCormick told ABC. If the smell was too much for people, you can imagine how much if could affect a fish, especially when they only grow up to 14 inches long like the damselfish.

Damselfish’s bodies are covered with taste bud cells on the front, which they use to learn about predators.

“Taste and smell are incredibly important if you’re a fish,” McCormick told Phys.org. “The smell that comes off [of a] degrading coral reef is actually masking some of the important chemicals that the animal is using to inform its decisions.”

The scientists trained the damselfish to recognize the scent of a new predator by pairing that scent with another chemical damselfish release when under attack, BBC also reports. Only the fish in the healthy reefs were able to learn the new predator’s smell and hid among the coral upon coming into contact with it. In the dead reefs, the fish continued exploring, leaving themselves vulnerable.

“If the process of cataloguing and avoiding predators is hindered in some species by coral degradation and loss, then much of the diversity of reef fish could be lost too. Many reef fish need specific habitats that only healthy coral reefs can provide,” said Uppsala University researcher Dr. Oona Lönnstedt.

“If dead coral masks key chemical signals used to learn new predators, the replenishment of reefs could be seriously threatened,” she added.

MORE ON WEATHER.COM: Coral Bleaching