Australia barely had time to recover from record fires in early 2020 before the Great Barrier Reef experienced its third massive coral bleaching event in the past five years. Only five of these have occurred since records began in the 1980s. High water temperatures and marine heat waves, caused by climate change, are making coral bleaching an almost regular occurrence in some parts. of the world.
Coral reefs are among the most vibrant ecosystems on the planet, but they are also very sensitive to stress. Meteorologists predict 2020 is likely to be the hottest year on record, further threatening discoloration on reefs around the world. But it is not only the coral itself that suffers.
Reef fish exposed to high temperatures tend not to behave normally. Underwater noise and pollutants, like agricultural pesticides, can have the same effect. Juvenile fish exposed to this type of stress are less able to identify and avoid predators. But scientists are not sure exactly why this is.
In our new study, we discovered that a double whammy of higher water temperatures and exposure to pesticides may be affecting the development of baby reef fish, with consequences for the entire ecosystem.
Metamorphosis in coral reef fish
Metamorphosis may recall the transformation a caterpillar undergoes to become a butterfly, but it is also very common in coral reef fish. After hatching, most reef fish develop into translucent larvae in the open ocean, before transforming into juvenile fish as they are recruited for life on the reef.
This journey is a very important transition that is fraught with danger. Many predators like to eat reef fish while in this small and vulnerable stage.
Marc Besson, Author provided
During metamorphosis, fish quickly develop their eyes, noses, and lateral line system, a special set of organs that allows them to detect changes in water pressure. Reef fish depend on these senses to detect and avoid predators.
Recent studies have shown that the process is regulated by hormones in coral reef fish. So if human-made pressures as diverse as rising temperatures, noise and underwater pollution can cause juveniles to behave strangely and not avoid predators, maybe it all has something to do with it. with its hormones.
We decided to test this in a laboratory. We exposed the larvae of the surgeon who condemns fish to chemical hormones and hormonal blockers and found that it directly influenced the development of their eyes, noses, and lateral line systems. When the fish received a hormone blocker, their sensory systems developed more slowly, they were less able to identify a predator with the naked eye or smell, and predators caught them more easily.
Aldine Amiel / Kahi Kai, Author provided
Laboratory tests showed that exposure to a hormonal blocker can prevent fish from developing the defenses necessary for life on a coral reef. So how could they be advancing in the ocean, given that pesticides drain from land to coastal waters, where these chemicals can alter hormones in fish?
In other experiments, we exposed the fish to different temperatures and doses of a common agricultural pesticide called chlorpyrifos. The higher temperatures and high levels of the pesticide made the fish worse by avoiding predators, but when additional hormones were administered, the fish regained their abilities. This shows how climate change and pollution can affect how fish naturally regulate their development, and how important it is for them to be able to morph in a healthy environment.
Warmer water and pesticide exposure may seem like completely different things, but our results suggest that they affect metamorphosis in a similar way. This is troubling, because most species of coral reef fish metamorphose early in their lives, and if their environment changes too much, it could endanger entire coral reef communities.
Fortunately, our results suggest that these negative effects are primarily felt at higher doses, but are at the extreme of what is already being detected in reefs. That means we still have time, but our habits must change soon.
