As the sun rises over the Southern Great Barrier Reef, casting a golden glow across the lagoon at One Tree Island. Annabell Klinke, a PhD student at Macquarie University, is already in the water, notebook in hand, observing the small, black-and-white-striped humbug damselfish that dart among the branching corals. For Annabell, this isn’t just another day at the office – it’s a research expedition in one of the most iconic ecosystems on the planet, and one that is rapidly changing in ways few of us can see from the shore.
Annabell’s research is uncovering a startling link between coral reef degradation and fish intelligence. “Research shows that animals living in more complex environments, both on land and in the water, often develop better problem-solving and learning skills,” she explains. “Since coral reefs are among the most complex ecosystems on Earth, it makes sense to think that reef degradation could affect the intelligence and adaptability of reef fish, which might then impact their chances of survival.”
Her focus is on humbug damselfish, a small species that thrives among branching corals and forms social hierarchies. To test their adaptability, Annabell uses a reversal learning task. Fish first learn to associate a colour with a food reward, and then the rules are switched, with the “wrong” colour becoming the correct choice. “We also measured personality traits such as boldness, exploration, anxiety, aggression, sociability, and curiosity, since personality can play a big role in how animals approach problems,” she says.
Findings from previous lab experiments are fascinating. Fish raised in simpler habitats were bolder and less anxious, starting tasks quickly, yet the more anxious fish actually performed better overall, needing fewer trials to figure out the new rule. “As coral reefs lose their complexity, fish personalities may shift in ways that affect how well they can cope with change. This could make them less flexible when faced with new challenges,” Annabell warns. She is now running similar experiments in the wild to see if these patterns hold true in nature, bringing her work out of the lab and into the very ecosystem under threat.
The urgency of her work is clear. In March 2024, the Great Barrier Reef experienced its fifth mass bleaching event in just eight years. Annabell explains that the biggest threat to coral reefs today is climate change. “Greenhouse gas emissions are heating up the oceans and making them more acidic. Warmer waters disrupt the partnership between corals and the tiny algae that live inside them, called zooxanthellae. These algae give corals both their beautiful colours and most of their food. When the partnership breaks down, corals expel the algae, turn white – a process called coral bleaching – and begin to starve. If heat waves last too long, corals can’t recover, and many die.”
Ocean acidification further weakens coral skeletons, making them brittle and easier to break during storms. On top of that, these tropical storms have increased in frequency and intensity due to climate change in the last decades. Pollution, sediment runoff, nutrient overload from farming, and destructive fishing practices add even more pressure, creating a perfect storm of threats.
But the stakes go far beyond the fish themselves. Coral reefs support millions of people worldwide, particularly in coastal communities and Small Island Developing States. “For many tropical coastal communities, reefs are an essential source of nutritious and affordable seafood. Fishing on reefs is not only about food or income – it also brings enjoyment, identity, pride, and a way of life,” Annabell says. Reefs also act as natural buffers, absorbing up to 97% of wave energy during storms and protecting nearly 200 million people from damage. They fuel a global tourism industry worth almost $36 billion annually and hold enormous potential for biotechnology, often called the “medicine chests of the sea.”
Despite the challenges, Annabell remains hopeful. “Nature has an incredible ability to adapt and surprise us, and I believe reefs still have a future, even if they look different from the past,” she says. Protecting reefs isn’t only about global initiatives – it starts with everyday actions. “Protecting reefs doesn’t just happen on a global scale, it also starts with the small choices we make every day. By thinking about our carbon footprint and making small, positive changes in our lives, we can help give reefs the time and space they need to recover.”
As the day winds down, Annabell records her observations on the reef, eyes scanning the water for any sign of change, while tiny damselfish weave through the corals. Her work highlights not only the fragility of these ecosystems but also the vital role of women in STEM leading the research that could help save them. “If we choose to act with care and hope, I believe the reefs of tomorrow can still be vibrant, beautiful, and full of life, just in a new way,” she concludes.
For Annabell, the lesson is clear: understanding how climate change is reshaping the intelligence and adaptability of reef life is just the beginning. With courage, curiosity, and dedication, humans can still make a difference – and sometimes, saving the smartest fish in the sea starts with the smartest humans on land.
Main feature image: Helen von Drenkmann