A new study by the University of Delaware's Danielle Dixson and Rohan Brooker has shown that butterflyfishes avoid coral that has come in contact with seaweed. The paper, which appeared this week in the Nature publication Scientific Reports, is the first to critically evaluate the impact coral-seaweed interactions will have on coral associated reef fishes, a key component of coral reef resilience.

In controlled patch experiments in Fiji, the study found that coral-feeding and coral-associated butterflyfishes overwhelmingly avoided corals that had contact with seaweeds. This was true regardless of whether the visual cue of the seaweed itself was present, leading the researchers to conclude that the coral-seaweed interaction produced chemical cues that were left behind even after the seaweed was removed. 

"Butterflyfish are kind of like the canary in the coal mine," said Dixson, an assistant professor in the School of Marine Science and Policy, which is housed in the College of Earth, Ocean, and Environment.

"When problems start to happen, they will be hit first because of their strong reliance on coral for food and shelter, so understanding their ecology is important before reefs become too degraded or just aren't there."

Co-authors on the paper include UD post-doctoral fellow Brooker, the paper's lead author, and Simon Brandl from Smithsonian Environmental Research Centre and James Cook University in Australia. 

Coral reefs in crisis

Since the 1980s, coral reef cover has decreased by 80-90 percent in the Caribbean and by 50 percent in the tropical Pacific. The dramatic changes are a result of the synergistic impacts of climate change, overfishing, eutrophication and pollution. Formerly healthy reefs with many species and complex coral-dominated communities are being converted at an alarming rate to wastelands dominated by seaweeds.

This shift is compromising delicate coral reef ecosystems, reducing reef productivity and resilience and also local food and tourism opportunities, and decreasing habitat for marine organisms. Yet, little is known about how this shift affects coral reef fishes and organisms. 

While micro-algae have a symbiotic relationship with coral, macro-algae such as seaweed compete with coral for space on the reef. Seaweed grows faster than coral, but plant-eating fish known as herbivores typically keep the reef ecosystem in check. 

As more reefs degrade, however, scientists continue to struggle to understand why. Top-down theories suggest overfishing of predator fish leaves fewer herbivores to eat the seaweed. Bottom-up viewpoints contend that rising temperatures and runoff from fertilizers on land cause the seaweed to over grow, throwing the ecosystem out of whack.