Healthy coral reefs are a paradise for an incredible variety of sea life, supporting extraordinary biodiversity in a dazzling rainbow of colors. But coral bleaching — caused mainly by warming seawaters — is making those same reefs a tough place for many types of fish to survive.

Coral bleaching occurs when fragile branching corals are shocked by sudden temperature changes. The stress causes the coral to expel the symbiotic unicellular algae that give the coral its coloration.

When that happens, the fish that depend upon the colorful coral as camouflage become much more conspicuous, and thus more vulnerable to predators. That means they’ve got to find new homes in increasingly crowded areas of remaining healthy coral, which isn’t always easy — especially for smaller fish.

"Almost within the second that they settle they'll start bullying one another and jostling for position, jostling for the best possible spot ... to try and survive in," says Mark McCormick, an associate professor at James Cook University.

"So what we find is that yes, it does happen and that size does actually matter when you're on a bleached coral because you're the one that actually gets the best possible spot. You increase your survival slightly by pushing the smaller fishes away from that precious piece of habitat," he said. 

Such bullying could dramatically change coral reef ecosystems. McCormick believes a decline in small fish could lead to a decline in the larger fish that eat them, and it wouldn’t be long before humans felt effects in the fishing and tourism industries.

Coral reefs are the new tough neighborhoods
Big fish bully smaller fish for habitat as coral bleaching ruins their camouflage.