Fish chilling out as our oceans heat up

JAMES Cook University scientists have logged more than 600 hours underwater to discover an early warning sign that could indicate whether coral reef ecosystems are in danger from warming seas.

The so-called canary in the coal mine is the iconic butterfly fish whose sensitivity to changing sea conditions makes a study of the fish the ideal way to monitor the general health of a reef system.

Researchers have found that when water temperatures heat up fish become less aggressive, providing the first clear evidence of coral bleaching serving as a trigger for rapid change in reef fish behaviour.

Focusing on 38 butterfly fish species, researchers from the Centre of Excellence for Coral Reef Studies considered the findings an early warning sign that reef populations are in trouble.

Led by Dr Sally Keith of Lancaster University the team examined 17 reefs across the central Indo-Pacific in Japan, the Philippines, Indonesia and Christmas Island.

“We observed that aggressive behaviour had decreased in butterfly fish by an average of two-thirds, with the biggest drops observed on reefs where bleaching had killed off the most coral,” Dr Keith said.

“We think this is because the most nutritious coral was also the most susceptible to bleaching, so the fish moved from a well-rounded diet to the equivalent of eating only lettuce leaves – it was only enough to survive rather than to thrive.”

Such changes may well be the driver behind changes such as declining numbers of fish.