A new study led by University of Victoria biologists shows that higher water temperatures linked to climate change halved a Pacific Ocean coral reef fish community.
The study is one of the first to assess the direct impact of heat stress on reef fish. Multiple agencies including the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation and the British Columbia Knowledge Development Fund supported this research.
“The devastating effects of climate change on corals are well known,” said UVic professor Julia Baum, the senior author of the study. “Much less is known about how increasing water temperatures directly affect the thousands of fish species that make their homes on coral reefs.”
A recently published study in the journal Ecological Applications shows that researchers counted 170,000 individual fish of 245 different species of reef fish at 16 different reefs on Christmas Island (Kiritimati) before, during and after the 2015-16 global marine heatwave that caused mass coral bleaching and mortality on reefs around the world.
Baum said corals are highly sensitive to the temperature of surrounding waters. In warmer water, corals release algae living in their tissues, making the coral turn completely white. This is known as coral bleaching and can lead to coral mortality.
Coral reef fisheries around the world are worth $6.8 billion U.S. dollars per year and are an important source of food and income for hundreds of millions of people in tropical island nations, according to a release from UVic.
“Our study shows that reef fish are also highly vulnerable to rising water temperatures,” Baum said. “Unless we intervene to limit climate change globally, we risk losing not only corals but entire reef fish communities.”
Researchers found that reef fish numbers dropped by 50 per cent when exposed to warmer waters. UVic said the researchers believe the fish moved to deeper waters to avoid stressful conditions.
“As many reef fish live close to their optimal or maximum temperatures, even a one to two degree rise can be stressful to them,” said lead author Jennifer Magel.
One year after water temperatures returned to normal, most of the reef fish community recovered to pre-heat stress levels. However, reefs exposed to high levels of human disturbance impaired recovery. Some reef fish also continued to decline as a result of the severe coral loss that occurred. For example, numbers of butterfly fish – which feed on corals – dropped by 80 per cent.
“Climate change may be a double whammy for reef fish,” Baum said. “As the ocean temperatures experienced during this heatwave become the new normal over the coming decades under climate change, we may see permanent changes in the distributions and abundances of fish communities on shallow-water coral reefs.”