Press release, 22 April 2020: Ocean warming halves coral reef fish communities. Higher water temperatures linked to climate change slashed a Pacific Ocean coral reef fish community by half, according to a new study led by University of Victoria biologists that is one of the first studies to assess the direct impact of heat stress on reef fish.  

 “The devastating effects of climate change on corals are well known,” says UVic professor Julia Baum, the study’s senior author. “Much less is known about how increasing water temperatures directly affect the thousands of fish species that make their homes on coral reefs.”

 In a study published recently in Ecological Applications, researchers counted 170,000 individual fishes of 245 different species of reef fishes at 16 reefs on Christmas Island (Kiritimati), before, during and after the 2015–2016 global marine heatwave that caused mass coral bleaching and mortality on reefs around the world.

 Climate change threatens the world’s coral reefs because corals are highly sensitive to the temperature of surrounding waters, Baum says. In warmer water, corals release algae living in their tissues causing the coral to turn completely white—known as coral bleaching—that can lead to coral mortality.

 Worldwide, coral reef fisheries are worth US$6.8 billion annually, and are a vital source of food and income for hundreds of millions of people in tropical island nations. Understanding the impacts of marine heatwaves on species of reef fishes could serve as a window into the potential long-term consequences of climate change-induced ocean warming for coral reef ecosystems and tropical coastal communities.

 Previous studies focused on the knock-on effects of climate change-induced coral bleaching on reef fishes, rather than on how the heat stress impacted the fish directly. “Our study shows that reef fishes are also highly vulnerable to rising water temperatures,” says Baum. “Unless we intervene to limit climate change globally, we risk losing not only corals but entire reef fish communities.”

 When subjected to warmer waters, researchers found that reef fish numbers plummeted by 50 per cent. The researchers believe that the fish moved to deeper waters to avoid the stressful conditions. “As many reef fishes live close to their optimal or maximum temperatures, even a one-to two-degree rise can be stressful for them,” says lead author Jennifer Magel, who recently graduated from UVic with a master’s in biology under Baum’s supervision.

 A year after water temperatures returned to normal most of the reef fish community had recovered to pre-heat stress levels, although reefs exposed to high levels of human disturbance impaired recovery. Some reef fishes also continued to decline as a result of the severe coral loss that occurred: numbers of butterflyfish, which feed on corals, dropped by 80 per cent.

 “Climate change may be a double whammy for reef fish,” Baum says. “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. These are above and beyond the impacts of the loss of corals on reef fishes.”

 The research was supported by multiple agencies including the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation and the British Columbia Knowledge Development Fund.

Photo Credit: Banner Photo (Kristina Tietjen)