Study says fish stocks along Atlantic, Pacific coasts unaffected by marine heat waves

A diver with nonprofit Reef Renewal USA cleans and maintains a coral nursery, Tuesday, Aug. 1, 2023, near Tavernier, Fla., in the Florida Keys. (AP Photo/Wilfredo Lee)

Marine heat waves haven't had a lasting effect on fish populations along Canada's Atlantic and Pacific coasts, says a study showing there is still a chance to act on climate change.

The study published Aug. 30 in the journal Nature looked at data from 1993 to 2019, which included 248 marine heat waves -- described as prolonged periods of warm ocean temperatures.

"The expectation is that whenever there are heat waves, the fish stocks will be affected, and that the bigger the heat waves, the bigger the impacts. But what we find is that the impact is not consistent," William Cheung, professor at the University of British Columbia's Institute for Oceans and Fisheries, and co-author of the study, said in a recent interview.

"What it suggested is that the relationship between heat waves and fish stocks can be more complicated."

The study looked at 82,322 samples of heat wave data from long-term scientific surveys of marine ecosystems in North America and Europe spanning the subtropics to the Arctic. The authors defined a marine heat wave as a period of five days or more with extreme higher-than-average temperatures.

The data included notable marine heat waves, such as "the Blob," which blanketed the Northeast Pacific from 2014-2016, and the 2012 one in the Northwest Atlantic. While "the Blob" caused a 22-per-cent loss of fish in the Gulf of Alaska, the 2012 marine heat wave led to a 70-per-cent gain in the number of fish in the Northwest Atlantic.

The authors said steep declines in fish populations -- "occasionally occurred after marine heat waves, (but) these were the exception, not the rule ... Marine heat waves have not driven biomass change or community turnover in fish communities that support many of the world's largest and most productive fisheries."

Cheung said while the study measured heat waves through temperature changes, there are other factors that affect fish, such as food availability, oxygen levels and salinity of the oceans.

"It may be that the relationship between temperature and those other changes in ocean conditions are more complex," he said.

But the study warns that as marine heat waves get longer and more intense, fisheries may eventually be affected because marine life is more vulnerable to warming than is terrestrial life. Marine organisms tend to live close to the temperature limits of what their bodies can sustain, while oceans, compared with land, have fewer "thermal refugia" -- areas with cooler temperatures than the surrounding ecosystem.

"As a future that is more than 1.5 C warmer looks increasingly likely, it is more critical than ever to develop a deeper understanding of what drives ecological responses to extreme climate events," the study said.

Marine heat waves have also been linked to widespread coral bleaching and die-offs of kelp forests and reef fish in shallow coastal seas, Cheung said. As well, he said, scientists have observed fish migrating to cooler parts of the ocean as temperatures have risen.

"But the thing is that not all species will have the same ability to shift their distribution; some species may be limited by geography," he said. "There are also other environmental conditions that may prevent fish from moving to even further, deeper waters."

Cheung pointed to sockeye salmon -- a species of fish important to First Nations and historically found in the coastal regions of British Columbia -- which seem to be changing habitat in search of cooler waters. This shift in species habitat will be felt by people who rely on the salmon, he said, as well as on other organisms that feed on those animals.

Scientists are carefully studying the rare flash of good news about the lack of major impact caused by marine heat waves. But Cheung said marine life may be limited in its capacity to absorb a constant pummeling of high temperatures.

"It really highlights the urgency to do climate actions now," he said.

"If we don't take any actions now, with continuous climate change, we know it will increase the intensity and frequency of marine heat waves and it is going to lead to a bigger, scary impact."

This report by The Canadian Press was first published Sept. 11, 2023.