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Seafood producers and conservation groups in British Columbia are monitoring an approaching marine heat wave that has the potential to keep the ocean warmer than usual into next year.

The phenomenon, considered significant but not extreme, has warmed some parts of the Pacific as much as 5 C above normal. Oceanographers say it could recede in the fall, as similar heat waves have done; however, a looming El Niño – the naturally occurring climate pattern that affects ocean surface temperatures every two to seven years – means warmer waters could also persist into next spring.

“The last time we had a large heat wave like we’ve got right now, and had El Nino looming along the equator, that was in 2014, and that led to ‘the blob,’ ” said Andrew Leising, a research oceanographer with the U.S. National Oceanic and Atmospheric Administration who has been tracking the weather event. The blob persisted from 2014 to 2016, disrupting the marine ecosystem, creating huge blooms of harmful algae and shuttering fisheries up and down the coast.

“So it leaves you to speculate: Will we have in 2024 another whole Pacific Northwest region-spanning blob like we did in 2015? It’s not out of the realm of possibility that we get another large heat wave that lingers through the winter,” Dr. Leising said.

Nico Prins, the executive director of the BC Shellfish Growers Association (BCSGA), said members have not reported any abnormal effects and that the association monitors such weather events.

“Generally speaking, a gradual increase in the ocean temperatures can be mitigated by adapting our grow-out techniques or breeding superior families,” said Mr. Prins, referring to strategies that can be used to produce more resilient shellfish. “It is the extreme and acute impacts that would harm our industry.”

A marine heat wave is defined as a weather event in which the ocean temperature remains above the 90th percentile for an extended period of time. The one currently off the West Coast developed in the Northeast Pacific in mid-May and grew as it travelled toward Oregon, Washington State and B.C.

Dr. Leising said Monday it is now roughly four million square kilometres in size, has reached the coast off Oregon and Washington State, and is among the 20 most severe he has seen in the Pacific since the early 1980s.

In a best-case scenario, it could recede in October or November, allowing ocean temperatures to return to normal over the winter months before El Nino brings warmer waters again, Dr. Leising said, noting that it could still have cumulative effects on marine animals. In a worst-case scenario, El Nino could extend the heat wave and keep waters warmer than usual through to spring.

Mr. Prins said shellfish growers have taken steps in recent years to mitigate the harmful effects of changing conditions where possible. For example, by starting oysters on a beach in the winter and exposing them to air and gradual warming, they can be trained to stay closed during warm summer months. Breeding programs can also produce oysters that are more resilient to warmer water, increased ocean acidification and other issues.

The association has also been working with the B.C. government’s Extreme Weather Preparedness for Agriculture Program to develop safeguards in the event of extreme conditions such as the 2021 heat dome, when billions of seashore animals were estimated to have died. In a similar future scenario, growers could take specific actions to prevent losses, such as deploying sprinklers on beaches to cool down their shellfish, Mr. Prins said.

Linda Sams, the sustainable development director at Cermaq Canada, said the salmon farming company feels prepared for the coming weather events. Like shellfish growers, salmon farmers can control conditions to some degree, deploying mechanical, biological and operational techniques to protect the fish, she said.

To mitigate warmer surface waters, staff can pump cooler water from greater depths into the pens, for example. Barrier technology can also help retain cooler water. Breeding programs can produce salmon that are resistant to warmer temperatures. And farms have some flexibility when it comes to harvesting or introducing smolts.

“Salmon farming is lucky. In a way, we’re in a much better position than many, just from our state of preparedness and the sophistication of our equipment,” Ms. Sams said.

Isobel Pearsall, director of the marine science program at the Pacific Salmon Foundation, said the blob’s impact on wild Pacific salmon was complex. The extended period of warmer ocean temperatures reduced nutrient supply, led to huge changes in the food web and wreaked havoc on many stocks. Sockeye salmon returns plummeted, with the knock-on effects lasting for years. Coho salmon in the Strait of Georgia appeared to fare well – “but you can imagine that you can get to a point where that’s just not going to be the case,” she said.

Warmer water is just one of several challenging conditions Pacific salmon are facing. B.C.’s severe drought conditions also threaten to strand rearing and returning salmon, and potential winter flooding could lead to further egg losses.

Ms. Pearsall said the foundation is in the process of implementing a crowdsourcing initiative to identify areas of the province that are under drought stress. It is also working with Fisheries and Oceans Canada to implement emergency measures in the face of acute events, such as erecting temporary shade screens and removing sediment from rivers and streams to create cooler and deeper pools during extreme heat waves.

Charles Hannah, a research scientist with Fisheries and Oceans Canada at the Institute of Ocean Sciences, tracks ocean temperatures using Environment Canada buoy data. He pointed to readings from several buoys in B.C. – off Kitimat, in North Hecate Strait and in the southern region of Queen Charlotte Sound – showing that temperatures this spring and summer have already peaked at several points “well outside what we think of as the normal variability” dating back to the mid-1980s.

“Is it unusual? Yes, if you look at the longer record. Is it unusual over the past five or 10 years? Well, I’m having trouble with what ‘usual’ is,” said Dr. Hannah, who is based in Sidney, B.C. “From my perspective, things are changing. Warmer than normal is kind of the new normal.”

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