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In 2014 and 2019, sea surface temperatures across a broad area warmed to well above average (0 in the middle of the key). On map, 3° C = 5.4° F // Map: NOAA Fisheries

Warm-water ‘blobs’ significantly diminish salmon, other fish populations, study says

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It’s no secret that salmon and other Northwest fish populations are expected to shrink as a result of a warming Pacific Ocean. But a new study suggests that the resulting decline in commercial fishing by 2050 could be twice as great as previously estimated by climate scientists.

The higher estimates of population declines were calculated by researchers at the University of British Columbia, who took into account occasional “marine heat waves” that can play havoc with the ecosystem. A recent example is the warm-water event known as the “blob,” which included ocean temperatures up to 7 degrees above average (Fahrenheit) during a two-year period beginning in 2014.

Current sea surface temperature anomalies (variations from average) for the Pacific Coast off North and South America. The temperature scale is different from the maps above.
Map: NOAA Coral Reef Watch, April 23, 2020

William Cheung, who led the new study, told me that previous estimates of declines in fish populations assumed that the waters would warm at a steady rate as a result of climate change. But the impacts are much greater, he said, when one considers the occasional shocks to the system caused by rapid warming. Climate-change models predict at least four additional “blobs” before the end of the century, although nobody can predict when exactly they will occur.

Cold-water fish subjected to warm water face a disruption in their normal body functions, reducing the size of the fish and increasing the risk of death. Warm water also can reduce the overall production in the food web, making it more difficult for fish to find suitable prey.

For the fishing industry, marine heat waves are not unlike a sudden pandemic such as COVID-19, William said. Fishing crews can adjust to normal fluctuations in fish populations, just as health-care providers adjust to flu seasons, but sudden and stronger disruptions can lead to more serious consequences.

“Last year, management agencies closed the Alaska Pacific cod fisheries (for 2020), because they had a suspicion that the blob was returning,” said Cheung, a professor at the UBC Institute for the Oceans and Fisheries. “There was concern that the already low Pacific cod population could be hit by a heat wave that could drive the fish stocks to very low levels.”

The 2019 “return of the blob” was not as long-lasting as the 2014-16 event, but waters off the coast are still warmer than normal.

The new study, published online in the journal “Scientific Reports,” combined climate and fish models to estimate the impacts of future “blobs” from Alaska to the Gulf of California. Findings suggest that the total biomass of fish will decline, and fish will move around to establish new distribution patterns. That will decrease the amount of fish available for harvest as well as changing the location where the fish can be caught.

William Cheung

While many studies have talked about fish stocks moving around in response to changing ocean temperatures, William said biomass decreases could be a more consistent indicator for assessing the impacts of marine heat waves on various species.

During a heat wave, the average biomass of sockeye salmon in the ocean off Alaska and British Columbia is expected to decline by more than 10 percent — in addition to a biomass decrease of 10 to 20 percent by 2050 under long-term climate projections.

Of 22 fisheries included in the study, only Alaskan Pollock in the Eastern Bering Sea is expected to increase significantly in biomass during marine heat waves. Pacific sardine and Japanese mackerel may show little change.

Because sardines do better in warmer waters, long-term models tend to project increases in sardine biomass along the West Coast over time, while anchovies, which prefer cooler waters, are projected to decrease. At the same time, such models predict that both species will expand their ranges northward, producing greater numbers in the Gulf of Alaska.

But the story is different when marine heat waves are added into the picture, according to the new study. Rapid warming can push temperatures to the limit for both sardines and anchovies, decreasing their total biomass in the Gulf of Alaska as well as along the West Coast.

The study found that the fish most impacted by a combination of long-term climate change and future “blobs” were pelagic (open water) species, followed by salmon and then bottom fish. Among the five species of Pacific salmon, the biomass of sockeye salmon is expected to decrease the most — 40 percent by 2100 throughout the study area. Coho are next on the list of affected salmon.

Pacific cod, sablefish and Pacific Ocean perch were the bottom fish projected to sustain the most losses throughout the area.

Worldwide, the frequency of marine heat waves has doubled since 1982, and climate models predict they will become more frequent and last longer in the coming years.

William noted that the study was based on a climate model that uses a high rate of greenhouse gas emissions (RCP 8.5). While recent temperatures seem to be following that high-emissions trend, emission reductions would have benefits for almost all fish populations. Still, any improvements in ocean-temperature trends will lag behind improvements in atmospheric conditions because of the heat-retention properties of water.

“Our results underscore the need for a reduction of anthropogenic greenhouse gas emissions – the fundamental driver of ocean
warming — to limit challenges from marine heat waves on fish stocks and fisheries,” William said.

The fact that marine heat waves can develop rapidly demands that scientists become better at short-term predictions, he said. Meanwhile, fisheries managers are challenged to develop plans that can respond quickly to changing conditions by reducing fishing seasons or moving fishing areas.

William plans further analysis of “blobs” across the globe, with a goal of developing projections of worldwide fishery impacts. That could lead to an economic analysis of future financial repercussions expected to result from sudden warming events in many locations.

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