In hot water: How warmer years might affect salmon populations

Salmon in the Nushagak District's Wood River in June 2019. (Photo by Isabelle Ross/KDLG)

Salmon in the Nushagak District’s Wood River in June 2019. (Photo by Isabelle Ross/KDLG)

There is a lot of worry and speculation about hot weather affecting sockeye runs on the east side of Bristol Bay this summer. But despite some of the hottest air and water temperatures on record, every district is meeting — or exceeding — expectations, both for escapement and harvest.

As the water temperature trends continue to rise over the years, the question at the fore is how warming water will impact salmon health, and whether it shapes the runs from year to year.

Jerri Bartholomew, Director of the Aquatic Animal Health Laboratory at Oregon State University, set the baseline for a discussion on the long-term effects of heat on salmon.

“When you try and predict what’s going to happen in an ecosystem, things just get complicated,” she said.

Trying to peer into the future of Bristol Bay salmon is a bit of a shot in the dark. Without actual data, it’s a lot of guesswork: trying to find places with ecosystems similar to the bay, then extrapolating conclusions based on what we already know.

Still, there are three pressing questions when it comes to salmon in warmer waters.

The first, basic question, is how temperature affects the fish — in both the short and long term.

“Temperature has a non-linear effect on salmon at all stages of the life cycle,” said Daniel Schindler, a professor in the University of Washington School of Aquatic and Fisheries Sciences and a principal investigator for their Alaska Salmon program.

According to Schindler, warmer temperatures have actually increased survival rates among salmon.

“There’s a very clear observation from the warming trend of the past decades,” he explained. “Juvenile salmon are growing faster in fresh water and spending more time in lakes and oceans.”

Schindler said the record-breaking runs of the past few years are likely due to the fact that warmer water has increased the survival rates of salmon smolt, because warm water increases food supplies for the young salmon.

The second issue is whether parasites in salmon are more widespread in warmer conditions. Studies conducted in the United Kingdom have found that fish parasites can grow up to four times as quickly in warmer waters. Bristol Bay fishermen have talked in recent years about a higher prevalence of parasites in local salmon.

However, Schindler said his studies have not shown that to be the case. One of his students recently looked at parasite infestation data from smolt leaving Bristol Bay rivers in 2010 and compared it to data from the 1950s, and found almost no difference in the number of incidences of parasite infestation.

“The expectation is that warmer waters lead to higher parasite infestation,” he said, “But the data we’ve looked at just don’t suggest that’s the case.”

This isn’t to say that there aren’t parasites around in Bristol Bay, or that there won’t be more as water gets warmer. But Schindler said that at this point, research doesn’t show that steadily warming waters have ballooned the parasite populations. That might be because of some of Bristol Bay’s unique water qualities — runs here are shorter and fish don’t have to swim as far.

Finally, fish diseases can be very temperature dependent. The third question regarding salmon health is whether salmon in Bristol Bay see more diseases as conditions heat up.

“Warmer waters have the potential to increase disease risk,” said Bartholomew of OSU. “If fish are migrating and then congregating, pathogens can be transferred from fish to fish and that can be bad.”

Bartholomew added that an increase in temperature does not equal a disease outbreak.

There are a multitude of factors that affect the spread of fish disease, and each differs based on the watershed and river system.

“In the last 50 to 70 years, The warming trend has translated into an increased abundance of salmon,” Schindler said. “The concern is where you tip off the other side, but we haven’t seen that, even in the last few warm years.”

In his estimation, only time will tell what’s too hot for the salmon.

Bartholomew provided an example of a place that might be exemplary of what years of hot, dry weather could do to Bristol Bay salmon. Although Bristol Bay may be different, she says we can draw conclusions from other places that have warmed and seen changes to their salmon populations.

“The emergence of whirling disease in Alberta, Canada, is probably a good demonstration of what we’re looking at,” Bartholomew explained.

“They went through a warm, dry period of a few years, and then saw clinical whirling disease after having never even thought they had it in Canada … You see habitats being permissive to things they haven’t been permissive to before.”

Salmon diseases can be more deadly in changing aquatic environments, but so far that hasn’t been the case in Bristol Bay.

For the time being, scientists seem reticent to say too much about what might happen as things get hotter. They know that when things get too hot for salmon, in the upper 70s Fahrenheit, they start to physiologically break down. But Bristol Bay hasn’t reached that point yet. In the past decades, salmon have come back stronger even in warmer waters.

Just as fisheries around Alaska are still discovering the effects of the unusual warming event of 2014 in the Pacific, also known as “the Blob,” Bristol Bay may not see the effects of this abnormally warm summer for years to come.

It’s up in the air — or rather, down in the water.

Alaska has a lot going on right now.

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