Skagway’s blue mussels were nearly wiped out last year

A sea of mussels.
Blue mussels before the mortality event. (Reuben Cash)

Reuben Cash says blue mussels are best served steamed with melted butter. But this year, he doubts you could find enough for a meal.

Cash is the environmental coordinator at the Skagway Traditional Council. He said that last summer, 70-90% of Skagway’s blue mussels population died in what he calls a “massive mortality event.”

“You can see all the shells from what happened this summer. And for a while, I mean, it was thick,” he said. “Where the high tide comes in was just probably four inches deep, and empty mussel shells and dying mussels with meat still on it — it was widespread.”

The Skagway Traditional Council samples local mussels to test for paralytic shellfish poisoning because they work well as an indicator species for the toxin. In late June, they realized something was wrong.

“We noticed that there was kind of a funky smell about a week earlier,” Cash said. “They kind of had a sweet, slightly putrid smell. And in the weeks leading up to that, temperatures were in the 60s, maybe the 70s.”

He said there was a heatwave in the Pacific Northwest further south that caused many blue mussels to die, but the upper Lynn Canal stayed relatively cool.

“I kind of discounted the temperature theory right off the bat because it just didn’t get hot enough,” Cash said. “Blue mussels are pretty tolerant of high temperatures — they can tolerate up to about 85 degrees.”

Cash also considered that it could have been some sort of pathogen, like a virus or bacteria affecting the shellfish, but blue mussels are highly resistant to them.

He said it was probably a combination of things that caused the mussels to die.

Dead blue mussels line the beach
Dead blue mussels line the beach at Nahku Bay. (Reuben Cash)

One thing is the salt. Typically sea water levels are 35 parts per 1,000.

“Starting in June, in 2021, it was down below one part per 1,000 — like, as fresh as river water,” Cash said.

And Cash said when the salt levels drop, blue mussels aren’t as tolerant of temperature shifts.

Another issue is sediment.

“Not only does the freshwater dilute the amount of salt that’s in the water, it also introduces a lot of sediment,” Cash said. “Sediment covers up the muscles. Now, they’re filter feeders, they’re not going to be able to function as well as they would if the water was clear.”

Last winter there was record snowfall in the mountains above Skagway and Dyea. Then the area went through a cool spring, which kept the snowpack in place later than usual. As temperatures warmed up, the snow melted and brought fresh water and heavy amounts of silt into areas like Nahku Bay.

Then around the summer solstice, an extreme low tide occurred.

“So they were exposed at the low tide for longer and with a little bit higher temperatures, probably being smothered by sediment with low salinity,” Cash said.

Normally, the mussels can handle any one of these things — but all of them combined? It may have been too much, said Cash.

He said the blue mussels that are left will be the strongest of the population and the most resilient. But he’s asking harvesters to avoid collecting mussels until the population rebounds.

“If you want blue mussels next year, hold off this year,” advised Cash.

Blue mussels tend to spawn mid-summer, so it may be late summer before the area sees an increase in their numbers.

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