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Blue Mussel Population Nearly Completely Wiped Out, Researchers Try to Find Out Why

Dead blue mussels line the beach at Nauku Bay.
Reuben Cash
Dead blue mussels line the beach at Nauku Bay.

Last summer blue mussels in the waters around Skagway died off in huge numbers. The local tribal government is studying the phenomenon and has a working theory about how it happened. Now they are asking residents to hold off harvesting the shellfish until the population rebounds.

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 says last summer between 70% and 90% of the 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 it was I mean, it was thick. The whole rack line all along, 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,” said Cash.

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. But there was no evidence of anything happening just like the muscles were off in smell right? They kind of had a sweet, slightly putrid smell. And in the weeks leading up to that temperatures were in the 60’s, maybe the 70’s,” said Cash.

He says there was a heatwave in the Pacific Northwest further south that caused many blue mussels to die, but 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. Blue mussels are pretty tolerant of high temperatures, they can tolerate up to about 85 degrees,” said Cash.

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

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,” said Cash.

And Cash says 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. And so and 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,” said Cash.

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,” said Cash.

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

Cash says 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.

You can find a link to the original story here.