By Isabella Breda, The Seattle Times

Katherine Maslenikov carefully plucked a young walleye pollock specimen from a glass jar full of ethyl alcohol. The critter’s eye and insides were missing, some carefully packed in a nearby vial labeled “inner organs.”

Maslenikov is the steward of the millions of fish specimens dating back to the 1800s neatly organized in a massive library of jars hidden in the basement of the University of Washington’s fisheries teaching and research building.

In their flesh are thousands of parasites — worms and other critters that lived off their host — that could reveal a hidden effect of climate change. Researchers fished out more than 17,000 parasites of the nearly 700 fish they studied. But as they moved to newer samples, they found fewer and fewer of the passengers.

In a study published Monday, scientists concluded that warming water may have caused some parasite populations in Puget Sound to plummet. The research suggests parasites may be especially vulnerable to global warming.

“It’s really our first peek into what parasites have been up to over the past couple of decades,” lead researcher Chelsea Wood said. “It’s a warning. It suggests that there might be more loss of parasite biodiversity than we previously anticipated.”

The loss of parasites could have implications for the long-term stability of Puget Sound ecosystems, according to the study, now the world’s largest and longest data set of parasite abundance.

As a whole, the oceans are taking up more than 90% of the extra heat in the climate system: the atmosphere, the oceans, ice pack, land and living things, said Nick Bond, Washington state climatologist and professor at the University of Washington.

“Puget Sound is along for the ride,” he said.

This decline in parasite abundance was correlated with increases in sea surface temperature measured at the Race Rocks lighthouse, off the coast of British Columbia, where saltwater spills into the Salish Sea from the Pacific Ocean.

Researchers in 2019 began carefully slicing open the bellies of eight species of fish including herring, hake, rockfish, pollock and perch, collected from 1880 to 2019 and held at UW, identifying and tallying parasites along the way.

They found 85 types of parasites including arthropods, little crablike or shelled creatures, and “unbelievably gorgeous tapeworms,” Wood said.

There was no significant change in abundance of parasites that require one or two hosts. But, those with complex life cycles, or those who relied on three or more hosts, declined in abundance at a rate of about 40% for every roughly 1.8-degree Fahrenheit increase in surface water temperature.

That decline is likely because climate change is affecting the parasites’ host species. Rising water temperatures and increasing amounts of carbon absorbed by the ocean are damaging marine habitats and the fish, crustaceans and shellfish that rely upon them. But some species may continue to thrive, while others decline.