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.
If the parasite relies on some declining species as hosts, even if they had some thriving host species in their life cycle, they were ultimately likely to decline.
The researchers’ analysis showed an 11% average decline per decade in abundance of parasites with complex life cycles. Of 10 parasite species that had disappeared completely by 1980, nine relied on three or more hosts.
“There’s a limited amount that you can benefit from having a host that’s a winner,” Wood said. “But if you have a host that’s a loser there’s like infinite possible destruction that can happen.”
Why should people care about parasites?
Think back to the early 1900s, Wood said, when many predators were perceived as vermin. Take wolves, for example. They were a threat to livestock, and potentially to people.
But when they were removed from Yellowstone National Park, the ecosystem became a Tilt-a-Whirl. Elk populations soared and decimated vegetation that other species relied upon. Beaver and bison were left without some of their main food sources.
When wolves were reintroduced in the 1990s, a wave of green returned to the park.
Parasites play a similar role in the stability of an ecosystem, Wood said, and without them, host species populations would explode.
Biodiversity, or the variety of living species, is the best means to slow the effects of climate change. Healthy ecosystems are more resilient. But a changing climate is also one of the main drivers in the loss of biodiversity.
“There’s something about having top predators to kind of weed out those sick and weak that it’s important to have that kind of competition there,” Bond said. “Those stressors, and certainly all biological systems, have evolved with parasites.”
There are conservation efforts for many other species impacted by a changing climate and human intervention, but not yet parasites.
Of the three environmental variables examined in the study — pollutants, fish density and sea surface temperature — only temperature could explain the overall change in parasite counts.
More studies need to be performed outside Puget Sound before any generalizations can be made about the broad impacts of climate change on parasites in the ocean. It’s unclear if the decline could be happening in freshwater ecosystems, or other saltwater ecosystems, Wood said. She has already begun studying the impacts in the Gulf of Alaska.
“If this is a general pattern,” Wood said, “this suggests to me that we should be investing more heavily in parasite conservation.”
