Warming oceans have destroyed marine parasites, study finds

Warming oceans have destroyed marine parasites, suggesting they may be especially vulnerable to climate change, researchers say.

The University of Washington study analysed more than a century of preserved fish specimens, assessing long-term trends in parasite populations. Critically, it found that the number of fish parasites fell between 1880 and 2019 as Puget Sound – the habitat surveyed and second largest estuary in the mainland US – warmed considerably.

Chelsea Wood, associate professor of aquatic and fishery sciences at the University of Washington, said: "People generally think that climate change will cause parasites to thrive, that we will see an increase in parasite outbreaks as the world warms.

"For some parasite species that may be true, but parasites depend on hosts, and that makes them particularly vulnerable in a changing world where the fate of hosts is being reshuffled."

This decline is concerning for ecosystems, with the study marking the world’s largest and longest dataset of wildlife parasite abundance. If this level of decline occurred in species people “care about”, it would likely trigger widespread concern, the scientists involved in the study said.

"Our results show that parasites with one or two host species stayed pretty steady, but parasites with three or more hosts crashed”, Dr Wood continued.

"The degree of decline was severe. It would trigger conservation action if it occurred in the types of species that people care about, like mammals or birds.

"Parasite ecology is really in its infancy, but what we do know is that these complex-lifecycle parasites probably play an important role in pushing energy through food webs and in supporting top apex predators."

The study primarily focused on eight species of fish common in the collections held by natural history museums. Importantly, fish, reptile and amphibian specimens are preserved in fluid, which preserves any parasites living inside the animal at the time of death. Elsewhere, mammals and birds are preserved with taxidermy.

Various parasites were discovered, including arthropods – animals with an exoskeleton – and tapeworms. The study found 17,259 parasites, comprising 85 types taken from 699 fish specimens.

"Our result draws attention to the fact that parasitic species might be in real danger”, Dr Wood concludes, remarking that Puget Sound has experienced a major decline in parasites.

"And that could mean bad stuff for us - not just fewer worms, but less of the parasite-driven ecosystem services that we’ve come to depend on."