There are less and less marine parasites. Contrary to appearances, this is a bad sign

Warming sea and ocean waters have a serious impact on hundreds of animal and plant species. As a result of changing thermal conditions, the ranges of occurrence of fish and marine mammals are shrinking, and their populations are decreasing at an express pace. The research, the results of which were published in “Proceedings of the National Academy of Sciences”, show that this trend applies not only to the most famous, popular and spectacular species. The researchers say that ‘if it were species that people care about, such as mammals or birds, conservation measures would certainly be implemented.’

Researchers from the University of Washington studied preserved fish specimens from the collection of the Burke Museum of Natural History in Seattle. All of them were fished from Puget Sound on the northwest coast of the United States. It is one of the largest such ecosystems in the country. The oldest individuals were from 1880, and the latest from 2019. The researchers carefully dissected the preserved fish to see if parasitic organisms were present, and then identified and counted the parasites. It is the largest and longest-running collection of data on parasites anywhere in the world.

In total, scientists counted more than 17,000 parasites belonging to 85 types. Statistical analysis showed that their number decreased over time. Most of them were found in the oldest individuals, and the least in those from the 21st century. They found that in the century from 1880 to 1980, 10 species disappeared completely from the waters of Puget Sound. For parasites that depended on three or more hosts (most of the species found belonged to this group), the average decline was 11 percent per decade. Parasites with one or two hosts have remained fairly stable.

“The decline was serious,” admits Chelsea Wood, lead author of the study.

The authors of the study considered three possible causes: changes in host abundance in Puget Sound, pollution levels, and thermal conditions in the surface water layer. The variable that best explained the decline in parasites turned out to be the water surface temperature, which in the bay increased by 1 degree Celsius between 1950 and 2019.

‘If this has gone unnoticed in such a well-studied ecosystem, what is happening elsewhere?’ Wood asks. “I hope our study inspires others to … find out if this trend is unique to Puget Sound, or if it’s something that applies to more ecosystems.”

Wood explains that the disappearance of so many parasites is not good news at all. On the contrary, it is a disturbing signal that something is wrong with the ecosystem.

“Parasites with a complex life cycle probably play an important role in regulating food relations within the ecosystem and in supporting dominant predators” – he explained. ‘The results highlight the fact that parasitic species may be in real danger, which could mean difficulties for us as well,’ he warns.

Wood added that most people believe that climate change will increase the number of parasites and that as the world warms we will see more cases of parasite infections. This, however, does not always turn out to be true.

‘There may indeed be a trend among some species, but parasites are host-dependent,’ he explains. – This makes them particularly vulnerable in a changing world where even the fate of their “hosts” is uncertain.