An investigation of microplastics in marine mammals from German waters has revealed that the tiny particles were present in nearly all the deceased harbor porpoises examined by scientists, although more research is needed to determine precisely what consequences this could have for the animals' health.
The researchers additionally found that harbor porpoises from the Baltic Sea had higher levels of microplastics in their intestines than seen in porpoises from a neighboring population in the North Sea — a worrying development because the population dwelling in the Baltic Sea is considered critically endangered. The team reported the findings May 10 in Frontiers in Marine Science.
Scientists have raised concerns that microplastics may cause inflammation, contain toxic chemicals or attract other pollutants that can accumulate in fatty tissue and organs. Examining top predators such as marine mammals can give scientists a snapshot of how microplastics are moving through an ecosystem, said Carolin Philipp, a marine biologist at the University of Veterinary Medicine Hannover Foundation and first author of the new study.
"Since the harbor porpoises prey on different fish species, and these fish species prey on smaller fishes or invertebrates — species like crustaceans or mussels, for example — we have a whole overview of the food web," she said.
Harbor porpoises are small and mostly solitary, Philipp says. They're the only cetacean — the group that includes whales, dolphins and porpoises — regularly found in the waters around Germany, and the only resident cetacean in the Baltic Sea more generally.
It's well-documented that zooplankton, fish and other animals low in the food web consume microplastics, which include plastic fragments less than 5 millimeters (0.2 inches) in size. Philipp's team and other scientists have also measured microplastics in marine mammals such as seals and whales, which feed on these smaller animals. Still, knowledge is limited about how much microplastic marine mammals are consuming and accumulating in their bodies and what health consequences can ensue, she says.
To gather more information, Philipp and her colleagues examined the intestines of 30 harbor porpoises that were found along the German coastline between 2014 and 2018. These included animals from both the North and Baltic Seas that had become stranded or entangled in fishing nets.
The researchers placed intestinal samples from the carcasses in double-layered nylon washing bags with mesh sizes of 300 micrometers and 100 micrometers, respectively, in the inner and outer layers. The team ran each bag through a washing machine at 60 degrees Celsius (140 degrees Fahrenheit) with detergents to get rid of the blood, feces and soft tissue remains from the porpoises' last meals.
This left the team with intestinal tissue and hard items such as sand grains, fish bones and plastic. The researchers separated the microplastic fibers and fragments from the naturally occurring materials, then counted and measured them under a fluorescence microscope.
Philipp and her colleagues found microplastics in the intestines of 28 of the 30 harbor porpoise carcasses. The researchers counted 401 particles in total that were greater than 100 micrometers in size, and the highest number of particles they identified in an individual porpoise was 48.
The number of plastic particles the porpoises carried didn't significantly differ between male and female, or adult and juvenile, animals.
The researchers also saw that porpoises that seemed to be well nourished when they died had higher levels of microplastics than ones with poor nutrition. Possible explanations for this observation could lie in the amount or kind of prey the porpoises were eating or the habitat in which they hunted, Philipp says.
When she and her colleagues examined the composition of the particles, they identified several common polymers used for packaging material, plastic bottles, fishing gear, clothing and many other products. The team also noticed a paint chip in a porpoise from the North Sea, an area with high ship traffic.
For this analysis, the researchers only examined a small number of porpoises. Studying more animals over time will be important to determine how representative the findings are and to pick up trends in microplastic exposure.
Philipp and colleagues plan to investigate whether tiny plastic particles are transferred from the gastrointestinal tract to other organs, and if pollutant concentrations in harbor porpoises are tied to how much microplastic the animals have ingested.
More research is also needed to explore how much plastic of different sizes the animals eliminate in their feces and how much accumulates in the body.
Still, there were some hopeful signs. Philipp said, "Since two individuals showed no microplastic occurrence within their GI tract sample, I was delighted."
And although the implications for conservation are important, the findings may also have relevance to efforts to understand how microplastics impact human health.
"Because we have the chance to investigate carcasses of different marine mammals, it gives us a chance to have a closer look at the impacts of the particles themselves to mammal species," Philipp said.
The study, "First evidence of retrospective findings of microplastics in harbour porpoises (Phocoena phocoena) from German waters," published May 10 in Frontiers in Marine Science, was authored by Carolin Philipp, Bianca Unger and Ursula Siebert, University of Veterinary Medicine Hannover Foundation; and Sonja M. Ehlers and Jochen H. E. Koop, Federal Institute of Hydrology and University of Koblenz-Landau.