There is a growing awareness that tiny pieces of plastic, known as microplastics, are now widespread in the world's oceans -- but we are only just beginning to understand its impacts on marine life.
Several recent studies by scientists around the world are revealing that microplastics, defined as pieces of plastic smaller than 5 millimeters, are now present throughout the ocean food chain, and are being eaten and transferred from tiny zooplankton, to fish, and upwards to the turtles and seals that feed upon them.
One recent study by researchers at the Plymouth Marine Laboratory analyzed scat from captive grey seals, which had been fed wild-caught Atlantic mackerel. One third of the mackerel and fully half of the scat samples from the seals contained microplastic, showing how these tiny synthetic particles move upward thought the food chain.
“Our finding that microplastic can be passed from fish to marine top predators is something we’ve long thought was the case but, until now, lacked the evidence to back our theory up," says Sarah Nelms, the study's lead author in a university news release. "We have shown that trophic transfer is an indirect, yet potentially major route of microplastic ingestion for these predators."
Microplastics have many potential sources. Larger plastic debris can degrade into smaller and smaller pieces, and synthetic fabrics like polyester fleece shed micro plastic fibers every time it is washed. Other potential sources include cigarette filters, toothbrushes, discarded fishing nets, tires, and microbeads, which are tiny pieces of manufactured polyethylene plastic that are added as exfoliants to health and beauty products, such as some cleansers and toothpastes.
"By examining scat from captive animals and the digestive tracts of fish they were fed upon," says Nelms, "we could eliminate the possibility that the seals were eating plastic directly and be sure that any microplastics we found in their scat came via the fish.”
A similar study on fur seals living in the wild by Cristóbal Galbán-Malagón, a professor in ecology and biodiversity at the Universidad Andrés Bello in Chile, also found plastic microfibers in the scat of fur seals on Guafo Island, a remote island off the Southwest coast of Chile. Galbán-Malagón, after collecting scat from wild fur seals and dissolving the organic materials using lye, found that 67% of the samples contained traces of plastic.
Of course, it's not just tiny bits of plastic that are contaminating the ocean environment. Researchers at New Zealand’s National Institute of Water and Atmospheric Research (NIWA) recently reported that they have recovered a working USB stick photos inside a chunk of seal scat that has been frozen in their lab since 2017.
Dr. Krista Hupman runs a volunteer network via LeopardSeals.org where helpful Kiwis can send in leopard seal scat for her lab to analyze. In November 2017 a local vet sent in a sample from Oreti Beach, Invercargill, which was frozen, and then thawed for analysis just three weeks ago. Inside the sample two volunteers discovered the USB stick, which, once dried out, worked perfectly well.
Somewhat ironically, once dried it was discovered that stick contained photos of sea lions at nearby Porpoise Bay. NIWA would be happy to return the stick, should the owner come forward.
“From our work over the years we have found microplastic in nearly all the species of marine animals we have looked at," explains Dr Penelope Lindeque, a lead researcher on microplastic at Plymouth Marine Laboratory, who was involved in a 2018 study that found synthetic plastic particles present in seven different species of turtles, collected in the Atlantic, Pacific and Mediterranean. The list of impacted creatures now spans the marine food chain from tiny zooplankton at the base of the food web, to fish larvae, and upwards to include mackerels, dolphins, turtles, and seals.
“Our study demonstrates how microplastic can be transferred from prey to predator and therefore passed up through the food chain," says Dr. Lindeque. "More work is needed to understand the extent to which microplastics are ingested by wild animals and what impacts they may have upon the animals and ecosystems.”
Interested in learning more about marine debris? You can check out our pages on the subject right here! And if you’re reading this from New Zealand and would like to help the effort to collect leopard seal scat, you can find out how here.