Image Credit: Oregon State University, CC BY-SA 2.0

Quantitative analysis of selected plastics in high-commercial-value Australian seafood by pyrolysis gas chromatography mass spectrometry (2020) Ribeiro et al., Environmental Science & Technology, https://doi.org/10.1021/acs.est.0c02337

The Crux

Plastic is one of those things that we hear about all the time these days. More specifically, we hear about how there is an absolute ton of it in the environment thanks to human negligence and the lack of concern that a large amount of people have for where their plastic goes when they are finished with it. Plastic isn’t like paper or metal, it takes a long, LONG time for it to break down. Plastic bags take anywhere from 10-20 years, but the normal time it takes for most plastic waste to decompose is about 1000 years. To put that into perspective, Leif Erikson led an expedition from Greenland to the coast of what is now North America in the year 1002. If his crew had some plastic with them and left it in the places they visited (typical tourists) there’s a good chance that it would STILL be there today.

I hope I’ve convinced you why plastic is bad, but another danger that plastics pose are microplastics, small bits of plastic that have come from a larger piece, all of which are less than 5mm in size. Our environment is full of them, and the ocean in particular has been saturated with microplastics. In 2014 a research expedition sailed from Bermuda to Iceland (a trip of 2500 miles/4023 km) and found microplastics in every single sample they took. And that was just plastic in the environmental samples they took, the real threat to marine life comes from what happens to all of that microplastic.

Because these microplastics are so tiny many marine organisms tend to eat them, either accidentally or on purpose, which can cause myriad health issues for those animals. While that is an issue for the animals that eat the plastic, food web dynamics mean that the animals that eat those animals can also suffer from the negative impacts of consuming plastic. And not just marine predators like crabs, squid, or sharks. Humans get about 17% of their animal protein from seafood. Today’s authors wanted to investigate just how much microplastic was being consumed by the typical seafood consumer.

What They Did

To analyze just how much plastic was found in commercially-available seafood the authors tested for the presence of six common plastic types in five species of seafood: farmed oysters (Crassostrea gigas), farmed tiger prawns (Penaeus esculentus), wild blue crabs (Portunus armatus), wild squid (Nototodarus gouldi), and wild sardines (Sardinops neopilchardus). Although some of these species were wild-caught and some were farmed, all specimens used in this study were purchased in a local seafood market and handled the same way that a typical customer would handle their food before cooking it at home. Plastic concentrations in seafood species was quantified by dissolving the samples, then filtering out any plastics from the resulting solution.

Did You Know: Accumulation and Sequestering

The amounts of plastic that we are talking about in this study may seem small, and they are. But the thing about contaminants like plastic is that they add up over time. A good parallel is mercury in fish, which like plastic doesn’t break down but instead builds up over time. If you didn’t know, mercury poisoning is a real thing and can be dangerous, which is why you shouldn’t eat fish like tuna too often. Plastics carry the same risks for both us and marine organisms, with small amounts building over time and increasing the chances of health problems. The authors in this study cite another study that estimated that the average consumer will ingest 5 grams of plastic a week. Small amount, right? Well, over the course of a year that adds up to just over half a pound (260 grams) of plastic.

What They Found

Plastic was found in each of the five species tested, ranging from less than 0.1 mg of plastic per gram of animal tissue for squid to ~3mg plastic per gram of animal tissue for sardines. The amount varied among the different species and was due to the animal tissue that was tested and where those animals live in the ocean. For example, squid tend to live ~300 meters below the surface, and microplastics tend to be closer to the surface. Also, when people eat squid they eat the mantle, but most microplastic will be found in the digestive tract and gills. Oysters, on the other hand, are typically eaten whole and thus any microplastic in their bodies, be that in their gills or digestive tract, will be consumed.

Problems

The methods used in this study to test for the presence of plastic require a certain minimum amount of plastic to work, and many samples either did not meet that minimum amount or did not have any plastic in them at all. Because of this, the statistical power of these results are reduced, which means that these plastic concentrations may over- or under-represent just how much plastic is in commonly consumed marine organisms. Additionally, samples of the crabs and sardines varied considerably in their plastic concentration, meaning that many more samples need to be tested in order to determine how robust the results of this study are.

So What?

While this may not have been the typical kind of ecological study that we tend to break down here on Ecology for the Masses, the results of this study are important to consider for multiple reasons.

First, there is the threat to human health, as plastics are not meant to be eaten and can be dangerous to us. The results of this study imply that the typical sardine serving contains ~30mg of plastic, which adds to other plastics that the average person consumes in their weekly diet, resulting in an estimated total of ~5g of plastic per week.

Second, we aren’t the only organisms that can suffer from plastic. Think of those heart-wrenching videos of sea turtles eating plastic bags because they look like jellyfish or the fish with stomachs FULL of plastic. This study has shown that humans are vulnerable to the same threats and problems that the entire marine food web is vulnerable to: the accumulation of plastic in our systems due to the contaminated food that we eat.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.