Millions of metric tons of plastic enter the oceans every year. Some of it is highly visible in the Pacific trash vortex, also known as the Great Pacific Garbage Patch, which lies between North America and Japan.
However, the most prevalent type of debris found in our oceans — microplastics — are less visible.
Microplastics are tiny bits of plastic less than 5 millimeters in length, which is about the size of a sesame seed. Nanoplastics, which are less than 100 nanometers in size, are also present in the marine environment.
A new study from scientists at the QUEX Institute, a research partnership between the University of Exeter in the United Kingdom and the University of Queensland in Australia, analyzed seafood from an Australian market for microplastics.
The scientists found microplastics in every sample of commercial seafood they tested.
Francisca Ribeiro, lead author of the study, says, “Considering an average serving, a seafood eater could be exposed to approximately 0.7 milligrams (mg) of plastic when ingesting an average serving of oysters or squid, and up to 30 mg of plastic when eating sardines.”
The authors recently published their study in the journal Environmental Science & Technology.
Analysis of the seafood samples
The researchers purchased five varieties of seafood: five wild blue crabs, 10 oysters, 10 farmed tiger prawns, 10 wild squid, and 10 wild sardines.
Before dissection, each sample was weighed and washed to remove any residue of plastic packaging. Only the edible part of each species was tested.
To extract any plastic present, the scientists placed each sample into a flask with an alkaline solvent and agitated it at 60 degrees Celsius in a shaker incubator. Once the solvent had completely digested the sample, the solution was analyzed for plastic.
The researchers then used a technique called pyrolysis gas chromatography mass spectrometry to identify the presence of five types of plastics: polystyrene, polyethylene, polyvinyl chloride, polypropylene, and poly(methyl methacrylate).
These plastics commonly appear in packaging, synthetic textiles, and marine debris.
While the team found plastic in all samples, Ribeiro says: “Our findings show that the amount of plastics present varies greatly among species, and differs between individuals of the same species.” The authors explain:
“Each of the analyzed seafood species of this study has different biological, physiological, and anatomic features and lives in different compartments of the marine environment, which influences the uptake and potential accumulation of microplastics.”
The study found:
- 0.04 mg of plastic per gram of tissue in squid
- 0.07 mg in prawns
- 0.1 mg in oysters
- 0.3 mg in crabs
- 2.9 mg in sardines.
All the samples contained polyvinyl chloride. The largest concentrations of plastic were composed of polyethylene.
“From the seafood species tested, sardines had the highest plastic content, which was a surprising result,” says Ribeiro. A grain of rice weighs about 30 mg, roughly the amount of plastic found in a sardine.
Co-author Tamara Galloway, from Exeter University, said, “We do not fully understand the risks to human health of ingesting plastic, but this new method will make it easier for us to find out.”
Roughly 17% of the protein humans consume worldwide is seafood. The findings, therefore, suggest people who regularly eat seafood are also regularly eating plastic.
Scientists have previously found microplastics and nanoplastics in sea salt, beer, honey, and bottled water. They can also deposit on food as dust particles.
How the plastic got there
The study describes how species differently consume food as a possible explanation of the varying amount of plastic they contain. It also suggests other potential sources.
The researchers say plastic may make its way from an animal’s gastrointestinal tract to its edible parts during processing — which includes gutting if performed incorrectly — and general handling. Plastics may also attach themselves to seafood via “airborne particles, machinery, equipment and textiles, handling, and from fish transport.”
Regarding the high concentration of plastic in sardines, the authors note the fish were purchased in bags made of low-density polyethylene.
Citing recent research that shows opening such a bag can result in the shedding of microplastics, they predict these types of packaging may be an additional and significant polluting mechanism for seafood.