Shelling out for mussels? Something in your dish may seem a bit fishy

Next time you sit down to enjoy a pricey plate of mussels, be sure to enjoy the free garnish of 10 plastic particles.

Some of them will be as big as 2mm, but most will be almost invisible to the naked eye, according to research just published by scientists at Cape Peninsula University of Technology (CPUT).

The number of microplastics in the Saldanha Bay farmed mussels tested by the researchers is far less than the 90 particles per 250g serving found in Germany.

But Conrad Sparks and his colleagues in CPUT’s department of conservation and marine sciences said this doesn’t mean there isn’t a potential threat to human health.

Writing in the Marine Pollution Bulletin, they said microplastics in the sea are colonised by micro-organisms, including pathogens, which may make them appear appetising to invertebrates.

“Microplastics in mussels may then also be vectors of pathogens, antibiotics and other contaminants that may pose a threat to human health,” they said.

However, this was not part of the study, which saw Sparks and colleagues Adetunji Awe and Jade Maneveld buying frozen and fresh mussels at four supermarkets and three wholesalers in Cape Town in July and August 2019.

They then cooked them, before counting and analysing the plastic fragments and filaments they each contained. The average number in the 30 mussels bought from each outlet ranged between 2.63 and 5.59.

Seven out of 10 microplastics were fragments, with the rest filaments, most were a dark colour and out of the 804 microplastics recorded, three-quarters were larger than half a millimetre.

Half the samples were polyethylene terephthalate (PET), the plastic used to make bottles, and latex made up the next biggest fraction. The researchers said this suggested protective gear such as gloves used during handling and processing of mussels could be sources of contamination.

Sparks said a study in 2014 estimated that a German who eats a 250g serving of mussels would consume 90 microplastic particles, but his study suggested in SA the number would be 10.

However, “with 179 tons of mussels produced annually in SA, this amounts to a total of 7.2 million potential microplastic particles that can be consumed when eating farmed mussels in SA”.

Mussels are regarded as good indicators of marine pollutants because they are filter feeders, with a throughput of up to 300ml of water a minute.

Five of the portions the researchers bought were from farms in Saldanha Bay, where more than 90% of the 1,700 tons produced annually is used for domestic consumption. Sparks said the other two samples probably came from there too.

“Mussel farming in Saldanha Bay started in the mid-1980s and takes place by raft culture, where mussels are attached to ropes hanging from rafts,” he said.

“Although in a controlled setting, mussels hanging from ropes are able to feed on particles in the natural environment.” These probably include plastic particles from the ropes and rafts, a question that needed more investigation.

One of the concerns Sparks and his colleagues raised was that ingestion of plastic could affects mussels’ growth rates and the productivity of aquaculture farms.

More broadly, “microplastics are detrimental to the health and wellbeing of the marine environment due to obstruction of digestive tracts and absorbed contaminants potentially being transferred into marine organisms”, they said.

“As resources consumed by humans, the passing on of microplastics in mussels poses human health risks.”

According to a 2016 study, every South African eats 7.5kg of seafood annually, on average.