The way in which sardines in the Mediterranean are now eating – a daily habit they have altered over the years owing to the gradual reduction in the size of plankton – is increasing the likelihood that they are consuming more plastic fibres than ever before.

Carried out by a team of researchers from the University of Barcelona, a new study suggests that while no direct negative effects of this microfibre ingestion have yet been recorded, it has highlighted the scale at which environmental changes are impacting pelagic fish and their interaction with plastic pollution.

The paper – now published in the academic journal Environmental Pollution – was led by researchers from the Health of Ecosystems and Aquatic Animals research group of the University of Barcelona to form part of a project investigating the impact of climate change and rising sea temperatures on fish species of interest across the Mediterranean coast. 

While the study has failed to draw firm conclusions right now on what is behind the overall decline experienced by wild populations of European sardine over the last few decades, it has suggested it is more than likely related to some rather dramatic changes in planktonic communities.

Because plankton in the Mediterranean, it would appear, are growing smaller.

The study notes that this change alone has been enough to influence the feeding behaviour of European sardines, forcing them to shift from capturing large particles of food to capturing what they can through the filter-feeding technique. The study implores that the fact these sardines are now ingesting increased levels of plastic fibres in their environment as a result of this feeding habit, cannot be ignored.

The team has reiterated throughout the paper that newly adopted filter-feeding behaviour now displayed by European sardine has played a major role in the recorded increases in ingestion of plastic fibres. 

This particular study was conducted using an experimental system that keeps wild fish in controlled conditions, allowing researchers to analyse the effects of factors that can influence the consumption of plastic fibres; notably their feeding techniques and the temperature of the water they’re in.

The study indicates that filter-feeding sardines ingest more fibres – on average 4.95 fibres per individual – than those that capture food particles directly. It was also observed that filter-feeding individuals had poorer body conditions, mainly due to the inefficiencies in filter feeding.

Water temperature does not seem to directly affect the amount of fibres ingested, but it does affect the time it takes for them to be expelled. At higher temperatures, the sardines are quicker to expel the fibres, generally – it was found – within 48 hours of ingestion.

However, Oriol Rodriguez, a researcher at the Department of Animal Biology, Plant Biology, and Ecology of the University of Barcelona warned that it would be “misleading” to think that an increase in water temperature is beneficial to the sardines.

“Climate change has caused important transformations in marine ecosystems, including the reduction of the size of the plankton in the Mediterranean,” said Rodriguez. “Sardines, which prefer to feed on larger organisms because they are more nutritious, are increasingly forced to rely on filter feeding because of the reduced availability of larger plankton.”

The study concludes that this change in their feeding behaviour not only provides them with less energy, but also increases the likelihood of ingesting more plastic fibres present in the water.

Although the authors of the study have not identified any direct negative effects related to the ingestion of plastic fibres in the sardines studies, they emphasise that changes in the environment – driven by factors such as climate change – could play a more significant role than previously thought in shaping how pelagic fish interact with plastic contaminants.

Some researchers claim that fish consumption does not represent a worrying source of microplastics for humans, suggesting that the amount ingested through products such as fresh sardines “is minimal compared to other sources” such as plastic packaging, synthetic fibres from clothing, or environmental pollution that can end up in our food.

This claim has been refuted by other studies which have found that toxic substances and dangerous germs can potentially be transmitted to fish through microplastics and the humans that eat the plastic-tainted fish. Their position is that reducing microplastics contamination is critical.

The research conducted by the University of Barcelona was funded by the national project PLASMAR (Research Challenges R&D&I project of the Spanish Ministry for Science, Innovation, and Universities.

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