Researchers from the Universitat Autònoma de Barcelona (UAB) demonstrate that the way sardines eat, altered due to the reduction in size of plankton, undermines the efficiency of their feeding and increases the likelihood of consuming more plastic fibres. Although no direct negative effects of this ingestion were identified, environmental changes may have a greater impact than previously thought on how pelagic fish interact with plastic pollutants.
The study was led by researchers from the Health of Ecosystems and Aquatic Animals research group (SEAaq ) of the UAB, in collaboration with researchers from the French Institute for Ocean Science and Technology (IFREMER) and the French National Centre for Scientific Research (CNRS). It forms part of a project on the impact of microplastics on fish species of interest in the Mediterranean coast.
The causes behind the decline experienced by wild populations of European sardine (Sardina pilchardus) in recent decades remain unclear. However, it seems to be mainly related to changes in planktonic communities. These changes influence their feeding behaviour forcing them to shift from capturing large particles, to filter feeding. Moreover, the fact that they ingest plastic fibres in their environment highlights this type of pollutants as an additional factor to consider when assessing the health of their populations.
In this study, the research team demonstrates that sardines ingest plastic fibres present in seawater and that their feeding behaviour is crucial in this process. To do so, they used an experimental system that keeps wild fish in controlled conditions. This allowed them to analyse the effect of factors that can influence the consumption of plastic fibres, such as feeding and temperature, as well as their subsequent expulsion. These data allowed inferring some of their possible negative effects.
The study indicates that filter-feeding sardines ingest more fibres (on average, 4.95 fibres per individual) than those that capture food particles directly (0.6). In addition, it was observed that filter-feeding individuals had poorer body condition, mainly due to the fact that this type of feeding is less efficient and not to the fact of ingesting plastic fibres.
Water temperature, however, 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 within 48 hours.
“It would be misleading to think that an increase in water temperature is beneficial because it accelerates the expulsion of plastic fibres”, says Oriol Rodríguez, researcher at the Department of Animal Biology, Plant Biology and Ecology of the UAB, who led the study. “Climate change has caused important transformations in marine ecosystems, including the reduction of the size of plankton in the Mediterranean. 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. 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 studied, 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.
Fish consumption not a concern when it comes to microplastics
Researchers claim that fish consumption does not represent a worrying source of microplastics for humans. “The amount of plastic fibres ingested through products such as fresh sardines is minimal compared to other sources, such as the use of plastic packaging, synthetic fibres from clothing, or environmental pollution that can end up in our food. Moreover, microplastics are concentrated in the stomach and intestines of the fish, a part that is usually discarded during the cleaning and preparation process”, explains Oriol Rodríguez.
Other studies carried out by the UAB research group on fish and crustaceans from the Catalan coast also confirm that these products are safe for human consumption, as they do not present worrying levels of microplastics, parasites such as anisakis, or other pathologies.
“The results obtained in this study offer a clearer view of the possible threats facing this very important species, both ecologically and for human consumption, and they add factors to be taken into account for future conservation efforts”, researchers conclude.
The research was funded by the national project “PLASMAR” (Research Challenges R&D&I project of the Spanish Ministry for Science, Innovation and Universities) and the UAB mobility grant ESTPIF 2022-29.
Oriol Rodríguez, Anna Soler and María Constenla, from the Department of Animal Biology, Plant Biology and Ecology of the UAB; Quentin Schull and Gilbert Dutto, (IFREMER), and Claire Saraux (CNRS) participated in the study.