Microplastics detected in farmed pond fish

Researchers have detected the presence of microplastics in sediment and fish samples from ponds. The study indicates moderate to high levels of pollution in the bed sediment of most ponds evaluated.

While previous studies have highlighted microplastic contamination in river water, sediment, and wild fish—even identifying these minute plastic particles in river- and ocean-dwelling hilsa—researchers have now detected microplastics in Bangladesh’s aquaculture ponds.

According to the research team, this study is vital for understanding how these particles accumulate in fish farming ponds and enter the aquatic food chain through fish. The researchers claim this is the first comprehensive study of its kind focusing on bottom sediment and fish in Bangladeshi freshwater aquaculture ponds.

The study was conducted in Trishal Upazila, Mymensingh. Microplastics were detected in sediment samples across all surveyed ponds, as well as within the fish samples.

These findings raise fresh concerns regarding potential risks to fisheries resources, the environment, and public health, while bringing to the fore questions about how microplastics enter these ponds despite being cultivated in controlled environments.

The study, titled "Microplastics in Freshwater Aquaculture: Evidence of Sediment–Fish Transfer and Associated Ecological Risk in Bangladesh", was published this month in the Environmental Pollution, an international scientific journal.

Fieldwork took place in Trishal Upazila, Mymensingh, a major freshwater aquaculture hub where commercial fish production relies on intensive and semi-intensive farming methods. Researchers collected samples from 25 selected fish farms during the winter season.

Professor Badiuzzaman Khan of the Institute of Environmental Science and Disaster Management at Bangladesh Agricultural University led the study.

He stated that this is the first study examining microplastic presence in freshwater farmed fish and could play a significant role in assessing potential damage to the nation's fisheries resources.

According to him, the primary objective was to determine the presence and characteristics of microplastics in pond bottom sediment and fish, alongside evaluating the relationship between fish morphological traits and microplastic ingestion, the chemical composition of the plastics, and the associated ecological risks.

Microplastics generally refer to plastic particles smaller than five millimetres. These tiny fragments originate from the breakdown of plastic bags, packaging, ropes, fishing nets, and other plastic items, spreading into the environment from various waste streams and plastic goods.

The study revealed an average of 11.04 microplastic particles per kilogramme of pond bottom sediment. The Pollution Load Index (PLI) used to evaluate sediment contamination levels ranged from 1 to 4.5. Based on this index, researchers classified the majority of sampling sites as moderately to highly polluted.

Pond bed sediment can serve as a major sink for microplastics. Compared to flowing rivers, the limited water exchange in aquaculture ponds allows particles to settle and accumulate at the bottom over extended periods. Consequently, benthic (bottom-feeding) fish and other aquatic organisms in contact with sediment face a higher risk of exposure.

However, the presence of microplastics in sediment does not imply that all fish are contaminated to the same degree. Particle ingestion levels depend on fish species, feeding habits, habitat depth, and the overall pond ecosystem.

Researchers examined a total of 150 farmed fish across six species, recovering 90 microplastic particles. The average concentration was 0.6 particles per fish.

Ingestion rates varied significantly by species. Sarputi recorded the highest average at 1.2 microplastic particles per fish, while Rui and Mrigal averaged 0.68 particles per fish.

In contrast, the study noted lower particle counts in samples of Catla, Common Carp, and Pangas.

Regarding habitat depth, bottom-dwelling species accounted for 52 per cent of all detected particles. Species inhabiting the mid-water column accounted for 35 per cent, while surface-dwelling fish accounted for 13 per cent.

Lead researcher Professor Badiuzzaman Khan noted, "Fish are fed large quantities of commercial feed. We are not saying all feed is contaminated, but certain feeds do contain these particles. Additionally, floodwaters often carry microplastics from other areas into the ponds. We primarily detected their presence within the digestive tracts of the fish."

Polyethylene (PE) and polypropylene (PP) were identified as the primary microplastic polymers. Polyethylene is commonly used in plastic bags, wrappings, and packaging, whereas polypropylene is used in containers, packaging, ropes, and other plastic goods.

Researchers highlighted these polymers as indicators of potential sources linked to aquaculture operations and domestic usage.

Tiny plastic particles can enter ponds through fish feed, feed sacks and packaging, fishing gear, water supply pipes, and surrounding household waste.

However, the specific polymer types identified cannot be attributed to a single source. Confirming exact origin points will require separate sampling and analysis of these materials and incoming water supplies.

Trishal is a major aquaculture hub. According to the Upazila Fisheries Office, fish are cultivated in 22,000 ponds across the upazila, producing 72,000 tonnes of fish annually.

Senior Upazila Fisheries Officer Shahana Nazneen acknowledged the pollution occurring in these operations, stating: "Pollution in fish ponds can occur due to poultry farming along the pond banks. We have already initiated enforcement drives and recently removed a chicken farm situated adjacent to a pond."

Because the study was limited to 25 ponds in Trishal, its findings cannot be generalised to determine contamination levels across all aquaculture ponds in the country.

Mahbubur Rahman, Associate Scientist and public health expert at the International Centre for Diarrhoeal Disease Research, Bangladesh (icddr,b), who has worked extensively on microplastics, told Prothom Alo, "The situation has escalated to a point where microplastics are now ubiquitous. There is no proper disposal mechanism, and recycling is virtually non-existent. We have already detected microplastics in the human body; there is no guarantee that they will not reach humans through fish consumption."

Concerns are growing regarding the health effects of consuming microplastics found in fish, alongside questions over whether high cooking or frying temperatures degrade these particles.

Professor Shimul Halder, Chairman of the Department of Pharmaceutical Technology at Dhaka University, told Prothom Alo that because polyethylene consists of complex molecular chains, standard cooking heat does not easily destroy it. As a result, microplastics can enter the human body via fish consumption, which he warned could adversely affect liver and kidney function.

Noting that the detection of microplastics in aquaculture environments is a serious concern, Professor Abdus Salam of the Department of Chemistry at Dhaka University emphasised that further research is essential to determine precise pollution levels, long-term impacts on fish, and the risks posed to human food safety.