Microplastics in water bodies can be ingested by very small organisms, which in turn are consumed by larger ones. As a result, the bodies of animals higher up in the food chain accumulate more microplastics and can suffer from amplified toxicity. This phenomenon is called bioaccumulation.
To understand microplastic bioaccumulation along the Goan coast, scientists from the CSIR-National Institute of Oceanography in Goa and the Academy of Scientific and Innovative Research in Ghaziabad recently examined the habitats and feeding behaviour of 251 fishes belonging to nine species of finfish and shellfish. These included mackerel, anchovy, oyster, clam, catfish, sardine, and other commercial varieties, which the team captured from various depths in the ocean’s water column.
In a study published in Environmental Research in August, the team identified 4,871 polluting particles in these fish, of which 3,369 particles were plastic polymers of 19 types. The scientists also found more contamination on the sea floor and in sediments at the bottom of the water column (in benthic realm) than in the open water column (pelagic realm).
These particles were mainly from degraded fishing material left at sea and wastewater discharged from human settlements.
Based on the findings, the scientists have indicated that the fish are suffering from disrupted gene expression, oxidative stress, reproductive damage, and lower growth. When people eat these fish, according to the team, effects on the human body could include immune dysfunction, higher risk of cancer, and toxicity to the brain.
Five key questions
Fisheries around Goa focus on the estuaries — ecologically critical zones that nourish young fish and which serve as feeding grounds for the older ones. Finfish and shellfish are commonly fished at estuaries. These fish kinds also feature frequently in Indian cuisine because they are accessible, affordable, and are rich in proteins.
Anchovies, sardines, and mackerel are small pelagic fish that play an important role in estuarine ecosystems, feeding on plankton and drawing in larger predatory fish. They are also filter feeders: they trap floating particles in the water and ingest them, and are thus more liable to imbibe microplastics.
These small fish are consumed by other larger ones, which in turn are prey for the elasmobranchs, or cartilaginous fish, including the sharks that dwell in shallow shelf waters. Microplastics thus move all the way up the food chain in a process called trophic transfer, eventually affecting the apex predators and human consumers.
The scientists analysed fish samples only from the Mandovi estuarine system, part of the Mandovi-Zuari system that contributes to 97% of Goa’s fish output. They treated the bamboo shark, an apex predator, as the focal species to identify the effects of microplastic accumulation.
The study fills a knowledge gap for five key questions for the region: microplastic contamination quantities in commercial fish, factors that increase the uptake of microplastics in fish, parts of the body that are the primary ingestion pathways, evidence of microplastics through ingestion in the bamboo shark, and risks of microplastic ingestion for fish and human health along the Goan coast.
Many shapes and colours
The scientists analysed 30 members each of mackerels, sardines, anchovies, bamboo sharks, sole fish, catfish, clams, and oysters, plus 11 of the rarer green mussels. These fish were grouped by their feeding levels: filter feeders and planktivores, secondary consumers, and carnivorous consumers. The scientists determined the microplastic concentration in their bodies by assessing the soft tissue.
Their analysis revealed that anchovies had the highest concentration among the pelagic species, at 8.8 microplastic particles per individual (MP/in). The catfish led the benthic realm at over 10 MP/in. The bamboo shark had the least: 3.5 MP/in. The water column itself contained 120 MP/litre.
The team found that the longer bodies the fish had, the fewer microplastic particles they accumulated. They also concluded that fish with habitats closer to contaminated sediments on the sea bed ingested more microplastics when they fed.
Among the finfish, the scientists found more microplastics in the digestive tract than in the gills, indicating accumulation through the consumption of contaminated water or prey. (As water moves through the fish, the particles are trapped in the gills and could lead to respiratory issues.)
They also identified four major shape-types of microplastics: fibres (53%), fragments (29.9%), films (13.1%), and beads (4%). Marine animals use colours to detect prey, and the particles came in nine hues: blue (37.6%), black (24.3%), red (12%), discoloured (8.7%), transparent (6.8%), green (4.4%), pink/purple (2.5%), yellow (1.9%), and orange (1.7%).
The types and colours of microplastics revealed their sources to be fishing gear, tire residue from roads, e-waste, packaging, and textiles.
Risk assessment
In all, the study categorised the region and ecosystem as low-risk but placed benthic life at more risk than pelagic. The paper also said 11 of the 19 types of polymers identified were highly toxic.
The study showed 66 of the 71 shellfish to have poor nutritional statuses. Previous studies have found the direct effects of microplastics on fish include lower fitness, protein, and fatty acids and lower nutritional quality. Indirectly, as the demand for such fish drops in the market, coastal populations could be at risk of losing their livelihoods.
“This is a good study that further supports several other independent findings,” Ravidas K. Naik, who also studies microplastics in marine environments at the National Centre for Polar and Ocean Research in Vasco da Gama and wasn’t affiliated with the study, said.
“There are microplastics in every place on earth, and such findings only underscore the urgent need for us to take action as a society to remove plastic from various environments, with better waste disposal and new research for biodegradable alternatives.”
Sandhya Ramesh is a freelance science journalist.
