Contaminated Fish Consumption in Kolkata and Impacts of Bycatch in Fishery 

SAIKAT K BASU

Fish remain a vital component of local diets, but multiple studies underscore the persistent risks from mercury, lead, pesticides, and pathogenic microbes in Kolkata’s supply. While fish from designated wetlands managed under monitoring frameworks may be considered safer, consistent vigilance, safer sourcing, and proper hygiene practices are essential to minimize health hazards.

Heavy Metals Contaminations: NGO studies from 2009–2012 at major Kolkata markets (e.g., Gariahat) found methyl-mercury levels often over 100% above international limits, affecting species like katla, bhetki, chital, pabda, and tangra. Weekly intake thresholds were exceeded in many samples—alarming even for adults and worse for children.   A GSI-led study in 2017 revealed lead concentrations up to ~17.8?mg/kg in fish sold in street markets—significantly above the Indian regulatory limit of 2.5?mg/kg 

Microplastics: A multiinstitutional survey (2018–2019) of fish from Kolkata’s waters detected microplastics—mostly fibres—in nearly 50% of fish guts, raising concerns about long-term ingestion risks in human diets.

Microbial and Pesticide Contamination: 

A 2019–2020 assessment of fish from peri-urban wholesale and retail markets (e.g., Baghajatin, Garia) found pathogens including Salmonella, Listeria, Clostridium, and Staphylococcus aureus, and presence of faecal coliforms in ice and water samples—indicative of poor hygiene practices. Earlier surveys (2001–2005) showed residual pesticides like totalHCH, DDT, Endosulfan, and Malathion in Gangacaught fish often above maximum residue limits (MRLs) 

Sewage-Fed Aquaculture (East Kolkata Wetlands):  A 2023 field study across 13 EKW sites measured heavy metals—such as Cd, As, Pb, Hg—in fish and sediments. Most fish muscle tissue contained metal levels well within WHO/FAO safety thresholds, except in isolated sites showing moderate contamination. Overall, fish from EKW were considered safe for consumption if regularly monitored. A complementary analysis of sewagefed pond fish indicated that lead accumulation in sediment and cadmium in sediment and organs may exceed aquatic life guidelines, though muscle tissue metal levels remained within WHO/FAO human-consumption.

Factors Driving Contamination

Air Pollution: High atmospheric lead—largely from diesel emissions—settles into streetsold fish and raw food items 

Industrial and Sewage Discharge: Rivers like Ganga and sewage-fed wetlands receive heavy metal and pesticidecontaining effluent from tanneries, chemical plants, and untreated sludge. 

Poor Hygiene Practices: Use of non-potable water for ice, inadequate sanitation in markets significantly increases microbial contamination risk .

Recommendations

Favor Fish Sources Carefully: Fish from heavily polluted areas (e.g. Ganga, inner city wetlands) have higher risk—opt for suppliers whose fish come from better-monitored water bodies.

Limit High-Risk Fish: Katla, bhetki, chital, pabda, and tangra should be consumed sparingly—especially by pregnant women, children, and heavy consumers—due to mercury and lead concerns.

Ensure Proper Handling: Buy from vendors who use clean potable water for ice and maintain hygienic storage. Avoid roadside shops with questionable cleanliness.

Avoid Monsoon Purchases: Bacterial contamination rises during monsoon season. Freshness declines faster; risk of waterborne pathogens is higher 

Advocate for Monitoring: Encourage local authorities to conduct regular heavy metal testing at fish markets, especially during seasonal changes.

Furthermore, Bycatch refers to the unintentional capture of species not targeted by the fishing industry—such as dolphins, turtles,  seabirds, juvenile fishes, snakes, rays and sharks, resulting in unprecedented decline of Non-Target Species. These species often die after capture, leading to population declines, especially among endangered and vulnerable species. Many bycaught species like sea turtles, some shark species, and marine mammals are already listed as threatened or endangered.

Continued bycatch accelerates their extinction risk, undermining conservation efforts. It leads to disruption of ecosystem balance. Predators, prey, and competitors are often caught unintentionally. Removing key species can destabilize food webs and lead to trophic cascades, affecting the entire marine ecosystem.

Bycatch often includes juvenile individuals of commercially valuable species, reducing their chances of reaching maturity and reproducing. This leads to long-term population declines and less sustainable fisheries. Species like albatrosses, sea lions, and dolphins are particularly vulnerable to certain fishing methods (e.g., longlines, gillnets). These species play unique ecological roles and their loss affects coastal and ocean biodiversity. Many species affected by bycatch migrate across international waters. Their decline contributes to global-scale biodiversity loss, not just in specific regions.

Solutions to Reduce Bycatch include use of bycatch reduction devices (BRDs), adoption of sustainable fishing practices, implementation of marine protected areas; 

and stronger monitoring and enforcement of fishing regulations. In conclusion, bycatch is a hidden but major driver of biodiversity loss in aquatic ecosystems. Addressing it is essential for protecting marine life, maintaining ecological balance, and ensuring sustainable fisheries.