The research team, led by scientists at the University for Development Studies in Tamale with collaborators from the University of Parakou in Benin and the University of Ghana, set out to fill a conspicuous data gap. Ghana ranks third in pesticide usage across Africa, and its northern region is dominated by intensive agriculture that employs roughly 80 percent of the population. The Libga reservoir, a medium-sized 48-hectare water body in the Savelugu Municipality fed primarily by rainfall and runoff from surrounding fields, and the Builpela reservoir in the Tamale Metropolis, replenished by the White Volta River during floods and precipitation from agricultural land, both act as collection points for water draining from farms and industry. Neither has ever been desilted, which means decades of accumulated sediment may continue to harbor chemical residues that cycle back into the food web.

Between November 16 and December 11, 2023, the researchers purchased freshly caught specimens of both species from local fishermen, selecting only visually healthy, undamaged fish to minimize confounding effects of disease or injury. Eighteen specimens of each species were collected per reservoir and pooled into one-kilogram composite muscle samples, following standardized protocols for euthanasia and handling. The team screened the tissue for 42 different pesticides spanning organochlorines, organophosphates, and a dozen other chemical classes commonly used in the region, using liquid chromatography-tandem mass spectrometry (LC-MS/MS), a technique prized for its sensitivity in detecting trace-level contaminants in complex biological matrices. Quality controls included solvent and procedural blanks, matrix-matched calibration, and spiked controls prepared from fish verified to be pesticide-free, with recovery rates between 85 and 112 percent and detection limits as low as 0.001 milligrams per kilogram.

Of the 42 pesticides analyzed, four turned up in the fish muscle: two organophosphates, chlorpyrifos and pirimiphos-methyl, and two organochlorine breakdown products, p,p’-DDE and p,p’-DDD. The latter two are metabolites of DDT, the notorious insecticide banned decades ago in most of the world but prized for its persistence. Organochlorines dominated the contaminant profile, accounting for 55 to 60 percent of the total pesticide load, with p,p’-DDE alone contributing 27.9 percent. Pirimiphos-methyl showed the highest single concentration, at 0.05 plus or minus 0.02 milligrams per kilogram in catfish from Builpela and 0.04 plus or minus 0.00 in Libga. Chlorpyrifos was detected in every catfish sample from Builpela, while p,p’-DDE and p,p’-DDD appeared in 100 percent of catfish from both reservoirs, a striking signal of environmental saturation.

The two fish species told subtly different chemical stories, and the divergence is rooted in ecology. Nile tilapia, a lower-trophic-level pelagic feeder that grazes largely on plant material, accumulated elevated levels of p,p’-DDD, reaching 0.03 plus or minus 0.01 milligrams per kilogram in Builpela specimens, but showed no detectable pirimiphos-methyl at Libga. African catfish, an omnivorous species with piscivorous tendencies that forages near the sediment, carried higher organophosphate burdens overall. The researchers attribute these patterns to differences in feeding behavior, trophic position, metabolic pathways, and possibly the age of the fish and the timing of sampling relative to peak pesticide application. Sediment-dwelling species like catfish are exposed to the hydrophobic, lipophilic DDT derivatives that concentrate in organic sediments, making them effective bioindicators of legacy contamination.

The presence of p,p’-DDE and p,p’-DDD is particularly telling from a toxicological standpoint. Because these compounds are photochemical degradation products of DDT, their ubiquity in the fish suggests that DDT was once used in the study area and has lingered in the ecosystem long after any formal application ceased, whether through historical agricultural use or illegal ongoing application. DDT and its derivatives resist microbial breakdown, bind tightly to sediments, and bioaccumulate in fatty tissues, allowing them to persist for decades and magnify as they move up the food chain. Comparable studies across West Africa and beyond, from Tono Reservoir in Ghana to Lake Parishan in Iran and the Ouémé River in Benin, have documented similar organochlorine fingerprints in wild fish, underscoring how stubbornly these legacy chemicals endure in tropical aquatic systems.

To translate contamination into human health terms, the team calculated estimated daily intakes based on a consumption rate of 0.2 kilograms of fish per day for a 70-kilogram adult, then derived hazard quotients, hazard ratios, and lifetime cancer risk values using United States Environmental Protection Agency frameworks. Monte Carlo simulations with 10,000 iterations were run to account for variability in body weight and consumption patterns. The results were reassuring on one front and sobering on another. All hazard quotients fell well below the 0.2 threshold, ranging from 0.004 to 0.046, and all hazard ratios stayed below 1, indicating negligible non-carcinogenic risk from current consumption levels. Every detected pesticide also had an estimated daily intake below the corresponding acceptable daily intake reference value.

The cancer risk calculations, however, told a different story. Lifetime cancer risk values for p,p’-DDE and p,p’-DDD, both classified by the US EPA as likely human carcinogens, fell between 10 to the power of minus 4 and 10 to the power of minus 6 across both species and both reservoirs, the range that regulators describe as an area of concern. For catfish from Libga, the LCR for p,p’-DDE reached 3.8 times 10 to the minus 5, and tilapia from Builpela registered 2.82 times 10 to the minus 5 for the same compound. The cumulative totals point to a potential long-term hazard for people who eat reservoir fish regularly over a lifetime. Organochlorine exposure has been linked not only to cancer but to endocrine disruption, type 2 diabetes, thyroid hormone alterations, and damage to neurological, immune, and reproductive systems, making chronic low-dose exposure a genuine public health concern rather than a theoretical one.

The authors are careful to note the limitations of their work. Sampling covered a single early dry-season window, two reservoirs, and a modest number of specimens, so seasonal peaks in pesticide application and broader regional variability remain unmeasured. The risk assessment relied on generalized consumption and body weight assumptions rather than local dietary surveys, and it did not account for other exposure routes such as contaminated water, sediment contact, or pesticide residues in staple crops, all of which could add to the cumulative burden. Still, the findings carry a clear message for policymakers in Ghana and other low- and middle-income countries where regulatory oversight of pesticides is often thin: regular monitoring of fish and water, public health advisories for fishing communities, stricter enforcement of agricultural chemical regulations, and promotion of sustainable farming practices are urgently needed. For the communities around Libga and Builpela, the fish on the plate today may be safe in the short term, but the chemistry accumulating in the reservoirs is writing a longer, more troubling equation.

Subject of Research: Pesticide residue contamination in reservoir fish and associated human health risks in northern Ghana

Article Title: Pesticide contamination and associated health risks in fish from Libga and Builpela reservoirs, Northern Ghana

Article References: Pesticide contamination and associated health risks in fish from Libga and Builpela reservoirs, Northern Ghana. (n.d.). https://doi.org/10.1186/s44329-025-00020-y

Image Credits: AI Generated

DOI: 10.1186/s44329-025-00020-y

Keywords: pesticide residues, organochlorines, DDT metabolites, bioaccumulation, Nile tilapia, African catfish, cancer risk, health risk assessment, northern Ghana, reservoirs, LC-MS/MS, food safety