Heavy Metal Presence in Fish from Fresh and Coastal Waters of Ghana

Paper: Gbogbo F, Arthur-Yartel A, Bondzie JA, Dorleku W-P, Dadzie S, Kwansa-Bentum B, et al. (2018) Risk of heavy metal ingestion from the consumption of two commercially valuable species of fish from the fresh and coastal waters of Ghana. PLoS ONE 13(3): e0194682. 10.1371/journal.pone.0194682

Bioaccumulation: A Serious Environmental Concern

Heavy metal: it’s not just an intense genre of rock music, but also an element high in density that can be incredibly toxic to humans at concentrated levels. While marine life faces many threats, an increasingly severe force has been the addition and accumulation of heavy metals in both coastal and freshwater environments. Metals, such as mercury, have an immediate and direct toxic effect on animals, and an indirect toxic effect on the other animals (and inevitably, humans) who consume these contaminated food sources. Just how do these pollutants travel through the food chain to end up in our bodies? The answer, and a major issue, is that they bioaccumulate: they become increasingly concentrated inside the bodies of living organisms over time, and then biomagnify as predators eat prey who have been exposed to the pollutant. Ultimately, aquatic animals at the end of the food chain can accumulate toxic levels of heavy metals, which poses a major risk for the humans consuming them.

Bioaccumulation and Biomagnification of Mercury- a Visual. Source: Wikimedia Commons.

Mad Hatter Syndrome- How do Heavy Metals affect humans?

Think back to Alice’s Adventures in Wonderland’s erratic Mad Hatter character. Many believed that author Lewis Caroll took inspiration for the Mad Hatter from hatmakers who suffered from heavy metal poisoning because of the mercury in their products. “Mad Hatter Syndrome”, or erythism, characterized the extreme neurological damage associated with heavy metal poisoning manifested by behavior changes, delirium, and personality changes. Heavy metals are extremely detrimental to the health of any organism because of their direct link to neurological disorders, kidney and skin damage, circulatory system problems, and cancer. While many industries no longer so freely use heavy metals, many communities still experience the risk of heavy metal poisoning from seafood. In 2017, the EPA advised pregnant women or parents of young children to avoid certain seafood choices (mostly seafood high on the food chain) because of their high, toxic levels of bioaccumulated and biomagnified mercury which could lead to serious adverse health effects. King Mackerel, Marlin, Swordfish, and some types of Tuna were among the top victims of highly concentrated  mercury– but how do other international commercial fisheries fare in the face of heavy metal pollution?

Heavy Metals in Ghana

Francis Gbogbo et al. (2018) assert that heavy metal contamination in marine environments is becoming increasingly common in developing countries, like Ghana, because of anthropogenic activities which affect both fresh and coastal waters. Anthropogenic activities are those in human origin that introduce environmental pollutants. Prevalent examples in Ghana include artisanal gold mining, electronic waste processing, industrial processes, domestic sewage discharges, and agricultural activities. The Ghanaian population was also shown to have higher levels of mercury in recent studies. Using this information along with the prevalence of fisheries in West Africa, Gbogbo et al. (2018) demonstrate the importance of examining different fisheries to uncover the potential severity of heavy metal contamination.

The Bigeye Grunt and Bagrid Catfish

Gbogbo et al. (2018) identify the bigeye grunt and Bagrid catfish as two particularly important commercial fisheries in Ghana. Bagrid catfish are omnivores found in freshwater environments, closer to the previously mentioned anthropogenic activities relative to bigeye grunts, which are carnivores and found in more coastal marine habitats, farther away from these anthropogenic activities.  Gbogbo et al. (2018) propose that because the Bagrid catfish lives closer to anthropogenic activity, it would bioaccumulate more heavy metal contamination compared to the bigeye grunt.

Gbogbo et al. (2018) collect their sample of bigeye grunts  from the Tema Fishing Harbor and Bagrid catfish from the Weija Dam on the Densu river to conduct their analysis. They assess the fish samples for their heavy metal content including levels of arsenic, cadmium, lead, mercury, zinc, and copper.  They also look for the presence of selenium in these fish, as high enough selenium concentrations can negate or inhibit the adverse health effects caused by mercury poisoning.

The Tema Fishing Harbour in Ghana. Source: Wikimedia Commons.
The Densu River in Ghana. Source: Wikimedia Commons.


A Minimal Risk but an Important Message

Comparisons of Heavy Metals in the bagrid catfish (Chrysichthys nigrodigitatus) and bigeye grunt (Brachydeuterus auritus). Letters indicate a significant difference. Source: Gbogbo et al. (2018).

Overall, neither species held high enough concentrations of any of the heavy metals to pose a health risk to humans. Among the seven heavy metals examined, selenium concentrations were the highest, lead and cadmium were below detection levels, and zinc concentrations were statically similar in both species of fish. Interestingly, selenium and arsenic were significantly higher in the Bagrid catfish than the bigeye grunt, while mercury was significantly higher in the bigeye grunt. Gbogbo et al. (2018) posit that heavy metals like arsenic tend to collect in larger quantities in the sediment of water beds, which means they tend to bioaccumulate more in demersal fish like the Bagrid catfish. On the other hand, mercury biogmagnifies in the bigeye grunt because of its carnivorous diet: eating only fish means rapidly collecting mercury from prey. As a pelagic species bigeye grunts also have more space to forage and hunt contaminated prey than the Bagrid catfish. Because of high levels of selenium, any marginal risk the mercury concentrations in either fish pose to humans are negligible. Ultimately, looking at the sample of bigeye grunts and Bagrid catfish supports the notion that contamination of bodies of water close to anthropogenic activities threatens species both in freshwater and marine environments, but also specifically species higher up on the food chain. While these two commercially important fish hold very low concentrations of heavy metals and pose a minimal health risk to Ghanaians, they are an important reminder of how our actions have an extremely significant impact on our surrounding freshwater and marine environments. Not only do heavy metals affect the ocean, but because humans are so intrinsically tied to marine life (especially through the commercial fishing industry), they end up affecting us as well.

Additional sources:
The Neuropsychiatric Sequelae of Mercury Poisoning: The Mad Hatter’s Disease Revisited
Eating Fish: What Pregnant Women and Parents Should Know

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