We studied mercury levels in Cape monkfish off Namibia’s coast

Consumers should bear in mind that the bigger the fish, the more likely that it will have a high concentration of mercury. Shutterstock

The presence of mercury in the marine environment is becoming an issue of concern. Once in the ocean mercury is converted into a more toxic compound known as methylmercury. In this form it’s absorbed by the digestive tract of creatures at all feeding levels in the marine ecosystem. In fish, reduced swimming activity, loss of balance, and possibly death have been linked to mercury contamination. Mercury has a very low rate of breakdown and excretion, which means it builds up – in a process known as bioaccumulation – through the entire food chain.

We conducted research to establish the levels of mercury in Cape monkfish, a commercially important fish species off the Namibian coast. This species also plays a key ecological role in the marine ecosystem. It feeds on other fishes and so regulates various fish populations.

The two conditions in which we found higher levels of mercury concentration were in larger fish and in fish in deeper waters. Some fish (0.57%) had higher levels of mercury above the World Health Organisation limit of 0.5mg/kg. However, most of the fish had mercury levels still below the World Health Organisation limit of 0.5mg/kg.

Our findings are important because although the Cape monkfish isn’t currently a risk to humans who eat it, this may change because of future industrial development in the region.

What we found

Cape monkfish is an important food source in Namibia and is also exported. The European Union is Namibia’s largest seafood export market. The fact that it’s a top predator increases its chance of accumulating mercury because, as research shows, moving up the food chain, the mercury will accumulate and increase.

Cape monkfish also have long life spans: over 10 years.

Our research compared the total mercury concentration between Cape monkfish muscle and liver tissue. We also related the mercury concentrations to the fish’s body size, depth and capture location.

We collected a total of 529 specimens of Cape monkfish from the three size classes; juveniles, sub-adults and adults, between 2016 and 2018. To check our laboratory results, we compared 50% of them with those obtained by the Namibian Standards Institution.

To establish whether the mercury concentration was spread evenly throughout a fish’s body we compared the concentration in its muscles and its liver tissue.

We found no significant difference in the two tissues. This implies that the muscle and liver tissues of Cape monkfish have statistically similar abilities to retain mercury. We expected significant differences because liver and muscles have different rates of metabolism.

The liver functions as a main storage organ and a detoxification site so we expected the liver tissues to contain higher mercury concentrations than the muscle tissues.

We did find that juvenile fish had lower contamination than bigger and older fish. This suggests that as the fish grows, the total mercury concentration increases. It also means that older fish are more threatened by mercury compared to younger fish.

We also found differences in concentration based on the location of the fish. One differentiator was lines of latitude: fish caught from areas around 19⁰S, 20⁰S and 26⁰S off the coast of Namibia had higher mercury levels than those caught in other areas off the Namibian coast. This might imply that there are mercury hotspots along these latitudes where sources of mercury are high, which can be linked to elevated industrial activities especially at 20°S and 26°S which a closer to the industrial towns of Walvisbay and Lüderitz respectively.

The concentration of total mercury in Cape monkfish was significantly higher in deeper water, though still similar in muscle and liver tissues.

What now?

When buying Cape monkfish to eat, consumers should bear in mind that the bigger the fish, the more likely it is to have a high concentration of mercury. Mercury can potentially be toxic to people when consumed. These threats may include renal failure, liver damage, cardiovascular diseases and even death.

Our findings should also be used to guide action taken by fishing companies operating around the 19⁰, 20⁰ and 26⁰S lines of latitude off the coast of Namibia. Fish caught from these areas should be screened for mercury and other heavy metals.

Johnny Gamatham and Stephanus Hamutenya also contribted to this article.