Tuna is a widely-loved food around the world, but it can harbour a toxin: methylmercury. According to a recent study, the levels of this toxin in Central and Southwest Pacific tuna depends not only on the size and species of the fish, but also their geographic origin. These results are crucial for advising consumers.
Mercury enters our diets when we consume fish like tuna. Mercury gas enters the atmosphere from volcanic activity, but most of it comes from human activities such as coal burning and gold panning?small-scale panning for gold in rivers. It progressively becomes deposited in oceans where a fraction of it is converted into methylmercury. This methylmercury then infiltrates the entire food chain, from plankton to the largest predators like tuna. However, methylmercury is a neurotoxin, meaning it is toxic for the central nervous system. The health risks are particularly high for foetuses and young children. Knowing concentration levels and the origin of methylmercury present in this widely-consumed fish is therefore an important public health issue.
Rarely exceeded safe heath thresholds
In this context, IRD researchers requested access to a sample base established by the Pacific Community (PC), an international organisation that works towards sustainable development in Oceania. Since 2001, samples of meat from tuna caught in a large area ranging from Australia to French Polynesia were recovered from fishing boats by observers who also collect information on the catch: its size & species plus the date and place of capture. “These samples have helped us study the variation in mercury concentrations of several species in the Pacific including: bigeye tuna, yellowfin tuna, and longfinned albacore”, explains Anne Lorrain, a marine ecology specialist with Marine Environment Sciences Laboratory (LEMAR).
Unsurprisingly, the highest mercury concentrations were found in the largest specimens(1) since the element naturally accumulates as tunas age and grow. However, safe thresholds for health are rarely exceeded. Only 1% of the yellowfin tuna and longfinned albacore catch and 11% of the bigeye tuna catch exceeded the maximum recommended levels?Recommended thresholds indicating the amount of tuna that can be eaten per week: 1µg/kg (and these were all primarily large individuals measuring over 1m). “Given the noted nutritional benefits, especially the omega-3 content that prevents certain cardiovascular diseases, you shouldn’t stop eating tuna entirely, just limit how much you eat”, states Anne Lorrain.
The importance of depth
Researchers also observed differences in mercury levels between species with bigeye tuna having higher concentrations than yellowfin tuna or longfinned albacore. Moreover, within a single species, the mercury levels can vary depending on the geographic origin of the tuna caught, sometimes reaching three times as much as other individuals or more. “We believe that the depth at which the tuna feeds has a significant effect on the mercury concentrations we find”, the ecologist explains. Bigeye tuna are known to feed at greater depths than the other species, especially if the temperature of the water allows it. For example, they do so south of the equator, such as around New Caledonia and Fiji, where waters are more homogeneous and are the same temperature even as you venture deeper down. According to David Point, a geochemist with the Geosciences Environment Toulouse (GET) laboratory, “the water furthest from the surface contains less water. This makes it more favourable for the development of bacteria that can transform mercury in the ocean into methylmercury”. Researchers now hope to confirm these results by conducting supplemental studies in the Indian and Atlantic Oceans. The goal will once again be to better assess the risks and benefits of consuming tuna caught in a specific area.
1. P. Houssard, D. Point, L. Tremblay-Boyer, V. Allain, H. Pethybridge, J. Masbou, B. E. Ferriss, A. P. Baya, C. Lagane, C. E. Menkes, Y. Letourneur, A. Lorrain, A model of mercury distribution in tuna from the Western and Central Pacific Ocean: influence of physiology, ecology and environmental factors, Environmental Science & Technology , 23 janvier 2019