Tuna, in both cooked and raw forms, is found on restaurant menus, in sandwiches, and on the family table. While tuna flesh is widely appreciated, it does contain a natural toxin known as methylmercury, which is generating a considerable number of questions and concerns. A multidisciplinary study (Houssard et al. 2019) has accurately mapped the methylmercury content of a number of tuna species of the central and southwest Pacific for the first time. This is the zone in which more than half of the world’s tuna is caught and subsequently exported, ending up on the plates of consumers. This study reveals that the methylmercury content of tunas depends not only on the size and species of the fish, but also on where they were caught. These results enable us to inform and advise tuna lovers.
To address the potential health risks associated with the naturally occurring methylmercury, the French Institute of Research for Development (IRD), the Pacific Community (SPC) and the University of New Caledonia (UNC) undertook research on the mercury content of a number of tuna species in the western and central Pacific Ocean. Using specimens collected since 2001 by Pacific national observer programmes and stored in the tuna tissue bank managed by SPC (Sanchez et al. 2014), IRD has in recent years performed more than 1000 muscle tissue mercury content assessments on yellowfin, bigeye and albacore tunas.
Importance of landed fish size
Based on flesh samples from tuna captured in a large area stretching from Australia to French Polynesia, it came as no surprise that the highest mercury concentrations are found in the largest tunas (Fig. 1). In tuna flesh, mercury essentially occurs as methylmercury, which is the organic form of mercury that builds up naturally as tuna grow older and bigger; this process is known as bioaccumulation. Methylmercury is eliminated by marine organisms at a slower rate than it is accumulated (see Box 1). For the great majority of fish tested, the values recorded remain lower than the recommended threshold of 1 mg of mercury per kg of fish (wet weight) (USFDA 2006; WHO/UNEP DTIE Chemicals Branch 2008), especially for tuna under 1 m in length (Fig. 1). It has also been observed that this threshold may be exceeded in some albacore and bigeye tunas over 1 m in length, while this is only infrequently true of yellowfin tunas over 1.2 m in length.
Figure 1. Mercury content trends as influenced by fish size in yellowfin, albacore and bigeye tunas in the western and central Pacific. The dotted horizontal line shows the recommended limit of 1 mg of mercury per kg of fish (wet weight).
Importance of depth and fishing zone
Researchers for the present study have also observed variations in mercury content between species, with bigeye tuna showing higher concentrations than yellowfin or albacore tunas (Fig. 2A). This difference between species can be explained by lifespan variations, different feeding habits and different physiological capacities, which, in particular, influence the respective depths at which they feed. Bigeye tuna live longer than the other two species and will, therefore, accumulate more mercury throughout their lifetime. In addition, bigeye tuna can dive deeper than the two other species, thus spending more time in the zone where natural methylmercury production in the ocean is highest (see Box 1). Bigeye tuna, therefore, show higher methylmercury concentrations than both albacore tuna, whose habitat is shallower, and yellowfin tuna, which tend to swim closer to the surface (Fig. 2B).
In addition to the effect of size and interspecific differences among tunas, our research made it possible to go even further by demonstrating that significant geographical differences also exist. In fact, within the same species, bigeye tuna for example, mercury content levels may vary by a factor of two or three or even more, depending on location. Around New Caledonia and Fiji for example, mercury levels are higher than they are at the equator (Fig. 3). Electronic tuna tagging research results (Evans et al. 2008; Fuller et al. 2015; Houssard et al. 2017) have shown that bigeye tuna dive deeper in New Caledonia than they do at the equator, again demonstrating that the deeper a tuna’s habitat, the higher the methylmercury concentrations it contains, because it feeds within a habitat where methylmercury production is higher (see Box 1).
Figure 2. A) Median mercury content levels in yellowfin, albacore and bigeye tunas in the western and central Pacific, and B) representation of the vertical habitat of tunas, with the curve of methylmercury content in the water in relation to depth.
Figure 3. Geographic distribution of mercury levels for yellowfin