Stable mercury levels and trophic position in three tropical tuna species in the South-West Pacific Ocean over the past 18 years

Humans are exposed to mercury primarily by consumption of marine fish, especially of top predators like tuna and billfish as methylmercury (MeHg) biomagnifies in food webs. Despite its relatively high MeHg levels, sometimes exceeding food safety guidelines (1 mg.kg^-1 fresh tissue), tuna is one of the most popular fish species consumed worldwide (> 6,000,000 tons/y), representing a commercial value of 41 billion $US/y. In the Western and Central Pacific Ocean in particular, tuna fisheries account for 54% of the global catch and is a major protein sources for local islanders. Several decades of anthropogenic mercury use and emissions to the atmosphere through fossil fuel combustion and artisanal gold mining have considerably modified mercury cycle and have created a global public health risk. Anthropogenic gaseous emissions are indeed thought to be responsible for a 3-fold increase in mercury concentrations of the surface ocean since industrialization. Yet it is still unclear how those emissions are converted into toxic MeHg in oceans and how it is transferred to the food webs.

In this context we studied temporal variability of mercury levels in the muscle of three tropical tuna species (Thunnus albacares, T. obesus and Katsuwonus pelamis) around New Caledonia and Fiji in the South-West Pacific between 2001 and 2018 (N=707). We used statistical tools based on temporal decomposition to investigate temporal trend and structure of mercury levels in tuna. As mercury is known to biomagnify with trophic position, temporal variations of nitrogen stable isotope (δ¹⁵N) values were also investigated as a proxy of the trophic position. Both mercury levels and δ¹⁵N values were highly variable between years for the three species but no significant temporal trend was evidenced Annual variations of mercury levels were then compared to oceanographic (e.g. Sea Surface Temperature), climatic (Oceanic Niño Index) and ecological (δ¹⁵N values) variables thanks to generalized additive models. It revealed that trophic position and vertical temperature structuration of the water column were important drivers of mercury variability in tunas. The constant mercury levels over time revealed here in the South Western Pacific ocean were finally discussed in regards to similar studies in tunas over the world.