Determination of Hg and Se levels in Atlantic Bluefin Tuna (Thunnus thynnus L.) muscle: evaluation of the potential health risk/benefit for human.

Mercury and Selenium represent two key elements for human: the first is a well-known toxic element (1) whereas the second one is an essential micronutrient, but also an antagonist of mercury toxicity (2). Due to these features, the knowledge about Hg and Se levels in the seafood, that represents one of the principal sources for human assumption of these elements, is recommended. Se/Hg molar ratio have been generally used for the evaluation of risks posed by Hg exposure from seafood consumption, but recently a new parameter, the Selenium Health Benefit Value (HBVSe), was introduced to better understand the quote of bioavailable Selenium that remains after its mercury-interaction (3,4). Atlantic Bluefin tuna (Thunnus thynnus) is a species of large interest for its high nutritional value and for its increasing commercial interest in the last years. This work aimed to determine Hg and Se concentrations in farmed tuna, sampled from the fish farm Fish and Fish Ltd (South– East of Malta), and to make a comparison with wild Thunnus thynnus caught in the Mediterranean Sea, to calculate the Se/Hg ratio and the HBVSe parameter: at our knowledge, this is the first study that calculate the HBVSe in farmed and wild Atlantic Bluefin tuna from the Mediterranean Sea. Mercury was analysed by Direct Mercury Analyzer DMA-1, whereas Selenium was determined by Graphite Furnace Atomic Absorption Spectrometry (AAS-GF) after microwave digestion of the muscle samples. Data were evaluated in relation to biometric parameters such as age, sex, weight, and length. Results showed that mercury content in all farmed specimens (0.61±0.20 mg/kg ww) was below the law limit (1 mg/kg ww), whereas the wild specimens have in most cases a Hg content over the limit (1.68±0.58 mg/kg ww) (Fig. 1). Selenium concentration was higher in farmed specimens (1.07±0.86 mg/kg ww) respect to wild ones (0.64±0.31 mg/kg ww). Selenium showed no relation with biometric parameters (due to its homeostatic regulation), whereas mercury concentration of wild specimens increased linearly with weight (r=0.5721, p<0.05). Concerning the risk/benefit values, a safe seafood could show a Se/Hg ratio >1, and a HBVSe >0. Se/Hg ratio and HBVSe of farmed tuna showed higher values (Fig. 2) than the wild specimens, with Se/Hg ratio of 5.48 vs 1.32, and HBVSe of 11.16 vs 0.29, respectively. These results demonstrated that, concerning mercury presence, farmed Atlantic Bluefin Tuna has a minor risk/benefit ratio, and it is safer than wild tuna. This study, the first in tuna from the Mediterranean Sea, paves the way to future studies on nutritional value of farmed fish even in the framework of Marine Strategy that aims to a good environmental status both for Descriptor 3 (The population of commercial fish species is healthy) and Descriptor 9. (Contaminants in seafood are below safe levels).