The effects of farming on mercury residues in South Australian southern bluefin tuna, Thunnus maccoyii

Methyl mercury is recognised as a naturally occurring neurotoxic metal residue that has the potential to accumulate to toxic levels in fish tissues – the primary environmental source of mercury exposure to human populations. The cumulative nature of methyl mercury results in those fish that are older and of higher trophic level being typically found to have the highest mercury content, and potentially pose the greatest threat to human health. Amongst those species recognised as potentially accumulating elevated mercury levels, tuna are one of the most frequently consumed and commercially available groups of fishes world wide. Levels of mercury in tuna vary according to species, size and geographical location. Moreover, recent research into farmed Southern Bluefin Tuna (SBT), would suggest farmed SBT is likely to have a lower mercury concentration then wild caught SBT. We investigated the temporal effects of farming on mercury accumulation and concentration in Australian SBT with an aim to assess the public health impact of wild caught and farmed SBT. Although all SBT complied with legal thresholds and were low by international standards, farmed product had a significantly lower mercury residue concentration than wild caught SBT of the same age class. Furthermore, mercury concentration in farmed SBT fluctuated with holding time and condition of harvested SBT. Estimated weekly intake levels of methyl mercury based on SBT consumption, would suggest that farmed SBT can be safely consumed on a more frequent basis than wild caught SBT while maintaining total dietary methyl mercury intake below the Joint FAO/WHO Expert Committee on Food Additives, tolerable weekly intake limits for children, women of childbearing age, and the general population.