New research is showing that in terms of heavy metals, farmed tuna are healthier to eat than wild ones.
A project by a Flinders environmental health PhD student Sita Balshaw has demonstrated that levels of mercury in southern bluefin tuna (SBT) are lower in farmed tuna than their free-swimming counterparts.
Ms Balshaw's study, run in conjunction with the Aquafin Co-operative Research Centre and Port Lincoln industry partners, investigated mercury accumulation in tuna to assess the safety of tuna farming practices.
Tuna farming involves the transfer of wild fish into sea pontoons, where they are intensively fattened over several months. With a steady diet of bait-fish, farmed tuna tend to bulk up quickly and Ms Balshaw said accumulation of very low levels of mercury is more than offset by their rapid growth rate.
"Selective control of bait-fish enables the industry to feed SBT a diet low in mercury," Ms Balshaw said.
"The rapid tissue growth experienced during culture outweighs the effects of mercury accumulation from feed, resulting in a dilution of mercury residues in fish tissues.
"So there's less mercury per kilogram of tissue than in a wild fish."
As well as conducting tests of whole fish, Ms Balshaw has measured mercury levels in different marketed tissue cuts of the tuna. She said the dilution of the mercury is more pronounced in the fatty tissues of the tuna, the cut most favoured by the Japanese.
"Manufactured feeds are not commonly used by the SBT industry, and because there has been research on the levels of contaminants in baitfish species from various sources, the farmers have an idea of the best varieties to use," she said.
While levels of mercury in both farmed and wild tuna are well below national safety limits, Ms Balshaw said there is continuing scientific debate about the effects on human health of the levels of mercury that accrue from regular seafood consumption.
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