Mercury in fish questions answered

Biggest and best tuna tend to have the most mercury, experts say - International Herald Tribune

My questions raised earlier this week about mercury in fish are largely answered in the above article.

Here are some points worth noting:

The higher the fish in the food chain, the more it collects mercury. Sharks often have levels of mercury over 2 parts per million, but their meat is rarely consumed in the developed world.

Except in Melbourne suburban takeaways, of course.

As to where this mercury in fish flesh is coming from:

Since tuna spend most of their time far out at sea, scientists believe that most of the mercury they absorb occurs naturally and is dispersed in the oceans by historic volcanic activity. Close to land, industrial sources can emit mercury as a pollutant. And since tuna spawn in shallow waters, local industrial pollution could have some influence on contamination of tuna flesh.

This answers my question about how orange roughy, a deep sea fish, acquires mercury in its body.

As to how dangerous the levels of mercury in tuna are:

The ill health effects of mercury in tuna and other large fish are a topic of active debate in the global scientific community, and many scientists caution against an exaggerated response - noting that fish is generally more healthful than red meat. In high doses, mercury is a neurological toxin.

But the health risks are greatest for pregnant women and nursing mothers who may pass mercury to their infants. When it comes to adults, "I think there is a little scare-mongering here," said Dr J.J. Strain a nutritionist at the University of Ulster who has been studying the health effects of fish-based mercury ingestion on young children in the Seychelles. "Fetal brain is at least ten times more sensitive than adult brain, so the health risk, if any, relates to pregnant women, not other adults."

And there is another complication to take into account too:

Research also suggests that the toxic effects of mercury is canceled out if it is ingested alongside selenium - and most ocean fish contain both elements. There's now a lot of evidence to suggest that "excess selenium over mercury equals healthy food; excess mercury over selenium equals potentially harmful food," Dr. Kaneka said.

"We shouldn't focus just on the negative risk of eating fish - yes, there are these contaminants, but there are also huge benefits. This is a work in progress and it's a very complicated risk/benefit analysis," Valdimarsson said.

Well, glad that's all sorted out. Or not, as the case may be.