Natural mercury in fish

Most of the posts on this blog are about bacterial food poisoning, so this post is a change on two counts.

Mercury contamination of fish is usually associated with industrial pollution of rivers and estuaries.  William Ray, of Radio New Zealand, today reported that experts have suggested that the levels of mercury in trout in waterways around Totorua are so high, people should limit their intake to one trout per month.  (I wish I could catch so many!).

Rotorua is one of New Zealand's most famous geothermal areas and the experts have suggested that the mercury is from natural hydrothermal sources.  However, it appears that runoff from farms in the area may also be involved.  The nutrients washed into the rivers and lakes cause algal blooms, which lower the dissolved oxygen content of the water.  This in turn allows transfer of mercury from mud into the water, from whence it enters the food chain.

Perhaps the most infamous mercury poisoning occurred in the Japanese city of Minamata, which gave the name Chisso-Minamata disease to the resulting neurological syndrome.  The Chisso Corporation chemical factory released methyl mercury into the Minamata Bay and this highly toxic compound was accumulated by fish and shellfish.

The Rotorua trout have also accumulated mercury, but Dr. Ngaire Phillips from the National Institute of Water and Atmospheric Research points out that the recommendation for consumption of no more than one trout per month is based on a lifetime consumption.