Heavy metal levels have been monitored in Derwent oysters, mussels and flathead for over 20 years by the zinc works at Risdon (now managed by Nyrstar), as a condition of their operating permit with the Environmental Protection Authority (EPA).
The DEP also measured heavy metal levels in several other recreational fish species (bream, estuary-caught trout and mullet) as part of a 2007 pilot survey in collaboration with the University of Tasmania and Nyrstar Hobart Smelter. This study was supported by a Tasmanian Government Fishwise grant and published as an Honour's thesis at University of Tasmania (Verdouw, 2009).
The survey results showed that average mercury levels in bream taken from the Bridgewater area were about three times the recommended food safety standard of 0.5 mg/kg. Mercury levels in sea run trout taken from the area between New Norfolk and Bridgewater in October 2011 were also found to be slightly above the food safety standard, as were mercury levels in flathead. Mercury levels in mullet were generally low.
Based on this pilot survey, precautionary health advice was issued in 2007 by the Director of Public Health (DHHS) to avoid eating Derwent-caught bream and to limit consumption of flathead and other Derwent-caught fish until further studies were carried out.
In 2009 the DEP received additional funding from the Australian Government's Caring for Our Country program to extend the pilot survey to a larger number of fish, including a broader range of recreationally targeted species.
Between 2009 and 2011 mercury levels were measured in seven recreationally-targeted fish collected from the Derwent estuary: bream, flathead, trout, Australian salmon, whiting, cod and flounder. This work was carried out in collaboration with the Institute of Marine and Antarctic Studies (IMAS), Nyrstar, DHHS, Inland Fisheries Service and community/recreational fishing groups Fishcare and TARFish.
The study confirmed that mercury levels in Derwent estuary bream are very high – more than twice the recommended food safety standards. Mercury levels in flathead and trout are slightly above the food safety standards. Mercury levels in Australian salmon, mullet, whiting, flounder and cod were below the food safety standards, however this was based on a limited number of samples, and should be considered indicative.
Photo: N. Barrett
Restoring and promoting the Derwent estuary