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Derwent Estuary Program

Dr Rosalie Woodruff MP

Dr Rosalie Woodruff MP  -  Tuesday, 22 May 2018

Tags: Ralphs Bay, Oxygen Levels, Petuna, Huon Aquaculture, Tassal, Fish Farms, Algal Blooms, Derwent River

Dr WOODRUFF (Franklin) - Madam Speaker, I rise to present to the House a presentation given by Dr Christine Coughanowr, who is the immediate outgoing CEO of the Derwent Estuary Program. She made a submission to the Storm Bay North Marine Farming Development Plan as to the environmental impacts the Derwent Estuary Program are concerned about and raise regarding the Storm Bay expansion for the three companies, Huon Aquaculture, Tassal and Petuna.

The Derwent Estuary Program, as most members would know, is a partnership between a number of government business scientists and community groups, particularly the Brighton, Clarence, Derwent Valley, Glenorchy, Hobart and Kingborough Councils, the state Government, Nyrstar Hobart, Norske Skog, Boyer, TasWater, TasPorts and Hydro Tasmania.

The program was established in 1999 and manages the Derwent Estuary which has a long-standing history of heavy metal pollution, with the some of the highest reported levels of zinc, mercury and lead in the world. It has been an incredibly successful partnership. It has been nominated and awarded the National River Prize.

More recently, the estuary, Dr Coughanowr says -

Has shown increasing signs of nutrient stress including nuisance algal blooms, seagrass loss and persistent low oxygen levels in some areas. Previous research has shown a strong link between nutrient loading, low oxygen and release of heavy metals from sediments. Therefore, a key element of the program's long-term management strategy for the estuary is to manage and reduce nutrient loads, particularly during summer months when the risks are highest.

She makes the point that marine waters of Storm Bay and the D'Entrecasteaux Channel drive the overall circulation of the Derwent Estuary. Those waters from Storm Bay set the background nutrient levels for the whole system as it creates a salt wedge that travels up from Storm Bay and D'Entrecasteaux Channel toward the Bridgewater Bridge, that far up the Derwent River. A significant change in nutrient inputs from Storm Bay could have far reaching impacts on Storm Bay and on the Derwent Estuary.

Dr Coughanowr was concerned that, given the size and scale of the proposed fish farm expansion and the connectivity of Storm Bay into the Derwent River, it is essential the three proposals currently being considered separately by the Marine Farming Planning Review Panel ought to be planned, assessed and managed together and ought to have a cumulative impact assessment. Without that, she maintains, it is difficult to understand the scale and timing of the three developments and the impacts they would have on the Derwent Estuary and River.

Of particular concern is the potential for increased nutrient levels in the bottom waters of estuary, that these drive the overall circulation of the estuary and the impact on the ecology as a result. Particular concerns would relate to Ralphs Bay, which is shallow and poorly flushed and may be susceptible drift algal growth, the spotted handfish in the lower Derwent and Ralphs Bay, the Derwent temperate reef communities, including EPBC-listed giant kelp communities, especially those along the Kingborough shoreline, the seagrass and macrophyte communities which are susceptible to algal overgrowth as well as shadings, and the heavy metal-contaminated sediments when low oxygen levels can remobilise toxic metals, including mercury, which are existing in the soil at the bottom of the benthic layer in the estuary, and this could have potential impacts on humans, fish, birds and other biota.

Dr Coughanowr pointed to the very large biomass. The numbers being produced for the dissolved oxygen levels for all the sewerage that is currently discharged into the Derwent Estuary from the different councils that discharge into the Derwent is estimated to be 327 tonnes per annum of dissolved nitrogen outputs. By comparison, the estimated dissolved nitrogen outputs the three fish farm proposals would provide would be 1892 tonnes per annum, nearly six times the Derwent sewage load at the moment. It would be more than 14 times the Derwent sewage load when the Storm Bay fish farm expansion gets to the full production level being proposed of 80 000 tonnes a year.

What we are looking at, they warn, is the impact of a 14-fold increase in the current nutrient levels coming from Storm Bay up through the D'Entrecasteaux into the Derwent Estuary and having an ecological impact remobilising potentially heavy metals in the Derwent Estuary, as well as having clear impacts on threatened species such as the spotted handfish and creating algal blooms.

This is a very serious issue that is being raised. The Derwent Estuary program recommends that the Storm Bay proposal be managed as a single system. It includes the impacts on the other nutrient-sensitive systems of the Derwent and Frederick Henry Bay. Given the scale of the proposed expansion the Derwent Estuary program is included in the assessment, modelling and monitoring programs. The impact of how the system responds to major storms must also be included.