A wide variety of marine pests have been introduced to the Derwent through ballast water discharges, hull fouling, fishing and aquaculture activities. Shipping is likely to be responsible for the majority of the estuary’s introductions. Over seventy introduced marine species have been identified in the estuary, with the greatest concentration found around the Hobart docks.
The following species are considered to be having the greatest ecological impact on the estuary. They are generally abundant and widespread.
Review of distribution, issues, recent actions & management options.
Weeds and their management
Over 71 species of weeds are found along the Derwent estuary foreshore, including 15 State-declared weeds and seven Weeds of National Significance (WONS). Boneseed, African boxthorn, blackberry and fennel are particularly widespread.
In 2010, the DEP carried out a Derwent weed mapping and prioritisation project, with support from an Australian Government grant. Weed data was compiled from multiple sources and a number of management priorities were identified. Sixteen sites were identified for further work, two of which have been progressed in collaboration with our partners (karamu control in the upper estuary and weed management at Bedlam Walls/East Risdon).
Rice grass (Spartina anglica) is a native of Europe and was introduced to a number of Tasmanian estuaries with good intentions, but adverse consequences. The Tasmanian Government commenced control efforts in 2002. The current distribution is now reduced to a few small areas, with annual surveys and control works coordinated by the DEP. See the 2017 Ricegrass Report for further information.
Karamu (Coprosma robusta) is another weed of particular concern for the Derwent. This New Zealand native is prevalent in the upper estuary near New Norfolk, and has the potential to spread into high value wetlands of the upper estuary. The DEP commenced control of this declared weed in 2010, with support from and Australian Government grant, and made good progress in reducing the extent of the infestation, with support from a number of partners. In 2017 efforts were renewed with the creation of a new Karamu Strategic Management Plan, designed by the Derwent Catchment Project with input from the DEP and other partners, and control work is progressing well.