What it will take to fix Kaipara Harbour

A new study by a consortium of researchers led by Streamlined Environmental Ltd has shown that the bulk of the sediment depositing each year in Kaipara Harbour is eroded from pastoral land, and it could cost up to $331 million a year to stop it. However, the study also shows that it is much cheaper and more efficient to set specific goals in freshwater and harbour receiving environments and then tailor mitigation to achieve those goals, than it is to do “blanket” mitigation across the catchment.

The researchers in the Kaipara Harbour Sediment Mitigation Study developed a model of Kaipara Harbour and its catchment for assessing costs and benefits of sediment mitigation. Currently, about 700,000 tonnes of sediment per year from catchment erosion is deposited in Kaipara Harbour, which is 6 times what it was before the arrival of humans, when the catchment was covered in native forest. As a result, the harbour is highly sediment stressed, with impacts on shellfish, seagrass, fish, birds and mangroves. “To really turn the clock back, large-scale catchment re-afforestation and rehabilitation of wetlands is needed”, said Dr Malcolm Green, the study leader. “However, significant ecological gains can be had at much less cost by focusing on-the-land mitigation in areas where it will have most effect in streams and in the harbour, and we can predict where those areas are with the model”. Ben Tait, policy specialist at Northland Regional Council, said the results were “sobering”. In nearly all of the sediment-mitigation scenarios that were analysed, sheep and beef farms were looking at the largest costs, mainly because these farms are located on the most erosion-prone land. The results will be used to plan soil conservation and erosion mitigation in the catchment, including the way costs might be distributed amongst stakeholders.

A summary of the results of the Kaipara Harbour Sediment Mitigation Study is available here.

The Kaipara Harbour Sediment Mitigation Study was funded by Northland Regional Council, Auckland Council and the Ministry for the Environment. The team consisted of Malcolm Green, Ngaire Phillips and Ashlee Dunsmuir (Streamlined Environmental), Adam Daigneault, John Dymond, Les Basher and Alexander Herzig (Landcare Research), Andrew Swales, Andrew Hughes and Drew Lohrer (NIWA), and Christoph Matthaei (University of Otago).