Care
Mangrove management
Mangroves in Ohiwa Harbour
Image: BOPRC
Care > Mangrove Management
If there is one plant in the Ōhiwa Harbour that is able to become the subject of heated debates, surely it is the mangrove, or manawa, a native plant that has arrived in New Zealand relatively recently – about 14,000 years ago. The increasing spread of this highly adaptive estuarine plant probably results from the increase in sediment in the harbour in recent times. It has worried some people in the Ōhiwa community for many years. Among their concerns were the impact on shellfish beds and fish breeding, the obstruction to natural drainage and the reduced access to the harbour for boating and swimming.
As a result, the Ōhiwa Harbour Strategy partners started supporting the management of mangroves in 2008. In 2009 they developed an approach to management that included both dealing with the sediment flowing into the harbour from the land and controlling the mangroves themselves. The removal areas were carefully selected. It was well understood that mangroves, as a native plant, do have their place and provide habitat for other species, in particular the uncommon banded rail. In order not to disturb wildlife and considering dense mangrove stands act as coastal buffer zones, a removal boundary was drawn.
In 2011 Te Upokorehe, as one of the partners of the Ōhiwa Harbour Strategy partnership, was granted resource consent to control mangroves outside the mapped boundary. Te Upokorehe Resource Management Team and a small number of other local residents did the bulk of the hard work. With logistical support from the Strategy partners, each summer about ten working bees were held around the harbour.
Considering the value of mangroves the focus has been on the removal of seedlings and outlying plants from the outer fringe of established mangrove “forests”. Mangroves were cut with hand-held loppers into small pieces which are removed by the tide. At the end of 2020, volunteers completed a first “round” of the harbour. This means, since the start in 2011, outlying mangroves were removed from right around the harbour where access permitted the work. This included some fringes of Uretara, Ohakana and Hokianga islands.
A High Court decision in 2021 stopped this work. Why?
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Up to then it had been assumed that the National Environmental Standards for Freshwater (NESF) 2020 did not apply to saltwater wetlands (such as areas of mangroves). (The NESF provide very stringent protection for natural wetlands and do not allow any modification of them.) The Bay of Plenty Coastal Environment Plan allowed the Ōhiwa volunteers to do the work Upokorehe have been doing and this approach was supported by the Environment Court.
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In late November 2021, the Minister of Conservation and Forest and Bird appealed to the High Court. The judge had a different view from the Environment Court and declared any saltwater wetland (including mangroves) are subject to the NESF since they are natural wetlands. This decision over-rides the Environment Court and the Regional Coastal Environment Plan. In short, as a result of the High Court decision, mangrove removal in the Ōhiwa Harbour came to an end.
Now research into mangroves and coastal environments by Professor Karin Bryan at the University of Waikato has confirmed that comprehensive management approaches are needed. Professor Bryan points out that solutions need to be found within the larger scale catchments, for example the reduction of sedimentation loss from up-stream land-use practice. In fact, the key take-away of research on coastal restoration undertaken by Bryan and an international team of experts is: ‘Don’t blame the mangroves.’ The study, published in the journal Nature shows that ‘coastal mangrove removal initiatives cannot stop or mitigate mud-infilling of estuaries to restore previous sandy ecosystems’. By contrast, according to the researchers, ‘the removal of mangroves enhances estuary-scale sediment. This means human interventions like vegetation removal in estuaries can lead to counterintuitive results that actually impede restoration efforts’.
In light of this knowledge a change of attitude towards mangroves may be on the cards for some. Locally, the value of mangroves for biodiversity (including providing fish rearing, bird nest and feeding habitat), in reducing coastal erosion and seaward sediment build up is increasingly recognised. Mangroves also protect seagrass habitat from acidification by making seawater more alkaline.
In Ōhiwa Harbour the removal of mangrove seedlings remains a permitted activity where they have been controlled previously, where there are no mature mangroves in the area and provided Rules DD 19 – 25 of the Regional Coastal Environment Plan are considered. Mangroves cannot be removed from any of the three exclusion areas:
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Uretara Island Indigenous Biological Diversity Area A
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Motuoto Island Nature Reserve Indigenous Biological Diversity Area A
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Pataua Island Scientific Reserve and Extension Indigenous Biological Diversity