Tauranga Harbour is a popular place to live, work and play. This is causing our natural environment to come under increasing pressure. Bay of Plenty Regional Council has identified key issues that the harbour faces, and ways to manage them.
The Tauranga Harbour Integrated Management Strategy has identified mangroves as one of these key issues. Bay of Plenty Regional Council is working with landowners to reduce sedimentation which exacerbates mangrove spread. Council also supports Estuary Care groups in their restoration work and has commissioned an innovative new hovercraft mangrove seedling mower to help reduce mangrove spread.
See the report published March 2016 on 2011 Mangrove abundance in the Bay of Plenty (1.95MB, pdf).
For more information about mangroves read the Tauranga Harbour mangrove factsheet (yet to be updated based on March 2016 report).
Facts about mangroves
There are 70 species of Mangrove found worldwide. In New Zealand only one species is present, Avicennia marina subspecies australasica or Manawa, which is native to New Zealand and has inhabited New Zealand coastal environments for approximately 19 million years. The Manawa species is the most southerly growing species in the world, found in estuaries as far south as Kawhia Harbour on the west coast, and Ohiwa Harbour in the Bay of Plenty.
Mangroves generally live in shallow and low energy (calm) inter-tidal (dry at low tide) areas of estuaries or harbours. They provide a buffer against coastal erosion and storm surge and are a nursery ground for some juvenile fish species like short finned eel and yellow eyed mullet. They're also home to many native insects, birds, shellfish, snails, crabs and algae. Mangroves can protect the shoreline against large waves and storm surge and trap and stabilise the sediment, resulting in more shoreline protection from erosion. However, too many mangroves can create problems in the harbour.
Changing the way we use land in the Tauranga harbour catchment has caused more sediment and more nutrients to get into the harbour. This includes activities like subdividing land and clearing land of bush. The extra nutrients have caused more mangroves to grow to the point where they've taken over large areas of the open tidal flats, especially in the sub-estuaries ( i.e Te Puna, Waikareao and Waimapu estuaries).
Mangrove canopy cover (ha) over time within a number of estuaries in Tauranga Harbour.
Waikareao Estuary in Tauranga Harbour with mangrove and salt marsh habitats
When the mangroves spread, they increase sedimentation (sand build-up), by changing currents and dampening wave energy. The vertical root structure and low branches of the plant help it to trap fine silty sediment.
Increased sedimentation and mangrove spread is a major concern to communities living around harbours and estuaries who would like to see open water and estuary habitats maintained in a healthy condition. Mangroves are spreading in many of the harbours and estuaries they inhabit, including parts of Tauranga Harbour. Aerial photo comparisons (figure 4) over time show the expansion of mangroves over 44 years.
The 1959 Aerial photo of the Tauranga Harbour showing a) Welcome Bay and b) Matapihi sections of the estuary and the change in mangrove abundance over time. Red areas show the mangrove area in 2003 and yellow areas show mangrove distribution in 1959
We have set up an Estuary Care programme which is similar to Coast Care in the Bay of Plenty. Estuary care involves our staff working with community groups that are concerned about the increase in mangroves and silt. The programme has the support of all Bay of Plenty coastal councils, the Department of Conservation and other agencies.
We provide the Estuary Care groups with technical and other resources to form a functional community group that links with other groups in that catchment. They address environmental issues like biodiversity, riparian planting, animal and plant pest control, as well as mangrove removal.
Removing mangroves requires a resource consent so our Land Management staff work with the groups to help them gain consents to work in selected areas of their estuary, and to comply with monitoring and consent conditions.
See our research and monitoring page for research reports on mangroves and other harbour issues.
What you can do to stop further mangrove spread
There are some things you can do to help reduce the spread of mangroves in Tauranga Harbour:
- Minimise sediment runoff when carrying out earthworks
- Plant out any riparian areas to trap sediment and reduce nutrient runoff
- Join an Estuary Care group to help restore estuaries and their catchments. Bay of Plenty Regional Council supports these group to gain the necessary approvals. Otherwise ensure you have the appropriate permission from Bay of Plenty Regional Council - coastal areas are publicly owned and very sensitive and it is important things are done right.
If you want to get involved in the community groups or would like information on areas covered by Estuary Care programmes in Tauranga Harbour, please contact Bay of Plenty Regional Council.
In 2007, Bay of Plenty Regional Council commissioned an independent review of the way it engages with community groups and other agencies in the management of mangroves. Read the review (1.1 MB, pdf) by Catalyst Consultants and the Chief Executive's report on how the recommendations would be addressed.
The Regional Policy Statement and Proposed Regional Coastal Environment Plan establish new objectives, policies and rules for mangrove management in the region.