Tauranga harbour's overgrowing mangroves will be mowed by a unique hovercraft.

The innovative custom-built craft can cut about 600 hectares of seedlings annually while causing minimal environmental damage.

A schematic of the hovering mangrove mower.

The first of its kind will be used as it offers many advantages over other designs, says BOP Regional Council general manager of environmental delivery Warwick Murray.

'The more we looked at it the more we were attracted to it because of its really light footprint,” says Warwick.

'It will leave no footprint, have negligible effect on the benthic community and also has some advantages in it can move between estuaries, cover ground more readily, move a bit faster so operating costs will be less.

'It's still unproven as a technique, so we are trying to be innovative - trying adaptive ways of dealing with this, but it's a pretty exciting possibility really.”

The hovercraft with mower blades attached, is intended to prevent the further encroachment of mangroves by cutting down mangrove seedlings. It is not intended to tackle mature growth, says Warwick.

'Basically it's going to have exposed mower blades out front. All it needs to do is nick off the head of the mangrove seedling. Once you take the leaves off, then it won't grow again.

'It's a bit like a Flymo [lawnmower], but with the blades on the outside.”

The council ran trials using a tracked vehicle, which worked but it left tracks. The hovercraft proposal turned up when it was put out to tender, says Warwick.

In her Weekend Sun column this week, BOPRC deputy chair Jane Nees says mangroves in Tauranga Harbour have expanded exponentially over the last 50 years, and there are now more than 900 hectares of mangroves fringing the waterfront.

The expansion is blamed on increased levels of sedimentation in the harbour and a reduction in the frequency of hard frosts.

Mangroves spread via propagules, germinated seed pods, from mature plants which are dispersed by tides and currents to establish in open, sandy flat areas.

The more mangrove plants there are, the more mangrove propogules are produced – and the faster the mangroves spread.

'If we don't want to lose a lot more of our sandy flat and salt marsh coastal fringe to mangrove coverage, we have to address these spreading mangrove seedlings,” says Jane.

'The good news is once mangrove seedlings are cut above the roots, they don't regrow, so ‘cutting/mowing' is a good option for control.”

Pulling out mangrove seedlings by hand is a permitted activity under the Regional Policy Statement and Proposed Regional Coastal Environment Plan, but keeping up with the mangroves will take a lot of man-hours.

The regional council has obtained consent to allow it to use a machine to ‘mow' seedling-infested areas, providing environmental impacts are minimised.

The hovercraft should leave no discernable footprint on the estuary surface and provide minimal disturbance to birds and benthic animals.

The hovercraft is under construction and is hoped to be trialled before the upcoming bird breeding season, says Jane.

The mangrove mowing hovercraft is part of a number of increased marine biosecurity efforts, and new scientific research the Bay of Plenty Regional Council will be leading this year in its work to care for Tauranga Harbour.

The community has asked for more work to be done to keep the Tauranga Harbour healthy and accessible, says Warwick.

'Tauranga Harbour and its catchment are a priority for Council's work to care for the land, air and water in this region.

'We've budgeted $5.1m this year to continue and expand on our efforts to deliver work the community has asked for, including reducing harbour sedimentation and mangrove spread, improving water quality and managing sea lettuce.”

The regional council Annual Plan for 2014/15 was adopted by the Regional Council on 26 June, based on Year Three of its Ten Year Plan 2012-2022. The Plan sets out what activities the Council will undertake and how they will be paid for.

'We already have a comprehensive water, shellfish and environmental monitoring programme in place and we assist landowners, through advice and subsidies, to manage pests, protect natural areas and get the best out of their land while looking after it. Since 2012 we've also been working with landowners to implement 16 sub-catchment action plans, with the aim of reducing sediment and nutrient run-off into the waterways and harbour,” says Warwick.

'That work, along with our maritime safety, pollution response, estuary care group support and sea-lettuce clean-up efforts will continue as usual.”

Additional work planned for this financial year includes:

• trialling and refinement of sediment control methods for upper catchment use
• development and trialling of a hovercraft for low impact mowing of mangrove seedlings in previously cleared areas
• increased surveillance and advocacy efforts to prevent the spread of unwanted marine pests such as the Mediterranean fan worm and clubbed tunicate sea squirt
• Pollution audits with businesses in the Judea industrial area
• Scientific research to improve understanding about nutrient sources and cycles (that can affect abundance of sea lettuce and other marine plants and animals) and the impacts of sediment on marine life in the harbour
• providing all-tide access to Matakana Island through dredging of the Opureora Channel (subject to resource consent approval)
• Development of a shared strategy, in partnership with Tauranga Moana iwi and territorial local authorities, for future management of the harbour and catchment.

'Tauranga Harbour is a taonga (treasure) that many people in the community rely on for recreation, kaimoana (seafood) and business. We need everyone to help keep the harbour healthy by taking care with what they do on land and in the water.”