Jane Nees BOP Regional Councillor www.janenees.co.nz

Work on the removal of mangroves around Tauranga Harbour is continuing. The Bay of Plenty Regional Council holds a resource consent to remove approximately 90 hectares of mangroves – less than 10 per cent of the increasing number which fringe the harbour. Work began last year using a low ground-pressure machine with an attached mulching unit to clear mature mangroves. The plan was the resultant mulch would be dispersed by tidal action.

After removal of about 60 hectares of mangroves, monitoring of the mulching progress revealed that, in some areas, the mulch was not dispersing, resulting in anoxic conditions and a much slower than anticipated return to the previous sand-flat habitat. As a result, the regional council trialled the use of the beach groomer, used to collect sea lettuce, to pick up the mangrove mulch. It was obvious after the trial this method was not ideal and improvements in efficiency and cost-effectiveness would be required. As a result, it was decided to look at other ways of dealing with the mulch and also to remove the roots and pneumatophores left behind by the mulcher.

Recently, a new method was trialled in the Waikereo Estuary. This involved using a new bucket attachment with cutting blades for the existing low pressure digger, specially designed to remove all of the above ground plant matter. This was trialled for removal of existing mulch, but also for mature mangroves. The removed material was loaded onto a flat-bottomed drag barge, towed by a low pressure bulldozer and then carted to a green-waste collection site in Te Maunga.

The good news is the machine did a reasonable job of removing mature mangroves, taking the bulk of the above ground plant material and getting it offsite with very little disturbance of the estuary bed. The bad news was the forecasted cost of this method is significantly more than the cost of the existing mulcher because the bucket method is slower, requires offsite disposal of the mangroves and still needs some mulching to take out the aerial roots which are left behind to get a relatively clean surface and allow sediment remobilisation.

To maintain progress, it has been decided to continue using the original mulching machine for the remaining 25 hectares of consented mangroves. At the same time we will monitor the effects of the mulching operation, track and measure reversion back to a sand-flat habitat and continue to explore other more efficient options for mechanical removal of mangroves, including seedlings.

If you have any views on this or any other issue, please phone 07 579 5150 email [email protected] or visit www.janenees.co.nz