New way of managing mangrove mulch trialled

A new way to manage mulch left after mangroves are mechanically removed was trialled today at Waikareao estuary.

The Bay of Plenty Regional Council trial is the first step to investigate different ways to manage or remove the mulch as part of the wider mangrove removal project currently taking place in selected sites around Tauranga Harbour.

Chairman John Cronin said the trial was timely as concerns had been raised recently that mangrove mulch was not being flushed from some areas as quickly as hoped.

“I’m pleased today’s trial went ahead as this season’s work has just begun and it’s important to alleviate any concerns that the mulch is causing a smothering effect,” he said.

“We hope that by removing mulched material the estuary bed will revert to open mudflats quicker,” Mr Cronin said.

The trial involved the purpose-built mechanical mangrove mulcher being followed by a beach groomer. The groomer has been used to remove sea lettuce from Mount Maunganui beach and other areas around the harbour. The mulch will be taken off-site as green waste to be composted.

Data collected today would be analysed over the next week to test the efficiency of recovering the mulch.

“This has been, as expected, a good learning experience. We should be able to improve a couple of equipment issues which will help in the long-term,” Mr Cronin said.

The results of the trial will be considered by the council and if successful, will continue in other areas where mangroves will be removed this year, under consent. Firm ground and good access are expected to affect where this method can be used effectively.

A longer-term scientific monitoring programme is being undertaken independently. It is going to track how areas respond to mangrove removal over time.

Anoxic conditions will develop regardless of the mulch being removed as the mud below mangroves is already anoxic and contains very high levels of organic matter. This matter will continue to decompose.

Stories and photographic evidence from the locals around Waikareao show that the older mulched areas have been heavily frequented by birdlife including banded rail, ducks, stilts, and herons.

Tu Piahana, Ngai Tamarawaho and founding chairman of Waikareao Estuary Care Group relates his fishing experience around the mulch site over the past few months.

“Since the original mulching it is the first time in years we have been able to reliably catch mullet in the Waikareao, we have even caught parore which has been unheard of over the past ten years.”

Last season about 80 hectares of mangroves were mechanically removed from around Tauranga Harbour, under resource consent. This season about 30 hectares of mangroves will be cleared in the Waimapu, Welcome Bay and Wainui estuaries.