Unwanted Colonist - Forest & Bird

 24 Feb 2011

Blogger: Forest & Bird’s North Island Field Officer, Al Fleming.

The seemingly innocuous, sea-dwelling plant that is the mangrove is dividing communities as it spreads throughout the estuaries and harbours in Northland and the Bay of Plenty.  

Te ara, Tauranga

A host of environmental factors has turned these coastal hotspots into prime mangrove habitat.

In the past 68 years, mangrove cover in Tauranga alone has increased  by 2300%  (40ha to 920ha) turning vast open-water vistas into forested expanses.

Debate has raged for years as to whether to keep these blue-green forests, or to chop them down.

The reasons for the flourishing mangrove population can be put down to several factors- catchment erosion and sedimentation, higher nutrient loads and fewer frosts (possibly global warming!) are all implicated.

The disappearance of uninterrupted blue vistas, vital bird roosts and access to these watery playgrounds has prompted local estuary care groups to wage war on mangroves*. 

All mangroves cleared are taken off-site which has meant the humming sea-bed communities can still thrive, rather than become smothered by rotting forests.

Recently, the Bay of Plenty regional council has joined in the effort to remove mangroves and already they’ve earmarked 90 hectares of forest for removal. 

So far 60 hectare have been mulched, however the National Institure of Water and Atmosphere (NIWA) has raised the alarm because of the affect on estuarine communities    

These concerns prompted Forest and Bird members to inspect mangrove mulched areas at Omokoroa and Waikareao estuary.  These site visits confirmed that these areas were in very poor health.  Forest and Bird’s concerns are:

• sediment anoxic (the creation of oxygen depleted areas) associated with the presence of decomposing mulch.
• the resultant smothering of marine organisms.
• a lack of recovery within the mulched areas.

Other areas monitored by NIWA include Waikaraka, Te Puna and Matua estuaries.  We visited the Matua estuary which appears to have recovered to some degree; possibly as the area mulched was narrower, smaller and had greater  water flows. 

In an attempt to find an acceptable solution, Forest & Bird has made it clear to the Bay of Plenty Regional Council that alternative methods of mangrove removal should be trialled and the effect of those trials monitored in a robust manner.

It is notable that the 3 other ‘northern’ regional councils, i.e. Waikato, Auckland and Northland, are all watching Bay of Plenty Regional Council response to NIWA’s and Forest and Bird’s concerns and recommendations.  All of these councils are considering mangrove removal within their respective regions.  This includes Whangamata, Kaipara and Mangawhai harbours

- See more HERE

Managing Mangroves in Whangamata

Mangroves are found in harbours and estuaries. Whangamata’s mangroves have increasingly spread due to decades of sedimentation caused by land clearance and development. Waikato Regional Council has been working with the Whangamata community and other stakeholders to decide on the best approach to managing mangroves. For more Infomation click HERE

Mangroves in New Zealand - NIWA

In the past half century, mangroves have increased in extent in estuaries and tidal creeks throughout the upper half of the North Island.

While mangroves are native to New Zealand, and an integral part of functioning estuaries, their relative increase and association with fine sediments has resulted in many authorised (and unauthorised) ‘mangrove restoration’ trials. Unlike the rest of the world where mangroves are declining, in New Zealand these restoration trials involve the removal of mangroves, with goals of estuarine areas returning to prior sandier, unvegetated states, ideally with abundant shellfish resources.

Mangrove restoration programmes have been reasonably well documented by Environment Waikato and Environment Bay of Plenty, particularly those in Whangamata and Tauranga Harbours. NIWA is working with both councils to examine success of mangrove restoration methods and develop best practice for mangrove rehabilitation based on long-term analysis of locations where mangroves have been removed. We will look at identifying barriers to restoration after mangrove removal, for example sediments or organic material remaining from mangroves that result in long-term barriers for successful colonisation by shellfish. We are also looking at effects on neighbouring sandflats, shellfish beds, and seagrass meadows of the mangrove removal methods to make sure that the mangrove removals do not have negative effects on other important estuarine habitats.

For more information click HERE