Jenny and Tony Enderby: Why mangroves are an asset to us all

Most of us living around the northern New Zealand coast hold strong views on mangroves. We have spent years snorkelling and observing the underwater life around them, and our view is that they are an asset to the tidal estuaries and marine life.

Those responsible for the chainsaw attacks on Whangamata Harbour's mangroves hold a different view. They suggested mangroves were proliferating, creating the mud that blocked the harbour.

Rather than making estuaries shallow, mangroves are the end result. Silt, carried by heavy rain, will fill the upper reaches of estuaries even if mangroves are removed.

Mangroves probably appeared in New Zealand at the end of the last ice age. The native mangrove species, Avicennia marina, is also found in southern Australia.

When James Cook's botanists discovered mangroves in northern New Zealand's estuaries, they also found large amounts of gum. They weren't aware the gum was from kauri forests that lined the estuaries or was buried beneath them and named the mangrove Avicennia resinfera, the gum-producing mangrove.

In Cook's time, mangroves bordered the edges of the salt marshes and swamp forests that surrounded estuaries. Silt carried by heavy rain from an estuary's catchment was trapped by the sponge-like qualities of the forests and salt marshes.

The small amounts that reached the estuaries settled around the minimal mangrove trees on sand at the edge of the salt marshes. Almost no examples of this coastal forest, saltmarsh and mangroves remain.

The first changes to mangrove forests began after coastal forests were felled. Often logs were carried to the coast in rivers. Huge amounts of silt travelled with them into the estuaries, carrying away and smothering the mangroves and much of the life that lived around them. Once the forests were gone the mangroves regenerated. Settlers drained the swampy salt marshes for farmland, depriving the bird life that thrived in those wetlands, of a habitat.

As the water got muddier, fewer fish and invertebrates colonised those areas. Wandering stock also added to the problem in the tidal reaches and streams that fed into the estuaries.

Natural control of runoff was gone and tonnes of silt washed directly into the waterways. The proliferation of mangroves began.

Coastal development opened up northern New Zealand's pristine beaches in the last 50 years. The roads to many beaches wound around estuaries.

An easy answer to improve road access was to fill in the high tidal mangrove areas for causeways, boat ramps, rubbish dumps and parking areas, often leaving only a small channel for the tide to reach the upper reaches. This initially caused a decline in mangroves.

For many people who built baches at the time, this is their memory of the mangrove-free harbours that once existed.

The higher tidal areas, cut off by causeways and roads began to fill with silt, only much more quickly than is natural. Only the channels and areas below the mid-tide area remained open, as mangroves do not grow below the half-tide mark.

Nutrient-rich silt filled the higher tidal areas where the mangroves grew and they thrived.

Some began to worry about mangroves appearing in new places, such as the seaward side of causeways and reclamations - especially bach or home owners who looked across them.

People began to discreetly remove mangroves, hoping to return the area to sand. Generally the mud remained, continuously fed by more silt. Most estuaries no longer supported the chain of life they once did as many of the filter-feeding animals that acted like vacuum cleaners were smothered.

The Raglan community addressed the issue of controlling the run-off. They fenced streams and planted 500,000 trees around the harbour. The result was clearer water and increased shellfish and fish stocks.

Recreational fishing has improved and whitebaiters who were taking home half a cup of whitebait now take half a bucket.

Eelgrass flats cover the harbour, which teems with crabs, shellfish and wading birds.

There were unforeseen benefits too.

Local dairy farmers increased milk production by 20 per cent after streams were fenced and stock became healthier with fresh water to drink.

Without fresh supplies of silt washing down from the streams, mangrove proliferation slowed as sand replaced mud.

Whangateau Harbour, an hour by car north of Auckland, is one of the few largely unspoiled harbours near a population centre.

It supports much marine life, something known to those who dive or snorkel in its waters or fish from its banks.

The harbour is bordered by farmland and the holiday resort of Omaha. Its southern reaches, separated by the Omaha causeway, support kahikatea forest, ti tree and salt marshes bordering stunted mangroves on sandy flats that support eelgrass and shellfish beds.

Large numbers of birds live in the wetland and even more feed over the sandflats as the tide falls. The retention of the surrounding natural habitats, coupled with a low catchment, rather than any planning, have probably saved Whangateau.

Underwater it is surprisingly clear, even around the mangroves. In early summer thousands of small fish, mainly yellow-eyed mullet and kahawai feed around the mangroves.

The reason for the water's clarity is the presence of huge numbers of bivalve shellfish, like cockles, pipis and wedge shells in the harbour.

Along with other invertebrates like crabs, barnacles and anemones they collectively act as a giant filter, straining minute particles from the water, leaving it much clearer.

Fish, including snapper and kingfish, drift with the tide into the harbour. The estuary residents- parore, goatfish, stingrays, flounder and eels - feed around the denser mangrove areas on the high tide.

New Zealand's few areas of healthy mangrove forest support a huge diversity of marine life.

If the coastal system of forest, saltmarsh and mangroves could be re-established by fencing streams and planting trees, the mangroves would be controlled.

The bonus of more shellfish, fish and clearer water is a far better end result than just pulling out or chopping down mangroves and leaving a muddy wasteland.

* Jenny and Tony Enderby are freelance photojournalists who specialise in the marine environment.