Coastal allure Hauraki Gulf's main peril

A rush to live on the coast is probably the biggest environmental threat facing the Hauraki Gulf, according to a report released yesterday.

Sponsors of the report, the work of the Hauraki Gulf Forum, include the Auckland Regional Council and Environment Waikato.

It says other threats to one of the country's most popular stretches of water include mud and silt pouring into harbours, pollution, the introduction of exotic pests and erosion.

Pollution

Tamaki Estuary is now one of the most polluted water bodies in the Auckland region and many streams and rivers that drain into the gulf are in poor condition, the report says.

Swimming at beaches around the region is safe for a "vast majority of time" except for beaches next to built-up areas after heavy rain.

Sediment

Development, specifically earthworks, are a key factor in the amount of mud finding its way into upper-harbour estuaries in the gulf.

In the Auckland region alone, consents were issued for 614ha of earthworks in 1999 and 2000. By 2003, the rate of earthworks had more than doubled to 1570ha.

Development

The rush to the coast has seen an 18 per cent population increase in the Hauraki Gulf's coastal areas between 1991 and 2001, while the national average for the rest of the country over the same period was 11 per cent.

Coastal development "hotspots" expected to exceed national population growth forecasts up to 2010 include Orewa, Gulf Harbour, Whangamata and Whitianga.

Property sales on Coromandel Peninsula have more than trebled in the past four years. Whitianga's population is set to double by 2020.

More than 70 per cent of the beaches and dunes of the Coromandel Peninsula have houses on them.

Just 10 per cent of the Waitemata Harbour is undeveloped. On the North Shore, 88 per cent of the coastline is developed and 12 per cent is developed to some extent.

Fish and marine life

The gulf is the most intensely fished recreational fishing region.

The report concludes that after reaching a low point in 1985, the snapper population of the gulf is "rebuilding". Shellfish such as cockles and pipi are being depleted.

Pests

Around 39 foreign species now in the Waitemata Harbour include a sea anemone, a sea slater and a barnacle.

Three new invasive species detected in the gulf since a 1997 survey include the seaweed undaria.

Four species introduced in the past 30 years that have had a detrimental effect on the gulf's marine life include the Asian date mussel, the pacific oyster and the parchment worm.

Coastal hazards

Erosion, coastal flooding and climate change are the biggest threats to the gulf environment, the report says.

Coastal erosion has meant 100m of land has disappeared in areas worst-hit. Many of the gulf's cliffs are retreating. Sandstone cliffs are being eaten away at an average rate of between 4m and 8m every 100 years.