Posted on 5-4-2004

Mahurangi Harbour Choking
03.04.2004, By ANNE BESTON, NZ Herald

Beautiful Mahurangi Harbour is slowly dying, filling with mud and silt
that is suffocating marine life and threatening to create boating no-go
zones.

After almost a decade of scientific study, the state of the harbour shows
the diversity of its marine life declining with some areas now devoid of
shellfish such as cockles. "If this continues, in a few years the harbour
will lose its water clarity, shellfish will die, oyster farming become
less viable and fishing and boating will be affected," said Auckland
Regional Council land and water quality manager Eddie Grogan.

The harbour is a marine playground for Aucklanders, lying just south of
Warkworth and includes popular holiday spots. The upper reaches of the
harbour, including Hamilton's Landing and Te Kapa River, are in a worse
state than the shallower reaches of Manukau Harbour, said ARC
environmental scientist Dominic McCarthy. "We've been monitoring the
Manukau for 15 years and it doesn't show the same problems we are seeing
in the Mahurangi," he said.

Though sediment first began washing into Mahurangi harbour around 150
years ago after deforestation, land clearing, subdivision and farming mean
more soil washes into the harbour every year. Core sampling shows between
4mm and 7mm washes into the harbour each year compared with less than one
millimetre 150 years ago.

Mahurangi was more likely than other parts of Auckland to be hit by
intense storms, Mr McCarthy said. The shallow estuaries in the upper
reaches of the harbour were becoming more shallow, and deeper parts of the
harbour were also showing signs of stress. Horse mussels were struggling
to filter ever-increasing amounts of silt.

ARC presented its findings to Rodney District Council, which already has
some land-use rules in place, including designated areas deemed unfit for
subdivision. But ARC is putting $600,000 into a draft action plan to be
agreed on with residents and the district council. It should be ready by
July, and a dedicated coastal adviser will be appointed. "Essentially
Mahurangi is a bit of a pilot where we can put sediment controls into
action, but we want the community to agree and be part of it," Mr McCarthy
said.

Controls include planting around waterways to stop banks crumbling,
keeping stock out of streams and rivers, tighter controls on earthworks
and providing incentives to stop development in erosion-prone areas.

But Mr McCarthy warned damage to some parts of the harbour might be
irreversible. "If you asked if some places in the harbour would recover
completely, I would say probably not - but if you asked if we could
improve them, I would say yes," he said.