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.
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