Pauatahanui Inlet in best health for 22 years

Pauatahanui Inlet might become a foodbasket once more, with a new survey indicating the health of the harbour is at its best level in 22 years.

Cockle numbers in the Porirua waterway have grown from 227 million to 336m since 2010, a 21 per cent increase, and a rise of 87 per cent since 1995.

The figures come from a three-yearly survey run since 1992 by volunteer group Guardians of Pauatahanui Inlet.

The eighth survey was done in December 2013. Its results, analysed by the National Institute of Water and Atmospheric Research, were encouraging, said Guardians member John Wells, a former zoology professor.

Shellfish were good indicators of pollution because they filtered so much water, but cockles in particular had very short breathing tubes, so sank and drowned when fine sediment carpeted the harbour floor, Wells said.

Middens and anecdotal memories from iwi suggested the inlet once had much higher shellfish counts, and he hoped that, within 20 years, cockle numbers might return to their historical highs.

The results were a boost to members of a harbour strategy, launched in 2012 by Porirua, Wellington and Greater Wellington councils and Ngati Toa, said Keith Calder, harbour strategy co-ordinator at Porirua City Council.

"It's exciting, because you know you're doing the right thing. I hope the community is happy with that as well, because it's easy to be impatient."

From 1974 to 2009, 315 millimetres of sediment had settled on the inlet bottom, much of it from an accidental spill while the suburb of Whitby was being built in the 1970s, Calder said. A natural rate of 1mm a year could be achieved by 2031, he said.

"If you don't get on top of the sediment rates, everything else you do may not matter."

Other problems plaguing the inlet's ecosystem included vehicle fumes, nitrogen from ageing sewers, and heavy metals from galvanised roofing, car tyres and brake pads, Calder said.

A cockle count for the much more polluted Onepoto Harbour in Porirua was being planned by iwi, and could begin as early as December.

Cockle numbers were lower than in 2010 in some southern sites of the inlet, including Seaview Rd, Browns Bay and Duck Creek. Sea lettuce and sediment washed by currents was responsible, said Tim Porteous, biodiversity manager at Greater Wellington Regional Council.

"It has to be remembered that the inlet is a dynamic environment, so there has been improvement in some areas and deterioration in others, but overall the picture is very good," he said.

The Dominion Post