Warriors of the harbour: restoring estuarine health
Just 20 minutes north of Wellington, where SH1 meets the coast, lies Te Awarua-o-Porirua Harbour, the largest estuary in the lower North Island. This network of inlets, wetlands and shell banks was once a rich food basket for local iwi Ngati Toa. Veronika Meduna reports that the iwi has now joined forces with councils and NIWA to restore the ecosystem to its former health.
“The harbour was our Countdown,” says Taku Parai. “We used it as our food source. Shallow water food came from the inner harbour and the deeper food we wanted was out in Cook Strait.”
He says his older brother, now 87, still remembers walking the hills that surround the harbour as a child to collect conger eels and kaimoana.
“The pipis used to be big and beautiful and the best in the country when I was a kid, and I just caught the tail end of that opportunity and ability to get food from the harbour.”
A history of degradation
Back in 1888, Ngati Toa had the foresight to write a letter to the colonial government, warning that the harbour’s health would soon decline as more and more settlers were arriving. The iwi called for its protection in its pristine condition, but a decade later a hospital asylum was built, which resulted in waste being pumped into Porirua Stream.
“While we were gathering shellfish at this end, they were pushing it out into the harbour and our people started dying of typhoid,” says Taku Parai.
Then, in the 1950s, road construction straightened some of the coastline and truckloads of rocks and debris were dumped across flounder beds.
“And, after the war, we needed people to help with the reindustrialisation of the country and housing was built in Porirua East. Where did they put the dirt from that reclamation? Into the harbour.”
The continued degradation of the harbour and estuaries took away the integrity and mana of his people, he says. “The harbour to me is a huge issue and I won’t be satisfied until my grandchildren can go back to it and eat pipis.”
With ongoing changes in land use, which now include the construction of the four-lane Transmission Gully motorway, new residential housing and forestry plantations ready to be harvested in parts of the catchment, it was clear that there would be more impacts on the harbour and that something had to be done. To this end, back in 2012, Ngati Toa joined forces with three local authorities, NIWA and the community group Guardians of Pauatahanui Inlet to launch the joint Porirua Harbour and Catchment Strategy and Action Plan.
A plan to address the major issues
The aim is to implement a management plan for the harbour, and underpinning the endeavour is a robust science programme that addresses three major issues: sedimentation, pollution and habitat degradation.
“The biggest issue is sedimentation. If we could tackle that, we would reduce pollution and improve success rates for habitat restoration projects,” says Dr Megan Oliver, a senior environmental scientist at Greater Wellington Regional Council.
The two arms of Porirua Harbour—the Onepoto Arm and the Pauatahanui Inlet—are quite different systems. The Onepoto Arm has seen more development, while Pauatahanui Inlet is still convoluted, with stretches of natural coastline, but both parts have been receiving excess sediment from erosion-prone farmland, construction sites and forestry.
Pollution comes into the harbour as occasional sewer overflows and stormwater runoff in pipes and in the Onepoto Arm in particular, and littering can be an issue at the mouth of Porirua Stream.
Research by NIWA and Greater Wellington shows the two estuaries don’t share sediment flows and neither drains entirely during each tidal cycle, which means there are large subtidal areas and contaminants that are not completely flushed out. Contamination hotspots at the Porirua city’s main outfall show levels of zinc, copper and lead that exceed guidelines in some areas.
Using science to encourage restoration
To establish baselines for sediment flows and contamination, Greater Wellington has installed turbidity meters in the major streams that contribute most of the sediment, and, in 18 spots throughout the harbour, buried plates track the annual accumulation of mud and sand. Megan Oliver says the team can use this scientific evidence “to encourage local authorities to put stricter rules on developers and stormwater discharges”.
Warrick Lyon, a NIWA fisheries technician who grew up in the area, says another good indicator of changing ecosystem health is the size of the cockle population in the harbour. Back in 1970s, the then Department of Science and Industrial Research carried out a survey of cockles in both arms and estimated that the intertidal population was well over 500 million. Almost two decades later, the Guardians of Pauatahanui Inlet organised the first recount, and by then the population had more than halved, reflecting a period with significant subdivision development on the inlet’s shores. Since then, the cockle population has been on the increase again and surveys are carried out every three years to keep track of changes.
Apart from cockles, NIWA has been working with Ngati Toa to establish baseline data on other intertidal shellfish and fish species. The fish survey identified 21 species that live in the harbour or use it during certain times of the year. This list includes yellow-eyed and grey mullet, spotties, mottled triplefin, but also kawahai, flounders, eagle rays and rig.
Warriors of the harbour
“Porirua Harbour is an important estuary with habitats that are essential in the life history of many species,” the survey found. “It is used in many different ways by many different species. Some species are year-round residents, such as kahawai that use the harbour until they reach maturity; sand flounder of all sizes that leave only to spawn; and mottled triplefins whose entire life is spent in the harbour.”
Rig come into Porirua Harbour to mate and to have pups. These small sharks are a commercial species known as lemonfish, and they are often what you eat when you have fish-and-chips.
Warrick Lyon has managed to track rig during one summer, using an especially designed floating GPS tracking system, and he says the two estuaries are an important nursery for their young.
NIWA’s work was set up to build capacity and to encourage members of Ngati Toa to continue the surveys and the work to restore habitat. The earlier surveys from the 1970s found a rich diversity, with “huge schools of grey mullet”, but during the housing development along the inlet, average sedimentation rates reached between 8.5mm and 15mm
“The pre-European background rate is less than 1mm per year,” says Megan Oliver.
Complementing coast with catchment
To complement the strategy, Greater Wellington has established the Te Awarua-o-Porirua Whaitua, which represents the entire catchment area. Warrick Lyon is a Whaitua committee member and describes the process as collaborative and consensual, with the goal of developing regulatory and non-regulatory proposals for land and water management and to set objectives and limits on what can be taken from or discharged into the Porirua Harbour waterways.
NIWA urban aquatic scientist Jonathan Moores and aquatic chemist Dr Jennifer Gadd are also involved in the Te Awarua-o-Porirua Whaitua process with the “Modelling Leadership Group”. This multi-disciplinary group of seven researchers from throughout New Zealand is contracted by Greater Wellington to organise modelling to support the Whaitua Committee in its decision making.
Moores’ involvement is to provide expertise in modelling the effects of urban development, with Gadd providing expertise in Bayesian Network modelling. A Bayesian Network is being developed for this process which will allow an assessment of the effects of different management methods and development scenarios on water quality and ecological attributes. This work will feed into social, economic and cultural assessments, to enable a holistic assessment of the effects of policies that the Whaitua Committee recommends.
The Bayesian Network model will build on previous models developed in the MBIE-funded programme “Resilient Urban Futures” to predict the effects of urban development on freshwater quality and ecology.