Last updated 15:48, March 5 2018
A new study shows about 700,000 tonnes of sediment goes into the Kaipara Harbour each year.
Sheep, beef and dairy farms are blamed for the majority of sediment ending up in the Kaipara Harbour and it could cost up to $331 million a year to fix it.
Northern Regional Council (NRC) and Auckland Council commissioned experts to research a range of solutions to reduce the loss of sediment into the harbour.
NRC water policy specialist Ben Tait said there had been concerns about the large amount of sediment getting into the harbour and the effects it had for some time.
The study showed the current annual-average sediment load into the harbour was about 700,000 tonnes per year, compared to 120,000 tonnes per year in pre-human times.
Sediment plume dispersing into the Kaipara Harbour from the Hoteo River outlet after a small flood in March 2011.
It found the best option to reduce sediment in the Kaipara Harbour was to plant trees along the coastline.
This could reduce sediment by up to 88 per cent at a cost of between $255 and $331 million per year.
"The results were sobering," Tait said.
Sediment in the Kaipara Harbour reduces water clarity, making it difficult for fish to feed.
"What the results suggested, although there are uncertainties and assumptions behind them, is that to actually make a meaningful improvement in the ecology of the streams and rivers that drain into the Kaipara Harbour and also the Kaipara Harbour itself by reducing sediment, you would probably have to afforest significant parts of the catchment at a very, very large cost."
One of the study's researchers Malcolm Green said sediment had numerous adverse effects on the harbour's ecology.
"Sedimentation can smother habitats and shellfish beds," the estuarine, coastal and marine specialist said.
Estuarine, coastal and marine specialist Malcolm Green says it's important to control the sediment getting into the Kaipara Harbour.
"You get reduced light penetration and water clarity with high sediment loads that affects seagrass and the ability of fish and birds to feed.
"It's important to control it."
Green said meetings had been held with the councils, stakeholders and iwi to develop a number of options to reduce the sediment going into the harbour.
Some of the scenarios included fencing all streams in the Kaipara Harbour catchment for stock exclusion.
Rules around making farmers pay to fence their properties could be an option, but NRC had yet to analyse what it would do.
In nearly all scenarios looked at, sheep and beef farms faced the largest costs.
This was because about half of the sediment loss came from sheep and beef farms, the study showed.
About a quarter came from dairy farms.
Excluding stock and stabilising large tracts of highly erodible land could reduce sediment in the harbour by 41 per cent at a cost of $13m a year, the study said.
Federated Farmers senior policy advisor Richard Gardner said it would be looking to get the study "ground truthed" to make sure the results were accurate.
However, Gardner said the organisation recognised that there was an issue with sedimentation getting into the Kaipara Harbour.
"Even now, farmers are doing an awful lot about it by way of riparian planting [and] restricting livestock access to waterways," he said.
With the claim that sheep and beef farms were responsible for almost half of all sediment getting into the harbour, Gardner thought this was more to do with the slope of the land rather than land use.
"That's one of the things we will be looking to ground truth."
Auckland Council was proposing a water quality targeted rate in its Long Term Plan, something Federated Farmers would support.
Gardner said the organisation accepted that farmers would be expected to contribute to it.
Rodney Local Board recently awarded about $230,000 to 30 applicants to help improve water quality.
The applicants would receive up to 50 per cent of the cost to plant along river banks and build fences to keep stock out of waterways.
- Stuff
(Source: https://www.stuff.co.nz/)