Sediment study results were ‘sobering’
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 were concerns about the large amount of sediment getting into the harbour and the effects it had.
This included smothering shellfish beds and reducing water clarity.
The study showed the current annual-average sediment load into the harbour was about 700,000 tonnes per year.
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.
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,’’ he said.
‘‘You get reduced light penetration and water clarity with high sediment loads that affects seagrass and the ability of fish and birds to feed.’’
Green said meetings had been held with key parties to decide on scenarios to research.
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.
A new study shows about 700,000 tonnes of sediment goes into the Kaipara Harbour each year.