Company harvests ‘sea gorse’
Demand is growing for organic fertiliser made from an invasive seaweed that chokes out native species and clogs mussel farms.
Harvesting is under way in the Marlborough Sounds of the pest species wakame, commonly known as wakame seaweed or ‘‘the gorse of the sea’’.
Introduced to New Zealand waterways in the 1980s via ballast water from cargo ships, the weed is now widespread along the eastern and southern coastlines from Auckland to Bluff.
Although classified as an unwanted organism under the Biosecurity Act, wakame is edible and can also be used to make fish food and fertiliser.
Nelson-based company Waikaitu harvests the seaweed every year and chief executive Alex Pressman said this year’s collection could be the biggest yet. ‘‘We’re expecting to get about 100 tonnes this season, the average is about quarter of that,’’ he said.
Most of what was harvested would be made into organic plant care products at the company’s production facility in Tasman and sold on the domestic market. Pressman said consumers and primary producers were looking for cleaner alternatives to chemical products as concern over fertiliser use and nitrogen leaching grew.
‘‘As a result, demand for our products has increased and we’re expecting our biggest harvest from Marlborough and Otago Harbours,’’ he said.
‘‘Very little of what we make goes to home gardeners most of it goes to big growers in viticulture, kiwifruit industries and pastoral farming now,’’ he said.
Wakame is a seasonal seaweed and spends eight months of the year as a single cell organism either floating in the water or attached to a surface such as a rope or wharf, making it extremely difficult to detect and remove.
However, once the water temperature cools to around 14 degrees Celsius the weed grows rapidly.
In October and November each organism can spawn millions of juvenile single cell gametophytes, making harvest timing crucial. Pressman said timing for this year’s harvest had been a lot more precise thanks to new visual-recognition technology developed by AUT.
‘‘The problem we had to solve was to find the exact time when the wakame was fully mature.
‘‘But prior to its reproduction to maximise the efficacy of our finished products and achieve the best ecological impact,’’ he said.