A decision to open up the North Island for commercial farming of seaweed has drawn concerns that harvesting could affect other fishery industries.

This month, most of the North Island was opened for the gathering of beach-cast seaweed - the loose weed that is blown ashore in storms. Collection had previously been limited to selected areas.

But concerns have been raised that a quota system allowing harvesting below the waterline could harm the marine environment.

Seaweed Association chairwoman Jill Bradley supported the Ministry of Fisheries' move to allow more beach collection.

"Beach-cast seaweed is a natural occurrence, and [it] is a hand-harvested industry. The environmental issue is secondary, as all the seaweed is not removed - about 10 per cent is always left."

She said it could provide a new source of income for remote coastal communities.

The association was more concerned about the ministry's proposal to introduce a quota system for seven seaweed species, which could allow the cutting of the weed below New Zealand's waterline.

"The Quota Management System [QMS] is a world-leading framework, but it may not be the best option here because seaweed is the only fishery that has the potential to negatively affect all other high-value inshore fisheries." Ms Bradley wanted the ministry to wait until 2011 to make a decision, awaiting the results of a five-year scientific study.

Her family-owned company, AgriSea, is funding the study, which monitors the effects of harvesting on target species. Interim results from the project have been promising, showing that seaweed regenerated rapidly.

However, she said any harvesting of wild seaweed would have to be stringently managed to prevent environmental damage.

Forest and Bird New Zealand marine advocate Kirstie Knowles said commercial harvesting had the potential to upset fragile coastal habitats.

"When you start fishing down the food chain and putting your efforts into species that are at the foundation of food webs, it is a bit of a downward spiral."

Ms Knowles said beach-cast seaweed might look dead but it formed a habitat in itself, supporting myriad forms of life. It was a source of food, shelter and nesting material for marine and coastal species, including dotterels, pipits, gulls and other birds.

Two large areas of the east coast of the South Island will be opened up to commercial harvesting of kelp in October next year.

New Zealand Kelp, based in Christchurch, supported the introduction of a quota system for seaweed species. It sells edible seaweed which it gathers from Akaroa Harbour, Tory Channel and the Chatham Islands. It applied for a permit to harvest wild seaweed before the moratorium came into force.

Owner Roger Beattie said a $380,000 Canterbury University research project, which he half-funded, showed the harvesting of kelp was "as sustainable as mowing your lawn".

"QMS is a robust and scientifically verifiable method of assessing how much can be harvested per year. It gives people certainty over the long term, so they can put money into marketing, branding and research."

Mr Beattie said he was already in promising negotiations with Japanese investors and estimated that the edible seaweed market was worth tens of millions of dollars a year.

* Export potential

New Zealand has vast forests of seaweed which have been protected under moratorium for the last 18 years.

Seaweed grows quickly and is a resilient plant capable of surviving and regenerating in rough surf. New Zealand's seaweed has good export potential because it comes from relatively pollution-free waters.

Mostly used as a fertiliser ingredient, it is also a valuable medicinal, pharmaceutical and food product.