Molecular analysis has confirmed that an unusual seaweed that wrapped itself around an anchor in the Bay of Islands late last year was not the potentially invasive species known as mermaid's hair.

That was good news for Northland Regional Council biosecurity experts, but biosecurity manager — marine and strategy Sophia Clark said while the weed turned out to be the non-invasive Chaetomorpha linum, authorities were thrilled that it had been reported.

"It's still an unusual-looking seaweed, and we're really pleased the incident reporter took the time to let both us and the Ministry for Primary Industries know," she said.

The council had noticed an increase in members of the public reporting things they believed were unusual or could be a known marine pest over recent months, the seaweed coming up with an anchor off Roberton Island.

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Ms Clark said making a formal, positive identification of some marine pest species could be a reasonably lengthy, time-consuming and difficult process. In this case the seaweed's DNA had to be compared to other samples, one of which had to be sent from Australia. The effort was always worthwhile though.

"Marine pests have the potential to cause a great deal of environmental, economic and other harm across multiple fronts, so any reports that draw them to our attention are extremely helpful, even if it ends up being a false alarm," she said.

Meanwhile, as part of the NRC's efforts to monitor and prevent the spread of pests into new areas, council divers had inspected more than 1250 boat hulls in Northland over summer. As of last week those inspections had not revealed any new established marine pest populations outside the areas where they were already known, but with given Northland's more than 3200km of coastline and "vast" marine space it was impossible for the council to be everywhere at once, "and it's great to have eyes and ears out on the water via the public".

While mermaid's hair wasn't the worst marine invader that potentially threatened Northland, its presence would still be an issue, as it was a biofouling organism, she added. It could attach itself to boats and marine structures and build up relatively quickly in both mass and area, effectively pushing out native species like the green-lipped mussel.

Unwanted seaweeds were just some of many potential marine invaders, however. Information on them, and land-based pests, could be found at www.nrc.govt.nz/pestcontrolhub.

"It's really important to make sure you keep your hull and other boating equipment clean, and if you do pull up biomass on your anchor, make sure you don't transport it to your next destination," she said. "Remember, if it looks unusual, keep the sample on board and call the regional council on (0800) 002 004 or the Ministry for Primary Industries' 24/7 exotic pests and disease hotline, (0800) 809 966."