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Lake users putting waterways at risk

Boaties who aren’t cleaning their vessels and trailers between waterways are risking the spread of aquatic pest plants in Rotorua’s cleanest lake.

Bay of Plenty Regional Council staff found fragments of hornwort, one of New Zealand’s worst aquatic pest plants, in the shallows at Rotomā and found pest weeds attached to boat trailers at the lake.

“This is very disappointing,” Regional Council Land Management Manager, Rotorua, Greg Corbett says. “It means people aren’t checking and cleaning their boats, trailers and fishing equipment between waterways. This must be done immediately following exiting a lake or river and before heading for a new waterway, not just occasionally or once they’ve got their boat and trailer or fishing equipment home.”

A Regional Council staff member spoke to a boatie after seeing him backing a trailer into Rotomā to retrieve his boat with what appeared to be weeds attached to the trailer. It was identified as hornwort and the man said he had been in Lake Rotoehu, where hornwort is well established, the previous day. He had not cleaned his boat after exiting that lake.

Another trailer in the carpark at Rotomā was found to have another serious aquatic weed, Egeria densa, attached to it.

“Numerous fragments of hornwort were found in the shallows within the weed cordon in place at Rotomā, in a beach area where many boats launch,” Mr Corbett says.

“Rotomā is currently clear of hornwort and Egeria and we need to keep it that way. The weed cordon can only prevent so much – pest plants can very quickly become established from fragments detaching from boats, trailers or other equipment used in waterways.”

Weed cordons, constructed from netting and marked with buoys and markers, are in place in several Rotorua lakes and are designed to catch plant fragments that may detach from boats and trailers entering the water, minimising the risk of aquatic pest plant invasions.

Boaties can be prosecuted for transporting aquatic pest plants from one waterway to another.

“They risk undoing all the hard work being put into keeping these weeds out or, in the case of lakes where they have become established, contained. They’re a real problem in some lakes and the last thing we need is to have them spread to others through people’s carelessness.”

Aquatic pest plants, many of which can easily grow from just small fragments, develop into thick blankets, altering habitats for native species, smothering native aquatic plants, clogging intakes on jet boats, fouling propellers and preventing recreational activities, such as fishing and waterskiing.

Weed fragments are often unwittingly transported between waterways on boats and other equipment and can also harbour the eggs of unwanted pest fish species.

There are four main invasive weed species established in Rotorua’s lakes and contributing to degradation of water quality and loss of native biodiversity – hornwort (Ceratophyllum demersum), lagarosiphon (Lagarosiphon major), egeria (Egeria densa) and Canadian pondweed (Elodea canadensis). It is an offence under the Bay of Plenty Regional Pest Management Plan and the Biosecurity Act to move any machinery or vessel contaminated with these pests.

Keep pest weeds at bay

Always check, clean and dry any gear that comes into contact with the water between every waterway, every time. That includes boats, kayaks, rafts, canoes, paddle boards, paddles, boat trailers, vehicles and fishing gear, fishing attire such as waders, life jackets, wetsuits and footwear – even pets and children’s toys.

Give gear a thorough check and clean inside and out with decontamination solution (5 percent any common dishwashing detergent/nappy cleaner or 2 percent household bleach). On boats pay special attention to the outboard motor, anchor rope and anchor recess, bilge pump, mats, carpet.

About hornwort

• Scientific name is Ceratophyllum demersum.

• Considered one of New Zealand’s worst aquatic pest plants.

• A submerged freshwater weed with narrow, finely divided bright green leaves.

• Modified leaves, rather than roots, anchor the plant in bottom sediment up to 16m deep.

• Dense beds of hornwort can reach up to 10 metres in height.

• Present in a number of local lakes including Rotorua, Rotoiti, Rotoehu, Ōkāreka, Tarawera, Rotomahana and Ōkataina.

• Bay of Plenty Regional Council is currently trying to eradicate hornwort from Lake Ōkataina and investigating the feasibility of eradicating it from Lake Ōkāreka.

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