Lake Rotomanu to be drained

Lake Rotomanu will be emptied this week to allow scientists to get a full picture of the extent of the freshwater gold clam infestation.

The invasive clam Corbicula fluminea was found in the New Plymouth lake on 8 November, the first discovery in New Zealand outside the Waikato River. The lake was closed to motorised watercraft days later.

The Lake Rotomanu outlet will be opened tomorrow [11 Dec]. It will take about four days for the lake to drain. During this time the fish population will be harvested in partnership with local hap, who will utilise as many fish as possible.

Taranaki Regional Council (TRC) leads the newly established Regional Corbicula Coordination Group (RCCG) alongside New Plymouth District Council (NPDC), Ngti Te Whiti hap, Biosecurity New Zealand, Fish & Game, Earth Sciences NZ and others.

TRC Environment Services Manager Steve Ellis says before draining the lake the RCCG had to ensure the clams would not spread to the Waiwhakaiho River.

Over the last few weeks weve carried out testing and obtained advice on the salinity and flow of the river. We are confident any clams or larvae will be quickly washed the short distance out to sea.

Given that assurance the RCCG has asked lake owners NPDC to now empty the lake so we can get a good look at exactly what we are dealing with.

A team from Earth Sciences NZ will next week carry out a detailed survey of the whole lakebed to determine where and how widespread the clams are, how deep they have burrowed and estimate the age of the clams present.

That information will help inform the next steps in the response. In particular, whether an elimination attempt is feasible and if so, what the treatment options are and how much they would cost.

Mr Ellis says the financial cost of elimination is likely to be significant and at this point it is unclear how it would be funded and by whom.

To be clear, there is no guarantee we will attempt to treat or eliminate the clams, even if it is technically feasible.

We all want the best for the region and were all aware of the massive economic damage these clams can do, so were having those conversations as a matter of urgency. In the meantime, were looking forward to seeing what the lakebed survey uncovers.

The lake will need to stay empty for the summer, which Mr Ellis says was not a decision made lightly.

We know its a popular summer spot and that locals and visitors will be disappointed, but we hope they understand the reasons for it.

The invasive clam Corbicula is a massive threat to our infrastructure, economy, ecosytems and recreational use of all the regions waterways, so we need to act now to give ourselves the best chance of preventing long-term damage or recreational restrictions.

Testing at Lakes Rotokare, Ratapiko, Rotorangi and the Waiwhakaiho River have not found any clams, while eDNA testing of the water has also been clear. While encouraging, that does not mean they are not present.

Boaties, jet skiiers, kayakers and other lake users are asked to be extra vigilant this summer, so as not to unknowingly spread the clam or other freshwater pests. Juvenile clams produce an invisible sticky thread of mucus which attaches to surfaces, like boats and recreational gear.

Always follow the Check, Clean, Dry procedure when moving between waterways and if possible, wake boats should be kept to just one lake.

A washdown trailer is on its way to Taranaki, courtesy of Biosecurity New Zealand, and will be set up at popular lakes over the summer. Two dedicated Check, Clean, Dry ambassadors will also be on site to spread the message, supported by new signage, direct education with clubs and organisations and a social media campaign.