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Lagarosiphon discovered in Oreti
Apr 19, 2005

The recent discovery of lagarosiphon ((Lagarosiphon major, or South African oxygen weed) in the mid reaches of the Oreti River (west of Jospephville) has ES staff on high alert. Lagarosiphon was discovered by Fish and Game early in April and whilst it is known to be present in localised streams in Invercargill and in fish and duck ponds on private property, this is the first time the pest plant has been discovered in the Oreti.

“Anyone who’s seen it [lagarosiphon] in Lakes Dunstan or Wanaka, knows we simply do not want it in Southland” said ES biosecurity manager Richard Bowman. “Its major human impact is an impediment to recreation, but if left to establish, lagarosiphon has the potential to impact on biodiversity values by displacing other aquatic plant and animal life. It could also have a potential economic impact because its presence impedes water flow and will increase the costs associated with maintaining waterways.”

During his initial inspection, ES senior biosecurity officer Keith Crothers noted the presence of lagarosiphon in a number of side channels, lagoons and ponds within the Oreti River berm area and said it appeared to have been established there for some time. Council staff could only speculate at this time as to how it established in the Oreti.

“Lagarosiphon is an extremely invasive aquatic pest and given the right conditions, it is very aggressive, out-competing other plant growth and quickly colonising slow moving reaches of water” explained Mr Crothers. “In a braided system like the Oreti, with plenty of backwaters and ponded areas, it has found an ideal habitat.”

Until now, lagarosiphon had only been found in a number of small streams in the Invercargill area. Monitoring showed they posed little risk of spread by either natural or human assisted means. Lagarosiphon is believed to have been introduced originally to these streams by the owners of pet fish releasing them into the wild, including the oxygen weed in the fish tank. It was once possible to buy lagarosiphon as oxygen weed from pet shops, but since the release of ES’ Regional Pest Management Strategy, this trade is now banned.

“ES has made quite a major investment to keep lagarosiphon out” said Mr Bowman. “Our major efforts to date have been through our involvement with the multi-agency Lagarosiphon Public Awareness Programme. This focuses on raising people’s awareness of lagarosiphon in the southern South Island and how they can limit their role in its spread.”

Mr Bowman reported “a high level” of awareness amongst the region’s boaties about the inherent dangers associated with lagarosiphon. ES and DoC also undertake an annual survey to detect any possible incursions of the pest plant in ‘at risk’ sites like Lakes Te Anau, Manapouri, Monowai and Mavora. To date, all results have been clear.

Whilst lagarosiphon is recognised nationally as a serious aquatic pest, it is far more prolific in the South Island because of its preference for cooler water. Because if its highly invasive nature and tendency to dominate the habitats in which it establishes, lagarosiphon is deemed a ‘total control’ pest plant under the Regional Pest Management Strategy. Its establishment is human-assisted and only a very small fragment of the plant is required to effectively propagate. It is capable of producing extremely thick vegetative masses, from 6.5 m in depth right up to the surface. It is also very difficult to eradicate by either mechanical or chemical means, however, successful control has been carried out in some areas.

“ES staff are working with other stakeholder agencies, undertaking a survey to delimit the spread of the infestation in the Oreti. The results will provide a basis to assess the eradication and containment options” said Mr Bowman. “In the meantime, we’ll be erecting signage to alert people using the river to its presence and