Lake Weed and Wallabies

The ultimate aim of the Symposium Lake Weed and Wallabies was water quality in the Rotorua Lakes. Like the eight earlier LWQS symposia it focused on specific current issues. This is a summary of key points made during the symposium, focusing on action required to maintain and improve water quality. It is not part of the official record of the Symposium. Reference should be made to the Proceedings for clarification and quotations from any participant.

The current improvements in lake water quality, some of which may not be permanent, should not delay further catchment actions. Climate change is a real challenge. For Lake Rotorua there is possibly a ‘wall of phosphorus’ still to reach the lake from the enriched agricultural landscape.

For weed control in the US:

  • biological control is a preferred option where an effective control agent exists
  • chemical control is the most widely used and is effective and relatively inexpensive but regulatory issues are increasing
  • mechanical techniques are useful for immediate control but slow, expensive and may affect fish

The best time to control a plant is before we have it: i.e. by preventing incursion.

The best time to eradicate a plant is when a species first appears in a lake.

Early detection of weeds (including the use of ‘citizen scientists’), and a rapid response plan, are important. Weed harvesting is useful to meet very specific and limited nutrient reduction goals, but not to change the trophic state of a lake.

Exotic plants have only been in the Rotorua Lakes since the 1950s and some, like lake water hyacinth, have already been eradicated. We have the opportunity to treat our lakes in a similar way as islands have been treated for restoration of native wildlife.

Recognition is needed of the major threat of pest fish and the harm they would do.

Work on the Rotorua Lakes needs to be in line with national initiatives including the NPS.

The collaboration of public authorities and the community over restoration of the Rotorua Lakes, and its science base, are an example for the rest of New Zealand.

Some weeds are worse than others. Human access moves weeds around much more than waterfowl. Weed growth is not a result of lake water quality declining – weeds grow even in very pure lakes.

Because boats and trailers are such a significant factor in spreading weed, all boat ramps need weed control measures.

As far as weed control methods are concerned:

  • weed harvesting is useful for control for amenity purposes, but can only be supplementary for nutrient reduction
  • biological control of some weeds is possible and needs a push to get funding
  • Endothall is very effective for hornwort and lagarosiphon. (The South Island is now clear of hornwort.)

A resource consent for the use of Endothall in the Rotorua Lakes needs to be expedited urgently.

Since 1941 the proportion of active phosphorus in Lake Okataina sediments has jumped. It had been stable for five centuries and then had fallen after the Tarawera eruption. About 1935 the population of browsing animals in the catchment increased. The understory of the forest has been significantly damaged and regeneration after logging and landslides affected.

Most phosphorus is carried to waterways by very fine particles eroding from the surface of the land – even under pasture. Much of Lake Okataina catchment has topsoil derived from the very fine Rotomahana mud tephra, which is highly erodible. In recent years with lake level fluctuations the lake edge has eroded substantially. Heavy storm events have caused many landslides. Across New Zealand, natural influences, including storm intensity and soil features, influence erosion far more than do animal pests.

The question considered at the Symposium was which of these factors – browsing animals or storm events and lake level fluctuations – cause the erosion which led to an increase of active phosphorus in the lake. Research is urgently needed to determine the cause and to protect the lake.

Limiting pests to allow regeneration requires a sustained effort repeated every 3 to 5 years. This should be supplemented by intensive community led pest control in local areas of high quality forest. But control of Dama wallabies is essential for another reason – to prevent their spread into the eastern ranges or the Coromandel.

Around the world ‘citizen scientists’ are on the watch for new or unusual species. They can discover new weeds or pests more quickly than official surveys. Smart phones and apps enable identification of suspicious species to be made from a photograph on the spot – which is located by GPS – and reported.

LINZ does weed control in the Rotorua Lakes in response to public demand to keep facilities or beaches clear. Wanaka is regarded as the most successful lake they work on because there is a plan drawn up in conjunction with the community, and then implemented. For the Rotorua Lakes, more effective weed control through spraying requires plans and a business case for more funding.

RMA consents for weed and pest control involve a complex multistep process. It is a huge effort to obtain consents for particular herbicides for particular sites over and over again. Thus multipurpose comprehensive consents across all the lakes should be sought.