Today’s symposium topic is the issue of water weeds in the Rotorua Lakes and their management. I will discuss how and where these weeds came from and how they got into New Zealand. The weeds discussed are only a small sample of potential weeds that could
have additional or even greater impacts on Rotorua and New Zealand as a whole. I will discuss our legislation and how management at the border protects us from future weeds yet to enter this country. Unfortunately, it appears that our legislation has been too
effective at keeping out new plants, with no new aquatic plants legally introduced since the mid 1990’s but over 25% of aquatic plants in the aquarium/pond plant trade appear to have got here illegally. Several prosecutions have resulted from interception of smuggled
plants.
Once inside our border, there are several biosecurity measures initiated that have kept a number of our worst weeds at bay. These include banning the propagation, sale and distribution of 30 aquatic weed species, national and regionally run eradication
programmes for high-risk weeds, with the successful eradication of five species and great progress towards the eradication of around twelve other weeds. However, some of these plants, such as water hyacinth, are still illegally kept by some individuals and several new
infestations have originated from such sources. Despite this, the programmes have successfully kept the volume of deliberate spread for a wide number of aquatic weeds to a trickle.
As previous speakers will have discussed, humans are the major cause of spread of aquatic weeds, with contaminated watercraft and trailers, fishing nets, diggers etc. all contributing to this spread. Bay of Plenty Regional Council have been proactively managing their lakes, identifying which lakes are threatened by which weeds, and then carrying out surveillance in order to detect and manage any new weed incursions. To assist with surveillance, they have designed and constructed weed cordons that effectively net off access points such as boat ramps (the main sites of new weed introduction) and have successfully detected and prevented establishment of new incursions of the weeds hornwort and egeria in Lake Rotoma. Each summer holidays an awareness programme run under the national ‘Check, Clean, Dry’ banner promotes inspection for weeds at boat ramps around the region. Additionally, surveillance of ornamental ponds near to high-value lakes has led to the detection of a number of
potential weeds.
