The majority of current aquatic weeds were introduced to New Zealand via the aquarium and ornamental pond trade and these have been subsequently spread via deliberate spread and other human-mediated activities.
Management at the border
Many potential problem species not yet known to be present in New Zealand are still traded internationally. Two pieces of legislation, the Hazardous Substances and New Organisms Act (1996) and the Biosecurity Act (1993) have permitted effective management at the border that protects us from the impacts of future weeds yet to enter this country. It appears that our legislation has been effective at keeping out new plants, with no new aquatic plants legally introduced since the mid 1990's. However, surveys of the aquarium/pond plant trade and interceptions at the international border indicate illegal importation of aquatic plants with the potential for new weeds and hitchhiker pests to enter New Zealand without screening. Several prosecutions have resulted from interception of smuggled plants.
Once inside of our border, there are several biosecurity measures that have been initiated that have kept a number of our worst weeds at bay (Figure 1). These include banning the propagation, sale and distribution of 30 aquatic weed species, and national and regional 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 the waterweeds, such as water hyacinth (Eichhornia crassipes), are still illegally kept and several new infestations have originated from such sources. Despite this, the programmes have substantially reduced the propagule pressure, and therefore invasion risk, of these aquatic weeds.
Proactive lake management
Humans are the major cause of spread of aquatic weeds, with contaminated watercraft and trailers, fishing nets, diggers (for example) all contributing to this spread. The Bay of Plenty Regional Council has been proactively managing their lakes, identifying which lakes are threatened by which weeds, and carrying out surveillance in order to detect and manage 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 (Ceratophyllum demersum) and Egeria densa in Lake Rotoma. Each summer holiday 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 high-value lakes has led to the detection of a number of potential weeds.
New Zealand is rightly heralded as a world leader in the field of biosecurity. There are still many iconic lakes close to their pristine state, but these are under threat of invasion by many weed species. A number of proactive management activities are undertaken both nationally and regionally to prevent further weed incursions but it only takes one action or inaction to undo all of this.
More information
Presentations from the Lake Water Quality Society Symposium will be made available on the Lake Water Quality Society website
