Options

Hydrilla verticillata (hydrilla) is an invasive aquatic macrophyte, which has earned worldwide recognition as one of the worst weeds among submerged plant species. Hydrilla was introduced to New Zealand and first positively identified and recorded in Hawke's Bay lakes in the 1960s. Currently its distribution is limited to four lakes in the Hawke's Bay region.

Aside from the legislation, and the geographical isolation of the infested lakes, the confinement of hydrilla to the Hawke's Bay region has no doubt also been facilitated by the prohibition of motorised boats on Lakes Tutira and Waikapiro, the most publicly accessible of the hydrilla-infested lakes. Further containment measures have included the use of signage beside lakes with hydrilla to ensure public awareness of the plant. Weed mat has also been used in selected areas of public access sites to minimise the likelihood of hydrilla transfer (for example, Lake Opouahi).

Hydrodictyon reticulatum (water net) is a green filamentous alga. It is distinctive is appearance, for its individual cells are joined to form a six-sided mesh which makes up colonies. Water net was first reported in New Zealand in 1986 near Tauranga. By February 1989 water net had spread from the initial infestation site to Lake Rotorua and Lake Rotoiti, with its range continuing to expand through the Bay of Plenty and the Waikato region over the next few years.

from: Wells et al. (1999). Journal of Aquatic Plant Management 31: 49–54.

Lake Parkinson is a small (1.9 ha) dune lake near Waiuku, South Auckland, which during the 1970s suffered from declining water quality, the introduction of rudd and the development of weed problems that lead to a down-graded trout fishery. Egeria densa had become the dominant submerged vegetation, forming dense beds over 55% of the lake area deeper than 1m, which often reached the water surface.

Incremental stocking of 60 grass carp in the lake in 1976 and 1977 led to the apparent eradication of egeria, and a >99% reduction in the cover and biomass of other macrophytes in the lake after the summer of 1977/1978. Two years after grass carp stocking the only species recorded in the lake were occasional growth of Glossostigma elatinoides and Myriophyllum propinquum. Nine years after egeria was last recorded in the lake and five years after removal of grass carp from the lake, the submerged vegetation was dominated by tall growths of the native species Potamogeton ochreatus and Nitella hookeri.

Although eradication of adventive submerged macrophytes from lakes is rarely feasible by physical or chemical means once invasion is well established, Lake Parkinson has demonstrated that temporary heavy stocking of grass carp can achieve eradication of species dependent on vegetative reproduction from stem fragments.

from: Tanner et al. (1990). NZ Journal of Marine and Freshwater Research 24: 181–186.

In an effort to determine whether hydrilla could be eradicated, Lake Eland, which is situated on private farm land, became the site of a grass carp (Ctenopharyngodon idella) trial in 1988.

In 1988 there was 1ha of hydrilla covering the 1.5m–4.5m water depth zone. At this time 400 triploid grass carp were released. By April 1990, 99% of hydrilla biomass was gone, with only remnant stands at the south end of the lake. Two and a half years after the original release of the grass carp (April 1991) there was no trace of hydrilla weed beds in the lake. But in November of the same year, there was occasional spring growth from turions, tubers and stem fragments. More importantly, a newly formed tuber was discovered on a small plant in Lake Eland in April 1996, which indicates that eradication of hydrilla from Lake Eland and therefore  from New Zealand is a long term endeavour. In subsequent years (1997–2001) remnant hydrilla plants have still been located and viable tubers found in sieved lake sediment during annual lake surveys.

Although the idea of using grass carp for the control of hydrilla is not new, some of the results obtained from the field trial to date are, and have important implications for their continued use. Specifically, the continued presence of remnant hydrilla plants and tubers in Lake Eland and the production of new tubers by some of these small plants demonstrate the difficulties of eradicating hydrilla from a lake using grass carp alone. Nevertheless, the high level of control achieved has significant implications for future applications in minimising the risk of spread of the geographically isolated hydrilla throughout New Zealand. However, the potential to use grass carp in other hydrilla-infested water bodies to obtain such a high level of hydrilla control could be limited – by unfavourable public perception by and the problems associated with this approach, such as lack of selective plant control.