Rotorua lakes weed cordons - a freshwater biosecurity tool.

Invasive plants represent a significant biosecurity threat to our pristine environments. To reduce this threat, weed cordons have been installed at boat ramps in lakes Rotomā and Ōkataina. This has been undertaken as these lakes house minimal invasive species but have a high risk of incursion from other lakes in close proximity such as Lake Rotoehu. A two-part trial was undertaken on the netted weed cordons that have been installed in lakes Rotomā and Ōkataina — two of Rotorua’s cleanest lakes. This trial was designed to test the effectiveness of the weed cordons via a simulated incursion from a boat and trailer or fishing equipment. This trial was also repeated in an un-netted area next to the weed cordon, called a ‘ghost’ cordon. This was designed to test what would happen if there was no weed cordon present. The trial was undertaken during all wind directions to gain an accurate understanding of what would happen to the released fragments in all conditions. The results of this report revealed that overall, the boat ramp weed cordons in the three locations retained 84.9% of all fragments released in all wind conditions. This was in contrast to not having a weed cordon present, which resulted in 85.2% of all fragments released being either not found or found to be present outside the ghost cordon range. Some of the results were surprising with regard to wind direction and the effect on fragment transportation. More studies may need to be undertaken to determine what lake currents do to weed fragment transportation and whether future weed cordons may need to be altered to combat this. Overall, it was concluded that weed cordons in these lakes appeared to be working effectively as a biosecurity tool but should be complemented with other initiatives such as portable wash-down facilities, public awareness, pest surveillance and lake weed spraying. Recommendations for future research include continued trial work to test which areas of the weed cordons are the most vulnerable. Monitoring of lake currents around the weed cordons could also be undertaken in future trials to determine the extent (and strength) of lake currents and their potential to affect fragment transportation.