Hicks, B.J., Brijs, J., Heaphy, J. & Bell, D.G. (2008). The use of boat electrofishing for koi carp (Cyprinus carpio) control in the Kauri Point catchment. CBER Contract Report No. 69, client report prepared for Department of Conservation. Hamilton, New Zealand: Centre for Biodiversity and Ecology Research, Department of Biological Sciences, School of Science and Engineering, The University of Waikato.
Permanent Research Commons link: https://hdl.handle.net/10289/3793
The koi carp is an ornamental strain of the common carp (Cyprinus carpio) which is believed to be one of the most ecologically detrimental of all freshwater invasive fish species. They are widespread in the Auckland and Waikato region and appear to be spreading both north and south of these areas. The presence of koi carp in 3 ornamental ponds at Kauri Point, Katikati which is located in the western Bay of Plenty region was confirmed in late 2006. Because koi carp is designated an unwanted organism under the Biosecurity Act it was decided by the Department of Conservation (DOC) that an attempt to eradicate them from this locality would occur. One possible option for eradication of koi carp in this catchment was the use of and electric fishing boat from the University of Waikato. This boat operates by putting a pulsed DC current into the water column where it attracts and then incapacitates fish, allowing operators to remove them from the water with hand nets. The Centre for Biodiversity and Ecology Research (CBER) at the University of Waikato was contracted to attempt to eradicate koi carp from the three ponds at Kauri Point by boat-electrofishing. The objectives were (1) to survey the fish abundance, (2) to remove as mand koi as possible in an attempt for eradication and (3) to estimate the proportion of koi carp removed from the system by boat-electrofishing. On 21 and 22 April 2008, a total of 327 fish comprising of 307 koi carp (137.5 kg of biomass), 1 goldfish(Carassius auratus) and 19 koi-goldfish hybrids were captured and removed from the 3 ponds located within the Kauri Point catchment by a combination of electric fishing (307 fish) and gill netting (20 fish). The majority of these fish (299 koi, 1 goldfish, 19 koi-goldfish hybrids) were removed from the largest pond (pond A). Boat-electrofishing caught a wide size range of koi (70 mm to 510 mm) and at least four distinct size classes of koi were apparent. The high proportion of juvenile (<200 mm) caught along with reasonable numbers of mature males and females in pond A strongly suggests that breeding is occurring within the Kauri Point aquatic ecosystem. Results from analysing scales of a small sub sample of koi (n=34) also shows that there was a wide range of ages of pond A (ages 1 to 8 years old). Pond A had a relatively high density of 4.6 koi carp 100 m⁻² compared to pond B and C which had low densities of 0.5 and 0.2 koi carp 100 m⁻² respectively. No juvenile carp were observed to be present in ponds B and C. Population and total biomass estimates for koi carp in pond A prior to removal of fish were calculated to be 358±66 koi carp and 145.14 ±44.27 kg (mean ±95% C. I) respectively. 299 koi carp or 122.30 kg of biomass (71-84% of the estimated population) were removed from pond A over two days of electric fishing, leaving a possible 125 koi carp or 67.11 kg of biomass remaining in the pond. Boat-electrofishing proved to be a successful too for removing a large proportion of the estimated biomass of koi carp in the Kauri Point ponds. Eradication of koi carp by boat-electrofishing from this system was not possible due to poor water visibility (difficult to spot narcotised carp), limited time allocated and successful koi carp breeding occurring in the ponds. Viable options of koi carp eradication in the Kauri Point catchment would involve the partial draining and poisoning of the three ponds and the associated tributaries where koi carp are found.