In Waikato waterways male koi carp typically mature in two years and females in three years, with a lifespan in excess of 8 and 12 years respectively (Tempero et al., 2006).
Spawning typically occurs in water temperatures of 18-28o
C and in northern New Zealand has been found to occur from September to April given appropriate conditions (Tempero et al.
, 2006). In the Waikato, warm water temperatures mean repeated spawning is possible within one breeding season (Tempero et al.
, 2006). This ability to breed rapidly may have contributed to the increase in numbers of feral koi carp in Waikato waterways degrading freshwater ecosystems in the region. Hicks and Brijs (2009)
found that "[t]here was a wide size range of koi carp ranging from 60 to 470 mm suggesting that successful recruitment is occurring in the lake [Lake Ngaroto]" (p. 15)
. Although physical removal of invasive fish species is generally considered ineffective at achieving complete eradication, it can be used to hold a population to a level at which the ecological impacts are minimal (Ling, 2009). However, Tempero et al.
's (2006) research indicates that attempts to control the feral population of koi carp in the Waikato using selective netting are unlikely to be successful due to compensatory growth in the remaining population. This is because the females that are not culled are likely to produce a greater number of eggs and may spawn more than once in the season. Selective netting is therefore an ineffective management strategy for koi carp, as the population is likely to recover quickly
Hicks, B.J., & Brijs, J. (2009). Boat electrofishing survey of Lake Ngaroto (CBER contract report 111). Retrieved from http://cber.bio.waikato.ac.nz/PDFs/CBER_111.pdf
Ling, N. (2009). Management of invasive fish. In M.N. Clout & P.A. Williams (Eds.), Invasive species management: A handbook of principles and techniques (pp. 185-204). Oxford, United Kingdom: Oxford University Press.
Tempero, G. W., Ling, N., Hicks, B. J., & Osborne, M. W. (2006). Age composition, growth, and reproduction of koi carp (Cyprinus carpio) in the lower Waikato region, New Zealand. New Zealand Journal of Marine and Freshwater Research, 40, 571-583.