Boat electrofishing survey of the lower Waikanae River, Ratanui Lagoon, and Lake Waitawa

We conducted the first electrofishing boat survey of the lower Waikanae River, Ratanui Lagoon, and Lake Waitawa on 11-12 July 2006. We caught five introduced and five native fish species in 4.74 km of fished length from a total of nine sites. Assuming that the bow-mounted anodes caught fish within a 1-m radius, the width fished was 4 m, and the total area fished was 16,200 m² or 1.62 ha. We landed a total of 125 fish comprising five introduced and five native fish species at the nine fished sites (Table 3). Shortfin eels were the most numerous species but are mostly not included in this total. We did not bring eels on board because of the handling time this involved, except for two sites 263 and 264 where densities were 1.19 fish 100 m⁻², which is moderate density of eels compared to previous boat electrofishing one-pass estimates, perch, tench, and rudd were found at most sites in Lake Waitawa. Goldfish had a much more restricted distribution, and were caught at only one site. Four adult brown trout were caught in the lower Waikanae River. Because of their large size, adult brown trout and tench comprised a significant part of the fish biomass where they occurred. No koi carp were caught, probably because of a combination of low density and low water temperature. Water temperature at the time of fish (10.2-10.5 °C in Lake Waitawa) may have influenced the susceptibility to koi carp to electrofishing. Common carp are known to seek winter refuge in deep water at temperatures below 11°C. Electrofishing is limited to the upper 3 m of the water column and would be ineffective if koi carp, Cyprinus carpio) are also less active during periods of low water. Inactivity reduces the probability of encountering fish and lowers catch rates. The optimal period for sampling koi carp begins as the water warms in spring, when koi carp move into the littoral shallows to spawn and are therefore highly visible. Common carp in Australia begin to spawn when water temperatures reach 15°C and koi carp have been observed spawning in New Zealand in Lake Waikare as early as 11 September in a water temperature 15.3°C. However, it is encouraging that no small koi carp were caught because this is evidence that breeding is not occurring in Lake Waitawa or Ratanui Lagoon, where koi carp are known to occur (Ian Cooksley, DOC, pers. comm.). Previous fishing with the electrofishing boat in the North Island, in similar conductivities and habitats and with similar machine settings, has caught a full size range of eels, smelt, bullies, grey mullet, rudd, brown bullhead catfish, perch, tench, goldfish, and koi carp. Thus we believe that these results reflect an accurate picture of fish abundance in the Waikanae area but we acknowledge that fishing in spring or summer in higher water temperatures is likely to increase the likelihood of catching koi carp. This fish data have been entered into NIWA’s New Zealand Freshwater Fish Database as card numbers 15190 to 15198