A boat electrofishing survey was conducted on five ponds in the Waitakere District on 18 and 19 July 2007; Lake Panorama (0.7 ha), Paremuka Pond 1 (1.2 ha), Paremuka Pond 2 (1 ha), Danica Esplanade (0.5 ha) and Longbush Pond (0.4 ha). A total of 337 fish were caught from the 2.89 km total fished distance of the 5 ponds. The total area fished was 11,537 m2 or 1.154 ha. The water temperature for the 5 different ponds ranged between 10.8°C and 14.9°C. Water fished at Panorama Lake and Paremuka Ponds 1 and 2 had a maximum depth of approximately 2.4 m, while Danica Esplanade and Longbush pond were shallower at approximately 0.1-1.5 m deep. The littoral zones of the ponds were well vegetated as the Waitakere District council has done extensive planting of native plants such as manuka (Leptospermum scoparium), flax (Phormium tenax), cabbage tree (Cordyline australis) and a diversity of grasses and rushes. Throughout the five ponds a range of submerged introduced macrophytes were observed such as curly-leaved pondweed (Potamogeton crispus), hornwort (Ceratophyllum demersum), parrot's feather (Myriophyllum aquaticum) and water lily (Nymphaeaceae).
Figure 1. The electrofishing boat in action on Longbush Pond, Te Atatu, Waitakere City. (Photo by Warrick Powrie)
In Lake Panorama (NZMG 2653320E 6478520N), shortfinned eels (n = 30) were the most numerous species caught (130 fish/ha), followed by perch (n = 23) (100 fish/ha) and tench (n = 10) (40 fish/ha). The perch had a mean weight of 51 g and a range of 24 to 131 g. The tench had a mean weight of 386 g and a range of 4 to 855 g.
In Paremuka Pond 1 (NZMG 2654118E 6480061N) koi carp (n = 38) were the most numerous species caught (120 fish/ha), followed by shortfinned eels (n = 18) (50 fish/ha). The koi carp ranged in size from 35 g to 3 kg with a mean of 920 g.
In Paremuka Pond 2 (NZMG 2653970E 6480086N), koi carp (n = 30) were again the most numerous species caught (340 fish/ha), followed by tench (n = 24) (250 fish/ha) and shortfinned eels (n = 7) (70 fish/ha). Mean koi carp size was slightly smaller than Paremuka Pond 1 at 825 g with a range of 105 g to 2.6 kg. The tench were also smaller ranging in size from 3 to 505 g with a mean of 182 g.
In Danica Esplanade (NZMG 2658008E 6482504N) and Longbush Pond (NZMG 2658282E 6483000N), shortfinned eels were the most numerous species caught (140 and 550 fish/ha respectively), followed by mosquitofish (Gambusia affinis). There was more macrophyte cover around the edges of Danica Esplanade compared to Longbush Pond and this decreased the catch rate as a large number of eels in Danica Esplanade were sighted but were unable to be captured. No perch, tench or koi carp were captured in these ponds.
Figure 2: Shortfinned eels (90-420 mm total body length) caught at Longbush Pond, Te Atatu, Waitakere City. Photo: Jeroen Brijs.
Koi carp were only caught in the Paremuka ponds. The majority of koi carp were caught on the edges of the lake in macrophytes and rushes. Koi carp biomasses were highest in Paremuka Pond 2 at 261 kg /ha compared to 106 kg/ha in Paremuka Pond 1. Biomass is a more accurate reflection of the potential ecological impact of koi carp than their density. Previous results suggest that 21-73% of the total population is caught on the first removal, depending on water visibility. As we fished the area at each site only once, the estimates in this survey represent a minimum abundance, and true population sizes are likely to be 1.4-4.8 times greater. The density of eels in both the Paremuka ponds is also likely to be higher as a large proportion of eels were able to escape into the macrophytes before they could be captured in the nets. Mosquitofish were also observed to be living in both the Paremuka ponds. Of ecological concern for the Paremuka ponds is the dominance of the fish biomass by introduced koi carp, which have a deleterious impact on aquatic habitats. Another concern for these ponds is the presence of small koi carp (<200 mm), which suggests that natural spawning is most likely occurring, although recent releases of carp into the ponds is another possibility.
Table 1: Number of fish captured by the electrofishing boat in 5 ponds in Waitakere City on 18 and 19 July 2007.