Boat electrofishing survey of Horseshoe Lake, Hamilton

Horseshoe Lake is a supertrophic peat lake that sits with the boundaries of the Waiwhakareke Natural Heritage Park in the north-west of Hamilton City. Formerly, this area consisted of 60 ha of grazed pasture land. Beginning in September 2004 it was extensively replanted with locally sourced vegetation as part of a 4 year urban restoration project funded by the Foundation for Science, Research and Technology (further information can be found here). As part of the restoration efforts, a pest fish survey of Horseshoe Lake was undertaken by University of Waikato researchers in May 2005. Unlike previous surveys of Horseshoe Lake which were conducted using nets, this survey was conducted using the University of Waikato's electrofishing boat.

Figure 1: Horseshoe Lake, Hamilton May 2005 (Photo by B. Hicks).

Horseshoe Lake (NZMS 260 sheet S13 2706288E 6378965S) is approximately 3 ha in size, peat stained, with a Secchi depth of 0.5-1 m. The average depth is 2.5 m with a maximum depth of 3.2 m. There are a number of small ephemeral Drains feed the lake and it Drains from a 1 m wide Drain that runs north to Lake Rotokauri. At the time of sampling the riparian fringe of the lake was dominated by grey willow (Salix cinerea) and weeping willow (Salix babylonica); with small areas of wetland plants such as willow weed (Persicaria decipens) and water purslane (Ludwigia palustris). Subsequently the willows have been substantially eradicated and are in the process of being replaced with vegetation characteristic of the lake margin pre-human.

Horseshoe Lake was boat electric-fished for 3 hours on the 13 May 2005; covering an area of 5072 m2 or 17% of the lake area, focusing primarily on the perimeter of the lake and one pass through the centre of the lake. Total fish density for the perimeter was 1.5 m2 and total fish biomass was 155 g m2. Species captured included the native shortfin eel (Anguilla australis) (n = 77) and common bully (Gobiomorphus cotidianus) (n = 1) and the introduced brown bullhead catfish (Ameiurus nebulosus) (n = 28), rudd (Scardinius erythrophthalmus) (n = 2) and Gambusia affinis (abundant). Koi carp (Cyprinus carpio) and goldfish (Carassius auratus) were not caught, indicating their absence from the lake.

Figure 2: Typical brown bullhead catfish removed from Horseshoe Lake on the 13th May 2005 by boat electricfishing (Photo: B. Hicks).

Catfish were large in size, ranging from 310 to 415 mm TL (mean 378 ± 22.8 mm SD) with a mean weight of 782 g (± 148 g SD). This gave a population estimate for the lake of 81 catfish, with a biomass of 21 kg/ha. Therefore, after removal approximately 53 catfish remained in the lake with a biomass of 15 kg/ha. The most abundant large species was shortfin eel. Mean length was 380 mm TL (± 131 mm SD) and mean weight was 148 g (± 156 g SD). This resulted in a biomass of 136 g m2 in the littoral zone and an estimated population of approximately 6000 eels (300 kg/ha). There was also an abundant population of Gambusia, but density and biomass estimates were not recorded for this species.

The comparatively small number of invasive species and the absence of highly invasive and environmentally damaging species such as koi carp, goldfish and perch (Perca fluviatilis) means that Horseshoe Lake is a prime candidate for further restoration efforts. In addition, the robust population of shortfin eels should help with restoration efforts by preying on juvenile invasive species, provided they are protected from over-harvesting.