News and progress reports

Fish Population Survey of Horseshoe Lake (report available on PDF here 260kb)
We fished Horseshoe/Waiwhakareke Lake with the University of Waikato's electrofishing boat on 13 May 2005 as part of the research by the CBER team lead by Assoc. Prof. Bruce Clarkson to determine the best way to restore natural ecosystems in city areas.

The fish species that we caught were all found previously in the 2001 NIWA survey, but we provided additional information on species abundance per unit area. In 3 hours we fished the following three sites within the lake:

1. the entire perimeter of the lake;
2. a removal reach - short section of the edge habitat fished to estimate eel abundance;
3. the lake centre:



The GPS trails of the fishing are shown in the figure below:

The catfish were very large, with many about 400 mm long and 1kg in weight (Figure 1). We replaced all the eels, but removed the catfish and rudd. Perhaps surprisingly, there were no common smelt, which are common in other Waikato lakes.


Figure 1. Brown bullhead catfish from Horseshoe Lake.

Our catch suggests that shortfin eels are very abundant in Horseshoe Lake, and the size distribution shows that recruitment is occurring. There was a low density of large catfish with apparently poor recruitment, and rudd may also be sparse. The capture of catfish by the electrofishing boat was clearly highly effective because we caught 29 catfish in contrast to the 1 catfish caught in 2001 by NIWA. The good news is that we caught no koi carp or goldfish.

We can further examine the fish catches to make population estimates for Horseshoe Lake from the boat electrofishing. For shortfin eels, 4 removals in 53 m (212 m2) of representative shoreline gave successive removal catches of 17, 25, 19, and 16 fish (sum 77). This yields a population estimate of 326 eels, i.e., 615 eels/ 100 lineal m of shoreline, or 154 eels/100 m2. The estimate total biomass was 28.9 kg, or 546 g/ lineal m (137 g/m2). The total for the entire perimeter (987 m) is 6,071 eels, or 539 kg of biomass. This does not consider eels that might be in the deeper areas that were inaccessible to boat electrofishing.

For catfish, 2 successive removals covering the entire perimeter (987 m or 3,948 m2) yielded 16 and then 13 individuals, or 85 adult catfish estimated to be in the entire perimeter. Much of the shoreline was fringed with willows in the water, and thus was difficult to fish effectively, so the catfish total is almost certainly an underestimate. This yields a minimum density of 8.65 catfish/ 100 lineal m, or 2.16 catfish/100 m2. The estimated biomass was 71 kg in total, or 72 g/m (18 g/m2). Brendan Hicks