|dc.description.abstract||The population structure, relative fish abundance, age, growth rate, and diet of the brown bullhead catfish (Ameiurus nebulosus) in Lake Taupo were examined. The study attempted to evaluate the possible impacts of catfish on the trout fishery in the lake. Fyke nets were set within the southern region of the lake at sites consisting of three distinct habitat types (weedy, rocky, and sandy), and left to fish overnight. Sampling was undertaken between 28 February 1995 and 15 December 1995 with catches grouped by season.
A total of 6226 catfish were caught from 273 fyke nets set at the southern sites, which gave a mean CPUE of 23 fish ner⁻¹ nighr⁻¹• A further 27 fish were captured in the northern regions of the Lake Taupo as part of the distribution experiment.
Abundance of catfish was greatest at the weedy sites followed by rocky and sandy sites respectively. Catch rates at sandy sites were consistently low. Macrophyte communities provide catfish with protection during daylight hours and spawning, and a large food source from associated macrophyte fauna. The abundance of catfish at Motuoapa Bay bas remained relatively constant since 1986. Catfish are believed to have reached the carrying capacity of the bay forcing juvenile catfish to migrate in search of vacant territories. The availability of vacant habitats supporting dense macrophyte communities is a major factor restricting the maximum number of catfish in Lake Taupo.
The ages of catfish ranged from 1 to 8 years, with the majority of fish in their third season of growth. The conditions in Lake Taupo are favourable for rapid catfish growth. Somatic growth of catfish in this study was similar to that of Waikato and overseas catfish in the first two years and, whilst subsequently lower than the Waikato, was faster than other studies in the following years. The maximum length of catfish in Lake Taupo (359 mm F.L.) is less than that found in the Waikato (455 mm F.L.).
Catfish in Lake Taupo spawn between September and December at a similar season to fish from the northern hemisphere. The temperature threshold necessary for spawning to commence in overseas populations is considerably higher than that observed in Lake Taupo. Fecundity at Taupo appeared to be comparable with the Waikato and higher than overseas populations.
The diet of catfish was size and habitat dependent. Catfish from weedy sites fed upon gastropods, Trichoptera, cladocerans, and chironomids. Larger catfish were found to prey to a greater extent, koura, fish, and Odonata. Generally, catfish from rocky habitats bad a similar diet to catfish from weedy habitats however, large catfish fed almost exclusively on koura and, to a lesser extent, gastropods.
At present population levels catfish are unlikely to influence trout size or numbers, however, there is potential for negative impacts to occur if catfish numbers increase significantly, particularly at depth. The degree of piscivory observed in catfish was not considered sufficient enough to cause a large decline in prey species, especially bullies. The predation of koura by large catfish will potentially decrease its abundance within the littoral zone of rocky and weedy habitats.||