The biology of three endemic and one exotic species of fish (banded kokopu, Galaxias fasciatus; shortfin eel, Anguilla australis; longfin eel, Anguilla dieffenbachii; and the European perch, Perca fluviatilis) was investigated in Lake Rotokare. Little is currently known regarding the local ecology of the lake. This study improves the understanding of the biological processes in Lake Rotokare and investigates the characteristics of the lake’s water quality.
Fish sampling occurred in the lake using a combination of boat electrofishing, gill netting, and fyke netting. The tributary was sampled using night time spotlighting. Fish taken from the lake were used to determine length-weight relationships, size frequencies, CPUE, abundance estimations, fish biomass, stable isotope analyses, trophic levels, and perch diet. Water quality sampling was also undertaken to further build upon data from previous studies.
The majority of fish sampled in Lake Rotokare were perch, with a mixture of size classes, but dominated by a large juvenile size class. Perch density was high (16.55 fish 100 m-2). Longfin eels showed a cohort of large individuals with no evidence of recruitment occurring; while shortfin eels exhibited a distribution of size ranges with evidence of juvenile recruitment occuring.
Electrofishing showed evidence of banded kokopu inhabiting the lake, even at distance from the tributary outlet, suggesting the possibility of a lake fringe population of kokopu. Spotlight sampling in June and December revealed a stable kokopu and kōura population residing in the lakes’ main tributary. Water quality data showed an overall decrease in total nitrogen and phosphorous loads within the lake since 1979. However, the lake has shown strong thermal stratification over summer over multiple years (1977, 2013, February 2017 and December 2017), and remains in poor condition, with a eutrophic TLI 4 rating of 4.1. Frequent cyanobacterial blooms continue to occur during the summer season, resulting in closure of the lake for contact recreation.
Both dietary and stable isotope analyses showed indications that chironomid larvae and Daphnia sp. constituted the bulk of the primary production of the food web; juvenile perch were found to be the predominant food source of the three resident fish species. A three-end member mixing model was created to duplicate the theoretical food-web within the lake with chironomid larvae, dragonfly larvae, and juvenile perch at the base. Lipid treatment techniques were examined in this study for eel fin and muscle tissue. The results indicate that lipid treatment is needed for 13C isotope values for both longfin and shortfin muscle, and longfin fin tissue for accurate results. Mathematical equations were constructed to correct untreated fin tissue values into treated muscle values for 13C and 15N; avoiding the necessity for future lethal sampling methods.
