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Aquatic ecology of Lake Rotokare, Taranaki, and options for restoration

Lake Rotokare is a 17.8-ha natural lake in eastern Taranaki, located 12 km east of Eltham in the 230-ha Rotokare Scenic Reserve. In 2008, the Rotokare Scenic Reserve Trust completed construction of an 8.2-km predator proof fence around the reserve. Frequent algal blooms in summer have led to long periods of lake closure to boating and contact recreation. As there are few lakes in the Taranaki region, these closures are a nuisance to the local community. The objectives of this study were to quantitatively survey the fish community of the lake and to evaluate the lake water quality for the Rotokare Scenic Reserve Trust for the purpose of advising on options for lake restoration. Water quality has not deteriorated since 1976-1980, and, if anything, has improved. Secchi disc depth in 2013 (1.95 m) was very similar to measurements in summer 1980 (mean 1.93 m on 30 January 1980). Mean dissolved reactive phosphorus (± 95% confidence interval) was greater in 1976 (190±50 mg/m³) than mean phosphate concentration in 2013 (93±31 mg/m³, p < 0.05, Kolmogorov-Smirnov two-sample test). The thermocline was deeper in 2013 at 6-7 m compared to 3-4 m in 1977. This indicates that a much greater volume of the lake was oxygenated in February 2013 than in February 1977. Also, the intensity of stratification was less in 2013, as the dissolved oxygen concentration below the thermocline was 21027% compared to just 3% in 1977. This suggests that an improvement in water quality has occurred, probably as a result of stock exclusion. To sample the fish community, boat electrofishing was used at the total of six sites. The total length fished was 1,656 m, which was 6,624 m² in area. Eighty minutes of boat electrofishing caught 234 fish (217 perch, 16 shortfin eels, and 1 longfin eel). Fishing at night showed a 16-fold increase in the catch rate of perch (125 fish/10 min of fishing) compared to fishing during the day (8 fish/10 min of fishing). Perch dominate the fish community in Lake Rotokare and the biomass and density of eels are low, which is unusual for Taranaki water bodies. The mean density of perch was 4.49 fish/100 m², and the mean density for eels was 0.29 fish/100 m². The lower eel density may be a result of impaired access for eels or may be the result of predation by perch on migrant juvenile eels. There have been changes in the zooplankton community since 1980. The North American invader Daphnia galeata was not found in 1980, and appears to have now replaced the cladoceran Bosmina meridionalis and copepod Boeckella sp. We also found a diverse rotifer community.
Type of thesis
Hicks, B. J., Bell, D.G., Duggan, I. C., Wood, S. A. & Tempero, G. W. (2013). Aquatic ecology of Lake Rotokare, Taranaki, and options for restoration. ERI report 14. Report prepared for the Rotokare Scenic Reserve Trust. Environmental Research Institute, The University of Waikato.
Environmental Research Institute, The University of Waikato
© 2013 the authors.