The 2.3-ha lower Karori Reservoir is a warm, monomictic lake with a mean depth of 8.2 m, a maximum depth of over 20 m. Its 252-ha catchment consists of mixed regenerating shrub-hardwood forest The lake is formed by an earth dam that as completed in 1874 to provide a water supply for the city of Wellington, New Zealand. Redfin perch (Perca fluviatilis) were introduced into the reservoir in 1878 and have formed a more or less monospecific population ever since. The lower reservoir ceased to function as a water supply in 1992. When this happened, water through-flow also ceased and the water retention time increased from 5 days to about 300 days. As a consequence, the reservoir has been dominated by blooms of cyanobacteria. We speculated that the trophic cascade induced by juvenile perch predation on zooplankton was exacerbating the cyanobacterial blooms so we set about reducing the perch abundance by fishing over 5 successive years to test this hypothesis.
The response of age-0 perch (N caught = 690) to the first year (2007) of removal of all sizes of perch was a 3-fold increase in age-0 abundance (N caught = 2,071) and an increase of 21 mm in age-0 mean fork length in 2008 (67.5 mm) compared to the original mean in 2007 (46.8 mm). Perch removal in 2008 increased the number of age-0 perch in 2009 to 6 times the original abundance (N caught = 4,281) and the mean fork length a further 3 mm to 70.9 mm. A hydro-acoustic survey before and after perch removal in 2009 suggested that 54% of the perch population was removed. No fishing occurred in 2010, but further fishing in 2011 revealed that the number of age-0 perch had returned to close to the pre-fishing abundance (N caught = 885). The mean fork length of age-0 perch, however, was 67.6 mm, still 21 mm greater than the original mean length in 2007.
Our results suggest that partial removal of perch is unlikely to achieve the desired top-down restoration of a healthy zooplankton population capable of controlling algal blooms.