Top down or bottom up? Feasibility of water clarity restoration in the lower Karori Reservoir by fish removal

摘要: As part of an overall ecosystem assessment lower Karori Reservoir, Sanctuary, Wellington, a number variables are being monitored routinely, including temperature, nutrients, and phytoplankton zooplankton populations. Ammonium (NH4) tends to be the dominant species inorganic nitrogen most time except in late winter when nitrate (NO3) becomes dominant. Total concentrations place Sanctuary mesotrophic eutrophic category. The reservoir has very unusual fish community that is almost completely one species, European perch. Our preliminary estimate 20,000 22,000 perch especially age-0 age-1 make up over 80% population, suggests likely have large effect on size abundance. We removed approximately 4,000 from between 12 15 February 2007. This represents 18-20% total estimated present, weight (72 kg) 8-10% biomass. Age-0 may been under-represented catch, given we caught more than Alternatively, (2005) cohort particularly strong age class compared (2006) cohort. small age-2 survival 1 2 poor. dam, spillway, valve tower structures restrict recruitment other by acting as barrier upstream migration juveniles sea. low numbers longfin shortfin eels. eels naturally recruited this not stocked, but were released reservoir. Banded kokopu stream upper reservoirs sea-run origin, or they lake-locked population. should tested otolith microchemistry. Boat electrofishing effective way catch littoral perch, night-time boat was at least 2-4 times day-time fishing. Population estimates fishing possible because capture probability, for limnetic Other methods estimating abundance such echosounding could evaluated, mid-water trawling potential both remove fish. Opera house nets ineffective Fishing itself unlikely eradicate Reservoir. A concerted effort reduce point which their negligible, would require ongoing maintain One draw down lake poison few killed poisoning replaced stocking. Another option worth considering introduction eels, compete with prey upon without growing enough juvenile water birds. reliance cover branches aggregation devices might also explored control young year. Shortly after removal, observed reduction cyanobacterium Anabaena planktonica increase because: (1) temperature stimulated feeding growth rates zooplankton; (2) removal zooplanktivorous reduced predation pressure zooplankton. At stage, it determine relative importance each these processes.