Growth and production of perch (Perca fluviatilis L) responding to biomass removal

This study was carried out in the oligotrophic lake Munksjoen in South-East Norway. The lake is situated 569 m above sea level, its surface area is 48 ha and maximum depth 10 m. The fish fauna consists of perch, roach (Rutilus rutilus L.), pike (Esox lucius L.) and sparse populations of brown trout (Salmo trutta L.) and burbot (Lota lota L.). The water quality was poor due to acidification, with a pH level of about 5.0 until the summer of 1991 when 23 tonnes of limestone powder were added. Afterwards, pH > 6.0 was maintained, except for snowmelt periods when the pH dropped to 5.8. Liming was expected to improve recruitment, and to avoid overcrowding and stunting, the most numerous species, perch and roach, have been heavily fished with traps and gillnets annually since 1992. Density of the perch population was determined with the mark recapture experiment, and biomass and production in the period 1992-1994 were simulated by modelling. Biomass of perch was estimated to 9.5 kg/ha before the mass removal and to 2.8 kg/ha afterwards. The biomass reduction led to increased growth rate, and based on the model, 1-1.5 kg/ha of perch with a mean weight of 100 g may be caught annually.