Development of whitefish (Coregonus lavaretus) population and effects of large-scale compensation stocking in Lake Inari, northern Finland

Of the several forms of whitefish living in Lake Inari, the study concentrates on the economically most important one: the large sparsely rakered (LSR) whitefish (Coregonus lavaretus). The large, oligotrophic lake (area 1,102 km(2)) has been regulated for 50 years to generate hydroelectric power, and for 20 years, the subsequent damage to fish populations has been compensated for by extensive stocking. Stockings (0.5-18 million fish/yr) have been made with one-sommer-old fingerlings (8-10 cm). The whitefish also reproduces naturally, and thus, its population is a mixture of wild and reared whitefish. For stock assessment purposes, roughly 1,000-2,000 catch samples were collected annually, among which the share of LSR whitefish was 63-82 % during 1980-1994. Data on fishery, catches, and CPUEs were gathered annually. For estimating the recruitment of different year-classes and stock density, population analysis (VPA) was used. The growth of whitefish was particularly studied in relation to stock density. To evaluate the efficiency of stocking and the catch share, an average of 70,000 fingerlings/yr were nose-tagged during 1980-1986. The effects of stocking on whitefish catches turned out to be quire small due to high natural reproduction and density-dependent mechanisms affecting the growth and recruitment in the 1980s. In the beginning of the 90s, growth of whitefish increased because efficient fishing thinned whitefish populations. Simultaneously, the vendace (Coregonus albula) stock declined drastically. According to our calculations, the yield of the,rocking programme, based on taggings, was rather poor: 9-26 kg/1000 stocked fingerlings in the year-classes 1981-86. Therefore, according to results of population dynamics and taggings, the level of stocking should be reduced.