YEAR-CLASS FLUCTUATIONS OF VENDACE (COREGONUS-ALBULA) IN LAKE PYHAJARVI, SOUTHWEST FINLAND, DURING 1971-90

Many recent studies have stressed density-dependent regulation mechani sms in oscillating vendace (Coregonus albula) populations. Long-term s tudies on the population dynamics of vendace have been hampered by the difficulty of obtaining reliable estimates of year-class sizes. Data collected from Lake Pyhajarvi (Southwest Finland) during 1971-91 allow ed relatively accurate analysis of vendace abundance changes. in the w inters of 1979-91, daily catch statistics were collected from professi onal fishermen. Species, age, and size composition were analysed from regular catch samples. We estimated the size of the vendace population from the decrease in the CPUE during 10 winters. For the remaining 10 years, estimates were based on the relationship between the numbers o f a year-class in autumn and mean individual growth rate. A 2-yr cycle was the most conspicuous feature of the data implying density-depende nt regulation. We suggest that in Lake Pyhajarvi the basic model produ cing a persistent 2-yr cycle is the asymmetrical food competition betw een age groups.