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

Many recent studies have stressed density-dependent regulation mechanisms in oscillating vendace (Coregonus albula) populations. Long-term studies 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 allowed relatively accurate analysis of vendace abundance changes. in the winters of 1979-91, daily catch statistics were collected from professional 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 of a year-class in autumn and mean individual growth rate. A 2-yr cycle was the most conspicuous feature of the data implying density-dependent regulation. We suggest that in Lake Pyhajarvi the basic model producing a persistent 2-yr cycle is the asymmetrical food competition between age groups.