POPULATION REGULATION OF VENDACE (COREGONUS-ALBULA) IN LAKE PYHAJARVI, SOUTHWEST FINLAND

Previous time series analysis on vendance population dynamics in Lake Pyhajarvi, 1971-1990, revealed a 2-year cycle in year-class strength, implying powerful density-dependent regulation. Here we have extended this analysis by using multiple regression models to test whether the recruitment series is influenced by density-independent factors. We chose population size with a lag of 1 year as the density-dependent factor; the density-independent factors were the summer water temperature with a lag of 2 years (temperature sum for June, July and August, indicating the year-class strength of predators) and the temperature-derived length of the larval period of vendace. For the years 1972-1990 the coefficient of determination (r2) of this regression model was 0.77. We suggest that the basic mechanism producing a persistent 2-year cycle of vendance in Lake Pyhajarvi is the asymmetrical food competition between age groups. The abundance of predators in the lake and the warming of the water after the hatching of larvae in spring together determine the final year-class strength of vendace.