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 ch ose population size with a lag of 1 year as the density-dependent fact or; the density-independent factors were the summer water temperature with a lag of 2 years (temperature sum for June, July and August, indi cating the year-class strength of predators) and the temperature-deriv ed 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 cycl e of vendance in Lake Pyhajarvi is the asymmetrical food competition b etween age groups. The abundance of predators in the lake and the warm ing of the water after the hatching of larvae in spring together deter mine the final year-class strength of vendace.