Males die young, which may cause an Allee effect during a population collapse of the Vendace Coregonus albula

Abstract Objective We estimated the difference in mortality between mature male and female Vendace Coregonus albula based on a large data set of catch samples from 25 locations in Finland. We then used this estimate and age distribution data from Lake Etelä‐Konnevesi to illustrate how the sex ratio (females per one male) might decrease as the average age of spawners increases during a several‐year‐long period of recruitment failure. Methods We estimated mortalities first from sex‐specific age–ln(catch) curves and second from the average age‐specific proportions of different sexes. Result The estimate of the additional mortality of males was around 0.2–0.4, depending on the method of estimation, and assumed true proportions in the population at age 1 (two growing seasons), when Vendace reaches sexual maturity. When using the additional mortality estimate and age distribution data from Lake Etelä‐Konnevesi, the hypothetical sex ratio in the most extreme year was even as skewed as four females per one male. Conclusion If the lack of males per female spawner during a population decline reduces the per capita recruitment success of females, this is a depensatory density‐dependent effect, the Allee effect. This phenomenon may partly explain the rapid population collapses and contribute to 2‐year cyclicity typical of the dynamics of Vendace populations.