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, de-pending 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.