A matter of time—Efficacy of whitefish stocking in a large pre‐alpine lake

Abstract The tradition of stocking whitefish ( Coregonus spec.) is widespread throughout Europe and dates back more than a century in some lakes. Despite its broad acceptance as a management action, stocking success is less often studied and might change with shifts in ecosystems. Similar to many other large lakes in Central Europe, Lake Constance is stocked with millions of larvae each year at great cost. The last evaluation of this management practice took place in 2003, and since then several potential impacts on whitefish increased in magnitude (e.g. stickleback invasion, increasing water temperature). Forty‐two million dye‐marked larvae were stocked in 2016 and 36 million in 2018. Recaptures made 1.5 and 3.5 years after stocking revealed that around 20% of the 2016 and 2018 whitefish cohorts consisted of stocked whitefish. This amount is significantly lower than was found in previous experiments. To identify potential factors controlling recruitment, a Beverton‐Holt stock‐recruitment model showed that stocking did not significantly affect year‐class recruitment. We conclude that stocking is no longer cost‐effective, and we recommend that stocking should be reduced in the short term. Furthermore, our results suggest that explanatory variables that previously described recruitment variation of the whitefish population, such as April water temperature, may no longer apply.