New insights into the collapse of Upper Lake Constance whitefish (Coregonus wartmanni) and the impact of commercial fisheries management

Abstract

The whitefish Coregonus wartmanni is the key fishery resource in Upper Lake Constance (ULC), one of Central Europe’s largest lakes. A significant stock decline resulted in the closure of the commercial whitefish fishery in 2024. Reasons for the decline have been contested, with suggestions ranging from environmental changes to overfishing. As in many inland fisheries, management in ULC previously lacked standardized protocols for stock assessment, and relied instead on technical measures like regulating mesh size and net numbers. To assess stock dynamics and estimate biomass and fishing mortality over the past 25 years (1997–2022), a surplus production model was applied using scientific gillnet surveys and commercial catch data. The results confirm that the whitefish stock is at a historically low level. Apparently, in 2012, when the stock already showed signs of overfishing, invasion of the pelagial by non-native stickleback ( Gasterosteus aculeatus) triggered an ecosystem shift. Additional factors including oligotrophication, other invasive species and climate change also impact stock development, suggesting that reduced fishing pressure alone may not be enough to ensure short-term stock recovery.