Assessment of mortality during trap and transport in adult European eel

Many fluvial water systems suffer from reduced or completely disrupted connectivity due to human activities, causing negative effects for species and ecosystems. Artificial transport past migration barriers, so called trap and transport, can be used as a management tool to mitigate fish population declines. The efficiency of trap and transport is, however, rarely evaluated, in particular for downstream transport of catadromous species such as the European eel. In this study we analysed seven years of trap and transport data (2016–2022), encompassing nearly 58,000 transported adult eels, to evaluate mortality during all steps of the trap and transport process (i.e., mortality in fishing gear, during holding, and during transport). We found that mortality rates were generally low for all steps, 0.2–1.4 % in the fishing gear, 0.6–2 % during holding, and 0.03–0.17 % during transport (percent per year on average), compared to expected mortality in turbines (estimated turbine mortality in the investigated area: 70% to >99 %). The factors affecting mortality differed across the trap and transport steps, with year, season (day of year), temperature, and mortality in the previous step (i.e., indicative of general stress in the previous step, and/or poor condition) being the factors having an effect. We conclude that trap and transport may cause relatively low mortality compared to turbines, but since it requires maintenance and human interference, restoration of connectivity should be the long-term goal.