Abstract
Interannual fluctuations in the abundance of young‐of‐the‐year (YOY) European perch ( Perca fluviatilis ) were studied in two large French peri‐alpine lakes using a 12‐year dataset of late summer hydroacoustic surveys. Previous research has highlighted the importance of temperature. However, these studies often considered only monthly or seasonal averages. We analysed 12 years of data at the ecosystem level to identify the drivers of annual fluctuations in YOY perch stocks. We used traditional thermal indices based on average values, as well as thermal indices reflecting daily temperature fluctuations, derived from 36 years of in situ perch spawning data. Embryonic thermal conditions alone accounted for between 44% and 88% of the variation in YOY perch density across the lakes. An increase in temperature during the embryonic phase was beneficial, whereas significant daily variations had the opposite effect. In contrast, trophic factors and the adult stock did not exhibit consistent trends. Our study confirms previous findings using ecosystem‐scale analyses, long‐term data and novel thermal indices, validating in natura the central role of spring thermal regimes in shaping recruitment and improving our understanding of recruitment variability under climate change.