Winter wipeout? Post‐disturbance recovery of brown trout indicates strong population resilience to harsh winter conditions

Demographic resilience refers to the capacity of a population to resist and recover from disturbances in terms of both population size and structure. Resilience to severe disturbance events, such as heatwaves, floods, and cold snaps, is likely to vary with the life‐history characteristics of the population, such as migratory strategy, and demographic structure. In the winter of 1995/1996, a cold snap without insulating snow or ice caused supercooling and anchor ice formation in shallow, turbulent sections of small central Swedish rivers, where we analyzed long‐term density data from 11 resident and 11 migratory brown trout Salmo trutta populations. Our results suggest that deposited eggs in spawning areas were heavily affected, as young‐of‐the‐year (YOY) of nearly all sampled populations, regardless of migratory strategy, were found to be extirpated the following summer. For most populations, peak abundances of YOY were observed the year after extirpation, suggesting competitive release in the absence of a competing older cohort. However, migratory populations showed a stronger compensation effect than resident populations, likely due to the higher fecundity of migratory parents. All migratory and most resident populations reached pre‐disturbance densities after one year for YOY and after two years for older trout when the previous young cohort was recruited into the older age class. Migrating populations exhibited greater initial cohort ratio fluctuations, but both population types showed alternating strong and weak cohorts for 4–5 years after the extreme winter, likely due to inter‐cohort competition. This study shows that brown trout populations can recover within a few years after a single severe impact event, regardless of migratory strategy, if reproductive individuals still exist in the populations. Supplementary stocking could therefore await natural recovery, but conservation measures should be directed to the maintenance and restoration of local habitat refugia and river connectivity to reduce extirpation risk and support recovery.