Fish mass mortality events in northern temperate lakes are happening later in the year than in the past

Globally, mass mortality events are becoming more common across ecosystems and taxa. For fishes in northern temperate lakes, climate change is predicted to increase the frequency of summerkills and decrease the frequency of winterkills. We compiled reports of fish kills in Michigan inland lakes from 1936 to 2022 derived from historical records, management reports, and an online app for the public. Using this dataset of 525 reported fish kills in eight decades, we tested for changes in timing of fish kills. We found that fish kills in the early 20th century were most likely to be reported in early spring months, as lake ice thawed, while in the later 20th and early 21st centuries, more fish kills were reported through summer. Across our dataset, the median day of year fish kills were reported shifted later by more than 50 days. Shifts in the timing of these perturbations can alter population demography, and community resilience, and may have lasting and unprecedented impacts on lake ecosystems.