In this talk I explore the unusual mortality event on the Koyukuk River in the summer of 2019 associated with extreme warm water temperatures. Although the consequences of the 2019 die-off in either the short-term or long-term are far from clear, evolutionary change is possible to the extent that mortality acted non-randomly on heritable traits. After briefly setting the stage, I conduct and report on a preliminary analysis to test for size-selective mortality by comparing the lengths of sampled dead fish found en route vs. fish that survived to the spawning grounds and were sampled at weirs. I'll conclude with suggestions for next steps as we approach another spawning season in a rapidly warming world.