Much of the recent progress in the study of eco-evolutionary dynamics has occurred in fish models. In this review, I focus specifically on the mechanisms by which fish evolution affects ecological processes, highlighting outstanding questions and new frontiers. Fish evolution can alter the ecology of individuals by changing traits that underlie resource use or interactions with other species. Fish eco-evolutionary studies have focused on the ecological consequences of two kinds of effect traits (trophic traits and nutrient recycling traits), but questions remain about the relative influences of genetic and environmental factors. Other fish traits might also have large ecological effects, but we lack frameworks for identifying them, and it remains difficult to link the traits to their ecological consequences. The effect of fish evolution also depends on changes in fish populations, which might vary because of genetic or by environmental factors. Such changes can have dramatic impacts on the expression of e...