We investigate adaptive resource polymorphism in kokanee (Oncorhynchus nerka) from Jo-Jo Lake, Alaska by determining whether previously observed niche expansion occurs at the population or individual level. Utilizing morphological, genetic, and stable isotope techniques, we found no evidence of discrete trophic morphotypes as previously described, but instead found evidence for an ontogenetic diet shift. Carbon and nitrogen isotope data indicate a 40% decrease in the proportion of benthic feeding and an increase of 1 trophic position over the size and age ranges of adult kokanee, corresponding to a diet shift from consumption of macroinvertebrates in smaller individuals to piscivory in larger individuals. This novel piscivory in kokanee may result from predatory and competitive freedom resulting from the lack of limnetic predators in Jo-Jo Lake. Piscivorous feeding despite a phenotype-environment mismatch has resulted in large, piscivorous kokanee having up to 70% of their gill rakers damaged. Observed reductions in gill raker number relative to the putative ancestral population are convergent with expectations for piscivorous fishes, despite a presumed lack of standing genetic variation for piscivory in the sockeye salmon-kokanee species complex. Jo-Jo Lake kokanee are a distinctive example of adaptation in salmonids in response to ecological release. This unusual population highlights the importance of phenotypic plasticity in response to competition in shaping the adaptive landscape and altering evolutionary trajectories.