Despite 100 years of research testing the link between prey availability during the larval stage and year-class strength, field-based evidence for Hjort's "critical period" hypothesis remains equivocal. Here, we argue that a minority of past studies have relied on sufficient taxonomical knowledge of larval fish prey preference to reveal the potential effects of variability in zooplankton prey production on larval vital rates and year-class strength. In contrast to the juvenile and adult stages, larval fish diet and prey field are often poorly resolved, resulting in the inclusion of zooplankton taxa that do not actually contribute to the diet as part of the prey field considered by fisheries scientists. Recent studies have demonstrated that when accounting for prey selectivity, the expected positive relationships between preferred prey availability and larval feeding success, growth and survival are revealed. We strongly recommend that laboratories conducting research on larval fish trophodynamics take prey selectivity into account and acquire the necessary taxonomic expertise for providing valid assessments of the influence of prey availability on larval vital rates. We make the prediction that the proportion of studies supporting the existence of a "critical period" will increase proportionally to the progress of knowledge on prey preference during the early larval stage.