Classic offspring-size theory predicts that a single level of investment per offspring maximizes parental reproductive success in a given environment. Yet, substantial variation in offspring size is often observed among females within populations. Variation at this scale may occur because spatio-temporal variation in stabilizing selection prevents erosion of genetic variation. We tested whether patterns of size-specific offspring survival of Atlantic salmon (Salmo salar) varies across location and season within a short stretch of a natural stream by manipulating the emergence timing of juveniles from 12 families with different mean egg sizes and assessing their performance at two locations. The relationship between egg size and juvenile survival varied temporally and spatially; large eggs were advantageous for early emergers in one location, whereas egg size had no effect in the other. Furthermore, the performance of later emerging juveniles did not depend on egg size in either location, possibly because the early emergers had grown or established territories. Thus, selection on offspring size can be complex and vary across short periods of time and small geographic distances, thereby preventing the erosion of genetic variation expected under consistent stabilizing selection.