Egg size and fecundity are both positively associated with maternal reproductive success, yet maternal resource limitations result in a trade-off between these two traits. Exploring this trade-off, the extent of genetic and environmental influences on egg size and fecundity, and of correlations between these and other traits, and thus the effects acting within vs. among generations, is therefore a central goal in both evolutionary ecology and selective breeding. Using multi-generational captive Arctic charr (Salvelinus alpinus) records, we quantified genetic and environmental effects on, and correlations between, egg size and fecundity, body size (a proxy for growth) and condition prior to maturation, and body size at maturation. We estimated that genetic contributions to variation in egg size and fecundity are moderate to high. Egg size and fecundity do not significantly correlate at the genetic level but do correlate negatively at the environmental level. Growth prior to maturation and size at maturation are positively correlated with fecundity and egg size at the phenotypic level. Genetic correlations with growth are positive for both egg size and fecundity, but weaker for egg size. Contrarily, the environmental correlations with growth are of the opposite sign, also weaker for egg size, and increasing growth leads to decreasing egg size but increasing fecundity. Consequently, reproductive success can be optimised across generations via independent selection responses of egg size or fecundity and by correlated selection responses with body size. Ultimately, the egg size-fecundity resource trade-off in Arctic charr is resolved via growth-controlled phenotypic plasticity acting within generations.