Evolution often gives rise to a variety of individual life history phenotypes within populations; a characteristic frequently linked to enhanced population stability and resilience. Perhaps the most widely reported form of such diversity is partial migration, where populations comprise co-existing migratory and resident life histories. Divergent migratory strategies may confer differences in growth, age at maturity and survival and as such play an important role in shaping population demography. Using quantitative models developed with otolith chemistry and biochronology data, we will show how different migratory strategies are linked to different reproductive strategies in terms of male vs female spawning investment in a protandrous (i.e., male-first) sex-changing fish (barramundi Lates calcarifer). Wild Australian barramundi populations comprise estuarine/ marine residents and catadromous (i.e., migrate to freshwater) individuals, the latter occupying freshwater for anywhere from <1-10 years. Recent evidence suggests that growth rates are faster in freshwater compared with saline habitats, likely due to the underlying productivity of tropical Australian freshwater habitats. As female status is associated with large body size, extended freshwater occupancy may promote younger female maturation, inferring that freshwater habitats contribute disproportionately to population fecundity. Preserving tropical freshwater habitats is therefore vital to ensure the conservation of iconic barramundi populations.