Should I stay, or should I go? Prolonged residency in freshwater leads to greater size-at-age in a facultatively catadromous fish

Abstract

While freshwater habitat utilisation is known to promote growth of a facultatively catadromous barramundi (Lates calcarifer), the influence of residency duration on their growth has not been explicitly investigated. This study aimed to: characterise variation in the ontogenetic migration timing and freshwater residency duration, determine the impact of that duration on fish size-at-age, and assess the spatial variation in their migratory behaviours. A total of 530 adult barramundi were collected along the coast of North Queensland, Australia, in 2019-2020. Additionally, 33 juveniles and corresponding water samples were analysed to determine if microchemical fingerprints varied significantly among catchments. Based on otolith Strontium to Calcium ratios, catadromous barramundi demonstrated age-specific upstream migration (consistently during their first or second year of life), while downstream migration was highly variable, occurring during the second to twelfth year of life. Among catadromous individuals, approximately half utilised freshwater habitats for only one year, whereas the remainder stayed significantly longer (up to eleven years) before returning to the estuaries. Importantly, fish length-at-age was strongly correlated with the duration of freshwater residency, indicating a substantial growth advantage for individuals with prolonged stays in freshwater. However, we were unable to distinguish their natal origins. Consequently, we could not determine if these observed migratory patterns varied across river systems. Our findings underscore the importance of access to and persistence of freshwater habitats for catadromous barramundi. Promoting connectivity among freshwater, estuarine, and saltwater environments is likely to enhance individual growth and underpin the productivity of catadromous populations in estuaries.