Drivers of catadromous migration in barramundi (Lates calcarifer) in the wet/dry tropics of northern Australia

Abstract

Migration is integral to the life history of many fishes and enables individuals to
maximise fitness by exploiting habitats and resources that are geographically separated from breeding sites or vary seasonally in suitability. However, movement patterns are often highly variable among individuals, which may have profound implications for the productivity and resilience of populations and the ecosystem services they provide. Diadromous fishes, which undergo migrations between marine and freshwater habitats, are exemplars of such behavioural flexibility, which makes them ideal candidates to investigate the mechanisms that underpin life history expression.

The overarching aim of this thesis is to explore the causes and consequences of individual life history variation in a tropical, facultatively diadromous, sequentially hermaphroditic fish (barramundi, Lates calcarifer). Otolith microchemistry and biochronology data are analysed to elucidate individual migration behaviour and growth rates, respectively. My research reveals that diadromous individuals, which undergo juvenile migration to freshwater, obtain a growth advantage over non-diadromous, estuarine resident conspecifics. In turn, rapid growth rates lead to younger, but not smaller, female maturation, indicating that fast-growing individuals may obtain higher lifetime fecundity than slow-growing individuals. Adoption of diadromous versus resident life histories appeared to be influenced by climatic variation. Years of increased monsoonal activity were associated with a lower proportion of young-of-year barramundi undergoing diadromous migration, suggesting that favourable conditions within estuaries under these circumstances reduce the incentive for migration.

Overall, my findings underscore the importance of productive freshwater habitats for barramundi populations and highlight the complexity of trade-offs between life history
characteristics and individual fitness. I argue that the hydrological variability and spatial complexity which characterises northern Australian riverine ecosystems plays a key role in shaping the life history variation within barramundi populations, which in turn enhances the productivity of their commercial, recreational and subsistence fisheries. My thesis outlines the threats to tropical riverine and coastal fisheries imposed by increasing demand for freshwater resources emphasising the need to incorporate the preservation of life history diversity into the goals of sustainable development.

Date of Award	2023
Original language	English
Awarding Institution	Charles Darwin University (CDU)
Supervisor	David Crook (Supervisor), John Morrongiello (Supervisor), Alison King (Supervisor), Thor Saunders (Supervisor), Dave Morgan (Supervisor) & Sam Banks (Supervisor)