Students: Thirun Gawarammana (University of Adelaide)
Supervisors: Bronwyn Gillanders (University of Adelaide), Patrick Reis-Santos (University of Adelaide)
Partial migration, characterised by the coexistence of diverse migratory life histories (i.e., contingents) within a single population, is widespread among migratory fish taxa. The relative proportions of contingents are fundamental in shaping population dynamics including the resilience, productivity and stability of a population against environmental variability. This study leveraged life history data archived in fish otoliths and eye lenses to determine the effects of broad-scale environmental change on migrant proportions and assess the ecological outcomes of contrasting life history strategies in a partially migrating population of black bream (Acanthopagrus butcheri) in the Coorong Lagoon, South Australia. Analysis of lifetime otolith Ba:Ca profiles identified the presence of two distinct life history contingents representing estuarine resident and freshwater migrant A. butcheri. Generalised linear modelling demonstrated that the number of freshwater migrations per year was negatively associated with annual mean monthly rainfall and positively associated with annual mean monthly salinity. Although marginally non-significant, migrants had better mean body condition (Fulton’s K condition factor) compared to residents. Eye lens stable isotope ratio analysis revealed divergent lifetime trends in dietary history, with δ13C enriched in residents and depleted in migrants. Contrary to expectations of age-related ascensions in trophic level, δ15N declined with age in both resident and migrant A. butcheri. Overall, identifying the environmental drivers and ecological outcomes of diverse migratory life histories has important implications for managing partially migrating fish populations under an era of accelerated anthropogenic and climatic changes to aquatic habitats.
