Combining natural markers to investigate fish population structure and connectivity

Understanding population connectivity via fish movement and the appropriate scale for management is amplified by the use of tagging techniques. Natural fish markers provide unique signatures that reflect distinct environmental and physiological characteristics of populations, allowing us to determine the degree of connectivity among them. Different markers function within defined spatiotemporal ranges with specific advantages and limitations. We tested whether applying multiple markers would increase the power of assessing population structure and connectivity of black bream Acanthopagrus butcheri from 12 estuaries across southern Australia. We utilised a range of natural markers, including genetics, otolith shape, otolith isotopic composition, and otolith elemental composition, to determine the effectiveness of each marker independently and through integration. For this estuarine-dependent species, combining genetics and otolith-based techniques was complementary in increasing the accuracy of our results but not all marker integrations were consistently beneficial, highlighting the importance of appropriate marker selection. The maximum classification accuracy to collection site of 95% (a combination of genetics, otolith shape, and otolith isotopic composition) emphasised the species’ estuarine dependency and limited connectivity across the sampling range, suggesting location-specific management is needed for this commercially important species.