Population structure and movement of black bream (Acanthopagrus butcheri) in South Australian estuaries based on otolith chemistry

Abstract

The elemental concentration of otoliths provides an opportunity to discriminate among fish living in different habitats, and to estimate population connectivity, life history variation and potential movements. Understanding temporal patterns of recruitment and connectivity of estuarine fish populations, as well as life history variation is fundamental to elucidating population dynamics and informing effective management and conservation efforts. Estuarine associated species utilise sheltered habitats for growth and feeding, but also face fluctuating environmental conditions. Estuaries may be temporally variable sinks or sources of juveniles and have fish moving to other areas. Otolith chemistry provides a tool to reconstruct origins and quantify connectivity between juvenile and adult populations, as well as investigate movements of fish throughout their life history. The overall aim of this study was to assess connectivity and partial migration within black bream (Acanthopagrus butcheri) populations in South Australia. The key objectives were to: (1) Determine spatial and temporal variation in otolith elemental signatures to assess their usefulness for determining connectivity of black bream populations; (2) Retrospectively trace subadult fish from the Fleurieu Peninsula (FP) and Kangaroo Island (KI), South Australia, to their juvenile regions to gain an understanding of movements; and (3) Determine the proportion of migrant and resident life history types within estuarine populations and if these different contingents reflect a hybrid fish genotype. I examined spatial and temporal variability in otolith chemistry of juvenile 0⁺ black bream to discriminate fish living in different estuaries. Fish were sampled in up to 12 estuaries on KI (7 estuaries) and the FP (5 estuaries) annually over a five year period (2007–2011). Otoliths were examined to determine if individual estuaries or groups of estuaries differed in chemical concentrations (signatures or tags), and to examine inter-annual variation in chemical tags. Tags differed among individual estuaries and in some cases adjacent estuaries shared similar chemical tags. Differences in chemical tags were detected among all estuaries in two of the year comparisons. Similarities in otolith tags were detected for five of the seven estuaries in 2010 and 2011. Grouping estuaries with similar chemical tags enhanced the classification accuracy of fish to estuaries and therefore the ability to discriminate stock structure based on otolith chemistry. Temporal differences in chemical tags were detected among years for several estuaries; however, there were no clear trends in the differences between years. The results highlight that black bream from different estuaries or groups of estuaries have unique chemical tags that can be used to trace cohorts of fish. Such differences can be used to estimate connectivity, population movements, and the function of estuaries as nursery areas for this species in subsequent years. Otolith chemistry was then used to reconstruct origins and to quantify connectivity between juvenile and adult components of black bream populations. Trace element composition of juvenile black bream otoliths from broad areas representing estuaries from each of KI and FP were quantified for each of four years (2007-2010) and used as baseline data to retrospectively trace the juvenile region of 1 to 3⁺ year black bream. Such an approach was possible because there were differences in otolith element concentration of young-of-year black bream among broad areas. Through LA-ICP-MS analysis of the juvenile region of subadult (1 to 3⁺ year old) black bream, I was able to estimate the proportion of juvenile fish that recruited from these two regions through time, as well as temporal patterns of connectivity between KI and FP. My results suggest variability among cohorts of the same age, among age classes within a cohort and between regions in terms of self-recruitment. These results highlight the variable nature of connectivity among populations and recruitment to adult populations suggesting that management and conservation efforts may need to consider such variability. Partial migration, where some members of a population migrate showing life history profiles of salinity habitats was evaluated in six estuarine populations of black bream from KI, using otolith Ba:Ca ratios. Profiles of Ba:Ca across otoliths were used to broadly determine fish habitat use (freshwater, estuarine, marine) and assign euryhaline or stenohaline status. In addition, we evaluated whether migratory fish may represent hybrid fish between black bream and yellowfin bream (Acanthopagrus australis) using an eight locus microsatellite library. Partial migration was observed in all six estuarine populations, although for most estuaries there were greater numbers of migratory fish than resident fish. Hybrid fish were found in five of the six estuaries and comprised both migratory and resident fish. Both resident and migratory fish spent most time in marine waters, but migratory fish also moved to estuarine and freshwaters. Similar patterns were also seen for hybrid fish. The data highlight the different migratory behaviours of subadult individuals and show that hybrid bream had reached estuaries previously thought to be inhabited by pure black bream. Complex migratory patterns may allow populations to persist under adverse environmental conditions and in systems subject to change. Otolith chemistry data provided greater insight into patterns of connectivity between broad geographic regions and life history variation of black bream in South Australia. The variable nature of connectivity among populations and recruitment to adult populations suggests that conservation efforts will be more complex than protecting a single region. However, the temporally variable contribution of recruits and broad scale connectivity between juvenile regions suggests that no one region acts as a single ‘source’ of recruits, potentially safeguarding the species against the loss of functional nursery habitats. In the same manner, the ability of black bream populations to possess a suite of movement behaviours (likely in response to different conditions) may be beneficial in a dynamic environment such as an estuary since it allows the persistence of populations when faced with increasing habitat degradation. Overall, these findings infer that the adoption of a flexible life history strategy likely enhances species resilience to fluctuating environmental conditions and potentially adverse impacts.