Management of the ETBF is complex because of the cross-jurisdictional nature of the stocks and governance through the Commonwealth Harvest Strategy Policy and the WCPFC. Current assessments conducted by the WCPFC assume that these species comprise either single discrete stock units throughout the WCPFC area or across the Southern Hemisphere portion of the region and genetic methods used in the past have been unable to refute such assumptions. Biological information on growth rates and reproduction, movement data derived from tagging studies and spatial and temporal variability in catches of these species however, suggest that there is likely to be some structure to stocks throughout the WCPFC region and assumptions of single spawning populations may not be accurate.
More recently, traditional and next generation genomic methods have provided evidence of population structure in yellowfin tuna across the Pacific (e.g. Aguilar et al. 2015; Grewe et al. 2015) and provide some support to the hypothesis that yellowfin tuna fished by Australia’s tuna fisheries may be a localised stock within the Coral and Tasman Sea region. If yellowfin tuna and if any of the other principal species occurring in the ETBF do comprise localised stocks, this has obvious implications for the management both within national and regional contexts. Clarification of the connectivity and population structure of species in Australia’s Tropical Tuna fisheries with the broader WCPFC region is required for appropriate governance through the Commonwealth Harvest Strategy Policy and the WCPFC, to ensure any risks to regional stock biomass are minimised and to improve stakeholder concern over stock management.
The results of the current study are largely consistent with previous genetic investigations into the population structure of these four species. Consistency in results across years suggest that the groupings revealed here have some temporal stability across years across those sites where multiple years of samples were collected. Although results suggest the potential for two genetic groupings among albacore samples, assignment by the methods used here was statistically uncertain and resulted in some individuals not being able to be assigned to either group in the scenario with any confidence. Further sampling from the three locations included here as well as inclusion of samples from additional sites would also be required for resolving these uncertainties.
It should be noted that these results only apply to the sites included for each of the species in this study and therefore cannot be extrapolated across the wider western and central Pacific Ocean region with any certainty. Further sampling and analysis of sites across the western and central Pacific, including temporal replication of sampling, would be needed to investigate whether the results presented here are consistent with other locations across the western and central Pacific region or whether greater genetic differentiation is discernibly present. The resources required to support the attainment of broader insights into the connectivity of species across the WCPFC Area and connectivity between the ETBF and the western and Central Pacific Ocean will be dependent on current access to samples, the extent of further sampling required in order to attain broad spatial and temporal coverage of samples, the facilities and capability available for processing and sequencing samples and the capability available for data quality control and analysis pipelines.
As next steps, a second year of sampling for broadbill swordfish from New Zealand is planned and a preliminary small dataset from the Cook Islands (consisting of 24 samples) has been collected. These samples will be analysed and incorporated with the data from this project to provide further insights into the connectivity of broadbill swordfish across the western and central Pacific Ocean and presented to the WCPFC Scientific Committee in August 2021.