Connectivity of broadbill swordfish targeted by the Australian Eastern Tuna and Billfish Fishery with the broader Western Pacific Ocean

Australia’s Eastern Tuna and Billfish Fishery (ETBF) operates in waters off on the east coast of Australia and catches a number of pelagic species including broadbill swordfish. The distribution of swordfish is known to extend well beyond Australian waters and fish caught in the ETBF are considered to form part of a wider Western and Central Pacific Ocean (WCPO) population. Accordingly, regional stock assessments carried out for the Western and Central Pacific Fisheries Commission (WCPFC) assume a single interconnected stock. However, the specifics on the connectivity of swordfish caught in the ETBF with the WCPO is still a major source of uncertainty and tagging and fisheries data suggest that there may be some structuring of the species across the region. Using high throughput genetic sequencing of total genomic DNA derived from swordfish collected from the ETBF, including northern (Queensland), southern (Tasmania) and far eastern (Norfolk Island) regions, New Zealand and the Cook Islands, co-dominant single nucleotide polymorphism (SNP) markers were identified. Purpose built quality control pipelines allowed for the discrimination of poor quality and compromised (e.g. through DNA cross contamination) samples. Mixture models were then used to investigate the presence of one or more genetic populations for each species. Results suggest there is little genetic differentiation between swordfish caught at the locations sampled. However, given the low numbers of samples within years examined from eastern Tasmania and the Cook Islands, the results presented should be considered preliminary. Further sampling from these locations would be required for resolving the indicative nature of the results and further sampling from additional sites would be needed to investigate whether the results presented here extend to other locations across the western and central Pacific region.