Accessing samples from broadbill swordfish from two sites within the WCPFC area was particularly problematic and was exacerbated by a poor fishing season in 2019. This resulted in samples for broadbill swordfish consisting of samples collected from the ETBF (2 years), Norfolk Island (1 year) and New Zealand (1 year). The poor fishing season in 2019 also resulted in limited samples of striped marlin from New Zealand being collected in the second year of samples. The genetic groupings identified across bigeye and yellowfin tunas and broadbill swordfish suggest a substantial level of connectivity and mixing between each of the locations investigated, with little discernible genetic differentiation between areas. Results from albacore suggest the potential for two genetic groupings, however these were not able to be resolved by the methods used. The results from striped marlin indicate that there may be two genetic groups, with the ETBF, NZ and Hawai’i sharing the first group. The second group was identified only from samples collected from Hawai’i. The presence of two genetic populations of striped marlin in the waters of Hawai’i has been proposed previously and the results presented here lend further support to this hypothesis. The consistent absence in the ETBF and New Zealand of the second genetic group found in Hawai’i indicates a proportion of fish recruiting to the Hawai’i fishery do not contribute to the ETBF fishery and potentially represent a northern hemisphere population that doesn't migrate south of the equator.
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.