Bigeye tuna (Thunnus obesus) is a highly migratory species distributed throughout the tropical and subtropical waters of the Pacific, Indian and Atlantic Oceans. Genetic-based studies (e.g., allozymes, mtDNA and microsatellites) have not observed evidence of spatial structuring in bigeye tuna from differing locations in the Pacific Ocean, or between the Pacific Ocean and Indian Ocean sampling locations, suggesting that a lack of structuring is consistent with broad-scale panmixia among bigeye tuna in the region but also recognising that additional sampling and alternative techniques may be required to further investigate population structure. Based on genetic or non-genetic studies (e.g., tagging) it is suggested that the stock structure of bigeye tuna in the Pacific Ocean remains unresolved and is associated with high uncertainty (Moore et al. 2020). Regular stock assessment is developed by the Western and Central Pacific Fisheries Commission (WCPFC) and the same entity sets conservation and management measures for this stock in this region. In the most recent stock assessment report (Ducharme-Barth et al. 2020), as in the previous reports, a discrete stock unit within the domain of the model area (essentially the WCPO, west of 150ºW) was considered.

The Parties to the Nauru Agreement (PNA) is a regional binding treaty-level instrument to facilitate cooperation in the management of fisheries resources of common interest. The agreement requires the management of straddling/highly migratory fish stocks on a subregion basis through Regional Fisheries Management Organizations (RFMOs), and the WCPFC Convention (the regional fisheries agreement covering the WCPFC convention area). PNA members are simultaneously members of the WCPFC, so a single management unit is set for PNA-related profiles.