*

Australia’s Bigeye Tuna stocks are sustainable in the Pacific Ocean and depleting in the Indian Ocean. The separate biological stocks are managed by international commissions as a resource shared by many countries, including Australia.

Bigeye Tuna in the Indian Ocean, and Western and Central Pacific Ocean are considered to be two distinct biological stocks and are managed by separate regional fisheries management organisations. The Indian Ocean stock falls under the jurisdiction of the Indian Ocean Tuna Commission (IOTC) while the Western and Central Pacific Ocean stock falls under the jurisdiction of the Western and Central Pacific Fisheries Commission (WCPFC). These two commissions are international organisations established to manage highly migratory fish species within their defined geographic ranges. 

In the Indian Ocean, Bigeye Tuna is considered to be a single biological stock. This is based on genetic studies that indicate no evidence of intra-oceanic genetic differentiation within the Indian Ocean [Chiang et al. 2008; IOTC 2019; Davies et al. 2020; Diaz-Arce et al. 2020] and tagging studies that have demonstrated large-scale movements of Bigeye Tuna within the Indian Ocean [IOTC 2014]. 

In the Western and Central Pacific Ocean, defined as west of 150ºW, genetic studies to date also indicate a single biological stock [Grewe and Hampton 1998; Evans et al. 2021] but further investigations are being planned [Moore et al 2020]. 

Here, stock status is presented at the biological stock level—Indian Ocean and Western and Central Pacific Ocean.

Indian Ocean

The Indian Ocean biological stock of Bigeye Tuna is fished by Australian fishers endorsed to fish in the Western Tuna and Billfish Fishery (Commonwealth), and members of the Indian Ocean Tuna Commission. The assessments undertaken by the Indian Ocean Tuna Commission account for information from all jurisdictions that take Bigeye Tuna in this region. 

In the Indian Ocean, the 2019 update to the 2016 assessment [IOTC 2021] estimates that spawning stock biomass in 2018 was 31% of the unfished level (80% confidence interval 21–34%). The biomass of this stock has declined, but the stock is not yet considered to be recruitment impaired. This assessment also estimated that 2018 fishing mortality was above the level associated with maximum sustainable yield (MSY) (120% of fishing mortality at MSY; 80% confidence interval 70–205%). This level of fishing mortality increases the likelihood of the stock becoming recruitment impaired. 

On the basis of the evidence provided above, the Indian Ocean biological stock is classified as a depleting stock.

Western and Central Pacific Ocean

The Western and Central Pacific Ocean biological stock of Bigeye Tuna is fished by Australian fishers endorsed to operate in the Eastern Tuna and Billfish Fishery (Commonwealth), and members of the Western and Central Pacific Fisheries Commission. The assessments undertaken for the Western and Central Pacific Fisheries Commission account for information from all jurisdictions that take Bigeye Tuna in this region. 

The Bigeye Tuna stock in the Western and Central Pacific Ocean was most recently assessed in 2020 [Ducharme-Barth et al. 2020]. This assessment was accepted as a basis for management advice however, noting a range of concerns raised around the biological data/inputs and model complexity, an independent peer review was recommended [WCPFC 2020]. 

Based on the 2020 assessment, the median recent (2015–18) spawning biomass was 41% of the levels predicted to occur in the absence of fishing (80% confidence interval 27–52% across the grid of models used) [WCPFC 2021]. There was zero probability that the recent spawning stock biomass had breached the limit reference point (LRP) of 20% of the unfished level. The biological stock is therefore not considered to be recruitment impaired.  

The 2020 assessment also estimated that the median recent (2014–17) fishing mortality was 72% of the level associated with MSY (80% confidence interval 49–102% across the grid of models used). There was a 12.5% probability that the recent fishing mortality was above fishing mortality at MSY. This level of fishing mortality is unlikely to cause the stock to become recruitment impaired.

On the basis of the evidence provided above, the Western and Central Pacific Ocean biological stock is classified as a sustainable stock.

Bigeye Tuna biology [Froese and Pauly 2009; Farley et al. 2017, 2018, 2020]

Distribution of reported Australian commerical catch of Bigeye Tuna in 2021

Commonwealth – Commercial (catch). Catches reported for the Indian Ocean Tuna Commission and Western and Central Pacific Fisheries Commission are for 2021, the most recent year available. 

Commonwealth – Recreational. The Australian Government does not manage recreational fishing in Commonwealth waters. Recreational fishing in Commonwealth waters is managed by the state or territory immediately adjacent to those waters, under its management regulations.  

Commonwealth – Indigenous. The Australian Government does not manage non-commercial Indigenous fishing in Commonwealth waters, with the exception of the Torres Strait. In general, non-commercial Indigenous fishing in Commonwealth waters is managed by the state or territory immediately adjacent to those waters. 

Commercial catch of Bigeye Tuna - note confidential catch not shown

1. Blake, S, Bromhead, D, Patterson, H and Dylewski, M, 2022, Western Tuna and Billfish Fishery, in Patterson, H, Bromhead, D, Galeano, D, Larcombe, J, Timmiss, T, Woodhams, J and Curtotti, R (eds), Fishery status reports 2022, Australian Bureau of Agricultural and Resource Economics and Sciences, Canberra.
2. Chiang, H-C, Hsu, C-C, Wu, GC-C, Chang, S-K and Yang, H-Y 2008, Population structure of Bigeye Tuna (Thunnus obesus) in the Indian Ocean inferred from mitochondrial DNA, Fisheries Research, 90: 305–312.
3. Davies, C, Marsac, F, Murua, H, Fahmi, Z and Fraile, I 2020, Summary of population structure of IOTC species from PSTBS-IO project and recommended priorities for future work, online meeting, 7 to 11 December 2020, IOTC-2020-SC23-11E_rev1, Indian Ocean Tuna Commission.
4. Diaz-Arce, N, Grewe, P, Krug, I, Artetxe, I, Ruiz, J, Nikolic, N, Medieu, A, Pernak, M, Lansdell, M, Aulich, J, Clear, N, Proctor, C, Wudianto, Ruchimat, T, Fahmi, Z, Satria, F, Lestari, P, Taufik, M, Priatna, A, Zamroni, A, Farley, J, Davies, CR, Marsac, F, Fraile, I, Murua, H and Rodriguez-Ezpeleta, N 2020, Evidence of connectivity of bigeye tuna (Thunnus obesus) throughout the Indian Ocean inferred from genome-wide genetic markers. Hal – Open Science, 2020. Hal-03206834.
5. Ducharme-Barth, N, Vincent, M, Hampton, J, Hamer, P, Williams, P and Pilling, G 2020, Stock assessment of bigeye tuna in the western and central Pacific Ocean, working paper WCPFC-SC16-2020/SA-WP-03 (Rev 3), WCPFC Scientific Committee 16th regular session, online meeting, 12 to 19 August 2020.
6. Evans, K, Grewe, P, Foster, S, Gunasekera, R and Lansdell, M 2021, Determination of the spatial dynamics and movement rates of the principal target species within the Eastern Tuna and Billfish Fishery and connectivity with the broader western and central Pacific Ocean – beyond tagging, FRDC Project No 2016/018. Final Report April 2021.
7. Farley, J, Eveson, P, Krusic-Golub, K, Clear, N, Sanchez, C, Roupsard, F, Satoh, K, Smith, N and Hampton, J 2018, Update on age and growth of bigeye tuna in the WCPO WCPFC Project 81, working paper WCPFC-SC14-2018/SA-WP-01, WCPFC Scientific Committee fourteenth regular session, Busan, Republic of Korea, 8–16 August 2018.
8. Farley, J, Eveson, P, Krusic-Golub, K, Sanchez, C, Roupsard, F, McKechnie, S, Nicol, S, Leroy, B, Smith, N and Chang, S-K 2017, Project 35: Age, growth and maturity of bigeye tuna in the western and central Pacific Ocean, working paper WCPFC-SC13-2017/SA-WP-01, WCPFC Scientific Committee thirteenth regular session, Rarotonga, Cook Islands, 9–7 August 2017.
9. Farley, J, Krusic-Golub, K, Eveson, P, Clear, N, Rouspard, F, Sanchez, C, Nicol, S & Hampton, J 2020, Age and growth of yellowfin and bigeye tuna in the western and central Pacific Ocean from otoliths, Technical report SC16-SA-WP-02, WCPFC Scientific Committee 16th regular session, online meeting, 12 to 19 August 2020.
10. Froese, R and Pauly, DE 2009, FishBase, version 02/2014, FishBase Consortium.
11. Grewe, PM and Hampton, J 1998, An assessment of bigeye (Thunnus obesus) population structure in the Pacific Ocean, based on mitochondrial DNA and DNA microsatellite analysis, SOEST 98-05, JIMAR Contribution 98–320, Joint Institute for Marine and Atmospheric Research, University of Hawaii, Honolulu. 
12. Indian Ocean Tuna Commission 2014, Report of the seventeenth session of the Scientific Committee, Seychelles, 8 to 12 December 2014, IOTC-2014-SC-R[E], Indian Ocean Tuna Commission, Victoria, Seychelles.
13. Indian Ocean Tuna Commission 2019, Report of the twentieth session of the Scientific Committee, Karachi, Pakistan, 2–6 December 2019, Indian Ocean Tuna Commission.
14. Indian Ocean Tuna Commission 2021, Report of the 24th session of the Scientific Committee, online meeting, 6 to 10 December 2021, IOTC-2021-SC24-R[E], Indian Ocean Tuna Commission.
15. Moore, BR, Adams, T, Allain, V, Bell, JD, Bigler, M, Bromhead, D, Clark, S, Davies, C, Evans, K, Faasili, U, Farley, J, Fitchett, M, Grewe, PM, Hampton, J, Hyde, J, Leroy, B, Lewis, A, Lorrain, A, Macdonald, JI, Marie, AD, Minte-Vera, C, Natasha, J, Nicol, S, Obregon, P, Peatman, T, Pecoraro, C, Phillip, NB, Pilling, GM, Rico, C, Sanchez, C, Scott, R, Phillips, JS, Stockwell, B, Tremblay-Boyer, L, Usu, T, Williams, AJ and Smith, N 2020, Defining the stock structure of key commercial tunas in the Pacific Ocean II: Sampling considerations and future directions, Fisheries Research, Volume 230: 105524, October 2020.
16. Western and Central Pacific Fisheries Commission 2020, Sixteenth Regular Session of the Scientific Committee – Summary Report, electronic meeting, 12 to 19 August 2020, WCPFC17-2020-SC16, WCPFC.
17. Western and Central Pacific Fisheries Commission 2021, Scientific Committee – WCPO bigeye tuna (Thunnus obesus) – stock status and management advice.