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Australia's Striped Marlin stocks in the South-West Pacific Ocean and the Indian Ocean are both depleted. The two biologically independent stocks are managed by separate international commissions as a resource shared by many countries including Australia.

Photo credit: Riley Page

Striped Marlin in the Indian Ocean is considered to be separate from the stocks in the Western and Central Pacific Ocean, where separate stocks north and south of the equator occur. Here we consider the Indian Ocean stock and the South-West Pacific stock which are managed by the Indian Ocean Tuna Commission (IOTC) and Western and Central Pacific Fisheries Commission (WCPFC) respectively. These two commissions are international organisations established to manage a number of highly migratory fish species within their defined geographic ranges.

Here, stock status is presented at the biological stock level—Indian Ocean and South-West Pacific Ocean.

South-West Pacific Ocean

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

The most recent stock assessment for Striped Marlin in the SWPO (0–40°S, 140°E to 130°W) was in 2019 [Ducharme-Barth et al. 2019; Larcombe et al. 2022]. Influential changes from the previous (2012) assessment included use of standardised catch-per-unit-effort for the Japanese and Chinese Taipei fisheries, calculated using a geostatistical model, and updating the biological information on maturity and defining maturation as a function of length rather than age. 

The median results indicated that stock biomass (SB) was 19.8% of the levels predicted to occur in the absence of fishing in 2018. There was a 50.3% probability that the recent spawning stock biomass had breached the Commonwealth default limit reference point (LRP) (0.2SB₀). This was more pessimistic than the previous (2012) assessment, in which spawning biomass (2006 to 2009) was estimated to be 34% of the levels predicted to occur in the absence of fishing. The stock was estimated to be below the size required to produce maximum sustainable yield.  

Catch in the ETBF is reported to 2021 for all stocks, while international catch from the WCPFC is only available up until 2020. Striped Marlin catch in the ETBF increased slightly in 2021 to 207 t, while catch in the WCPFC (south of the equator) increased slightly from 1,053 t in 2019 to 1,100 t in 2020.

The most recent median estimate of the SWPO spawning biomass (SB) of Striped Marlin [Ducharme-Barth et al. 2019] is very close to, but just below, the 0.2SB₀ LRP adopted in the Commonwealth Harvest Strategy Policy and in the WCPFC for tunas (specifically 20% of the levels predicted to occur in the absence of fishing). The above evidence indicates that the biomass of this stock is likely to be depleted and that recruitment is likely to be impaired. 

The most recent median estimate of fishing mortality was below the level associated with MSY (F_MSY). 

There is no commercial catch of Striped Marlin by fishers operating under NSW jurisdiction, but it is a highly regarded gamefish species targeted by recreational anglers, although the total recreational catch is believed to be small relative to commercial catches [Ducharme-Barth et al. 2019]. Since 1974 over 32,000 Striped Marlin have been caught and tagged within the NSW Gamefish Tagging Program, mostly off the NSW coast. Estimating the total recreational catch is difficult as anglers targeting this species are a relatively small percentage of the broader recreational fishing population, so they are poorly represented in recreational catch surveys [Murphy et al. 2022]. Although 90% of Striped Marlin are released [Lowry and Murphy 2003], mortality rates for post-released fish captured using recreational fishing tackle off Mexico have been estimated as high as 26% [Domeier et al. 2003].

On the basis of the evidence provided above, the South-West Pacific Ocean biological stock of Striped Marlin is classified as a depleted stock.

[Kopf et al. 2011, 2012]

Distribution of reported commercial catch of Striped Marlin

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. Recreational fishing sectors in the Indian Ocean are South Australia, Victoria and Western Australia. Recreational sectors in the Pacific Ocean are New South Wales, Queensland and Tasmania.

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. Indigenous fishing sectors in the Indian Ocean are South Australia, Victoria and Western Australia.

New South Wales – Recreational Murphy et al. [2022].

Commercial catch of Striped Marlin - 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. Domeier, ML, Dewar, H and Nasby-Lucas, N 2003, Mortality rate of striped marlin (Tetrapturus audax) caught with recreational tackle, Marine and Freshwater Research, 54: 435–445.
3. Ducharme-Barth, N, Pilling, G and Hampton, J 2019, Stock assessment of SW Pacific striped marlin in the WCPO, working paper WCPFC-SC15-2019/SA-WP-07, WCPFC Scientific Committee 15th regular session, Pohnpei, Federated States of Micronesia, 12 to 20 August 2019.
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