Skipjack Tuna Fishery

Area fished, fishing methods and key species

Two stocks of skipjack tuna (Katsuwonus pelamis) are thought to exist in Australian waters: 1 on the east coast that is part of a broader stock in the Pacific Ocean and 1 on the west coast that is part of a larger stock in the Indian Ocean. The 2 stocks are targeted by separate fisheries: the Eastern Skipjack Tuna Fishery (ESTF) and the Western Skipjack Tuna Fishery (WSTF). These are collectively termed the Skipjack Tuna Fishery (STF), but the 2 stocks are assessed separately.

Historically, most fishing effort has used purse-seine gear (about 98% of the catch). A small amount of pole-and-line effort (when poling is used on its own) is managed as a minor-line component of the ETBF and the WTBF. Skipjack tuna are also caught as bycatch in the ETBF and WTBF longline fisheries.

Management methods

The Indian Ocean stock of skipjack tuna is managed under the jurisdiction of the Indian Ocean Tuna Commission (IOTC), and the stock in the western and central Pacific Ocean (WCPO) is managed under the jurisdiction of the Western and Central Pacific Fisheries Commission (WCPFC).

The domestic skipjack tuna harvest strategy consists of a series of catch-level triggers that invoke control rules (AFMA 2008). The control rules initiate closer monitoring of the ESTF and the WSTF, semi-quantitative assessments and revision of trigger levels. The catch triggers are set at different levels for the ESTF and the WSTF, based on historical catch of skipjack tuna in the domestic fisheries and regional assessments of stock status. Management action is only initiated when there is clear evidence of a significant increase in catches.

Target and limit reference points are not defined in the Australian skipjack tuna harvest strategy. However, target and limit reference points have been defined by both the IOTC (on an interim basis) and the WCPFC and are consistent with the requirements of the Commonwealth Fisheries Harvest Strategy Policy (HSP; Department of Agriculture and Water Resources 2018). Catches of skipjack tuna in the ESTF are currently limited to 30,000 t under WCPFC Conservation and Management Measure 2020-01. If the ESTF or the WSTF become active again, the Australian Fisheries Management Authority (AFMA) will review the Australian skipjack tuna harvest strategy to take account of both the revised HSP, and progress towards WCPFC or IOTC harvest strategies and allocations.

In the IOTC, a harvest control rule (HCR) was adopted for the Indian Ocean skipjack tuna stock in 2016 (IOTC 2016). The HCR seeks to maintain the skipjack tuna spawning biomass (SB) at or above the target reference point of 40% of unfished biomass (0.4SB₀), while avoiding the limit reference point of 20% of unfished biomass (0.2SB₀). Estimates from the stock assessment of current SB, SB₀ and the exploitation rate associated with maintaining the stock at 40% of unfished biomass are used to calculate the total annual catch limit for the following 3 years.

In the WCPFC, a limit reference point of 20% of biomass in the absence of fishing (0.2SB_F=0) has been adopted, with a target reference point of 50% of biomass in the absence of fishing (0.5SB_F=0) that is currently under review. Management strategy evaluation and testing of HCRs are ongoing.

Fishing activity

Globally, catch of skipjack tuna has increased steadily since the 1970s, and skipjack tuna has become one of the most commercially important tuna species in both the Indian and Pacific oceans. Catch in the STF increased for a short period from 2005 to 2008, peaking at 817 t in 2007–08. The catch was supplied almost exclusively to the cannery in Port Lincoln. However, the cannery closed in 2010, and there has been no catch in the STF since the 2008–09 fishing season.

a Fishery statistics are provided by fishing season, unless otherwise indicated. Fishing season is 1 July to 30 June. Value statistics are provided by financial year.
Notes: ESTF Eastern Skipjack Tuna Fishery. ETBF Eastern Tuna and Billfish Fishery. GVP Gross value of production. n/a Not applicable. TAC Total allowable catch. WSTF Western Skipjack Tuna Fishery. WTBF Western Tuna and Billfish Fishery.

Indian Ocean skipjack tuna (Katsuwonus pelamis)

Line drawing: FAO

Stock structure

Skipjack tuna in the Indian Ocean is considered to be a single stock for stock assessment purposes. However, a recent analysis of 368 individuals indicated pronounced genetic differentiation between north-west Indian Ocean (Arabian Sea) and other Indian Ocean locations (Davies et al. 2020). Additional sampling and analysis will be required to confirm the temporal stability of this pattern and the implications for future stock assessments.

Catch history

Total catch of skipjack tuna in the Indian Ocean increased slowly from the 1950s, reaching a peak of 610,000 t in 2006. After a period of decline, catches started increasing in 2013 and peaked again in 2018. Total catch in the IOTC area decreased from 608,906 t in 2018 to 549,822 t in 2019 (Figure 22.2).

Historically, effort in the WSTF has been low and was 885 t in 2007–08. There has been no fishing in the WSTF since 2008–09.

Note: IOTC Indian Ocean Tuna Commission
Source: IOTC

Stock assessment

The Indian Ocean skipjack tuna stock assessment was updated in 2020 using Stock Synthesis 3; it did not differ significantly from the previous assessment in 2017.

The assessment estimated that the stock biomass in 2019 was above the target reference point at 45% of unfished levels (80% confidence interval [CI] 38–50%). The estimated equilibrium annual exploitation rate associated with the stock at the target biomass was 92% (80% CI 67–121%). Recent average yearly catch over the past 5 years (2015 to 2019; ~506,874 t) is also within the estimated range of catch at the target reference point (C_0.4SB0 = 535,964 t; 80% CI 461,995–674,536 t).

The 2020 IOTC Scientific Committee noted that catches in 2018–19 exceeded the catch limit (470,029 t) for 2018 to 2020 that was calculated by applying the HCR. However, the 2020 stock assessment estimated a more productive recent stock (possibly due to environmental conditions) that supported those catch levels.

Stock status determination

The results of the current assessment indicate that the spawning biomass is above the limit reference point of 20% of unfished biomass. As a result, the stock is classified as not overfished. The equilibrium exploitation rate is below that associated with the target biomass; therefore, the current level of fishing mortality is unlikely to reduce the stock below the limit reference point and the stock is classified as not subject to overfishing.

Western and central Pacific Ocean skipjack tuna (Katsuwonus pelamis)

Stock structure

Skipjack tuna in the WCPO is considered to be a single stock for stock assessment purposes (Vincent, Pilling & Hampton 2019).

Catch history

Catch of skipjack tuna in the WCPO increased steadily throughout the 1980s because of growth in the international purse-seine fleet, before stabilising at around 1,000,000 t in the 1990s. Rapid increases in catch in the western equatorial zone have resulted in catches exceeding 1,500,000 t for each of the past 12 years and in 2019 was 2,045,970 t (Figure 22.3).

Historically, effort in the ESTF has been very low. Catch has only been registered once in the past 15 years, with 44 t caught in 2005–06.

Note: WCPFC Western and Central Pacific Fisheries Commission.
Source: WCPFC

Stock assessment

The skipjack tuna stock assessment for the WCPO was updated in 2019 using data to the end of 2018 (Vincent, Pilling & Hampton 2019). Key changes from the 2016 assessment included the adoption of a new 8-region model structure (compared with the previous 5-region structure); updated maturity and length information; and changes to a range of other inputs and assumptions, including treatment of tagging data, growth, length–weight and data weightings. The assessment grid included only the 8-region model structure (considered to best capture the biology of skipjack tuna) and 4 axes of uncertainty (steepness, growth, length composition influence and assumed tag mixing period), with some relatively minor down-weighting of the values on 2 axes (steepness and length composition influence).

The median estimate of the recent (2015 to 2018) spawning biomass was 44% (80% probability interval [PI] 37–53%) of the levels predicted to occur in the absence of fishing. The model predicted a 0% probability that the recent spawning biomass had breached the adopted limit reference point. The median recent fishing mortality (F) was 45% (80% PI 34–60%) of the level associated with maximum sustainable yield (MSY). The model predicted a 0% probability that the recent fishing mortality was above F_MSY.

Stock status determination

The results of the assessment indicate that the spawning biomass is above the limit reference point of 20% of the spawning biomass predicted to occur in the absence of fishing. As a result, the stock is classified as not overfished. The current level of fishing mortality is also below the level required to achieve MSY, so the stock is classified as not subject to overfishing.

Key economic trends

Vessels have not been active in the STF since the 2008–09 fishing season. In previous years, effort has depended largely on fish availability and domestic tuna canning facilities. No Australian canneries have active contracts for skipjack tuna.

Performance against economic objective

The absence of fishing since 2008–09 indicates limited economic incentive to fish in the STF. With no fishing, net economic returns from fishing are zero, so continuing the current low-cost management approach is appropriate.

In 2016, the STF received a 10-year exemption from export provisions (until 9 October 2026) and was accredited under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Approval is on the condition that AFMA reviews the fishery's management regime within 12 months of a level 2a trigger being reached.

The STF had previously undergone the ecological risk assessment (ERA) process up to level 3 (now called level 2.5). Based on this assessment, which considered finfish and chondrichthyans, no species was at high risk because of the low fishing effort in the fishery (Zhou, Fuller & Smith 2009). However, 25 species of marine mammals were identified as at high risk in the level 2 ERA process (Daley et al. 2007). The ecological risk management report for the fishery is therefore designed to achieve adequate monitoring to establish the level of interaction that may occur if effort increases, and to quantify the effect of the fishery on the marine mammal species identified as being at high risk (AFMA 2010).

In accordance with accreditation under the EPBC Act (see Chapter 1, 'Protected species interactions'), AFMA publishes and reports quarterly on interactions with protected species on behalf of Commonwealth fishing operators to the Department of Agriculture, Water and the Environment (DAWE). To date, no protected species interactions have been reported in the STF.

These reported interactions with protected species form part of the ongoing monitoring by DAWE of the performance of fisheries within their accreditation under the EPBC Act.

AFMA 2008, Skipjack tuna harvest strategy, Australian Fisheries Management Authority, Canberra.

——2010, Ecological risk management report for the Skipjack Tuna Fishery, Australian Fisheries Management Authority, Canberra.

——2015, Skipjack Tuna Fishery management arrangements 2015, Australian Fisheries Management Authority, Canberra.

Daley, R, Dowdney, J, Bulman, C, Sporcic, M, Fuller, M, Ling, S & Hobday, A 2007, Ecological risk assessment (ERA) for effects of fishing: Skipjack Tuna Fishery, report to the Australian Fisheries Management Authority, Canberra.

Davies, C, Marsac, F, Murua, H, Fraile, I, Fahmi, Z, Farley, J, Grewe, P, Proctor, C, Clear, N, Eveson, P, Lansdell, M, Aulich, J, Feutry, P, Cooper, S, Foster, S, Rodríguez-Ezpeleta, N, Artetxe-Arrate, I, Krug, I, Mendibil, I, Agostino, L, Labonne, M, Nikolic, N, Darnaude, A, Arnaud-Haond, S, Devloo-Delva, F, Rougeux, C, Parker, D, Diaz-Arce, N, Wudiano, Ruchimat, T, Satria, F, Lestari, P, Taufik, M, Priatna, A & Zamroni, A 2020, Study of population structure of IOTC species and sharks of interest in the Indian Ocean using genetics and microchemistry: 2020 final report to IOTC, paper submitted to the Indian Ocean Tuna Commission, Victoria, Seychelles.

Department of Agriculture and Water Resources 2018, Commonwealth fisheries harvest strategy policy, Department of Agriculture and Water Resources, Canberra.

IOTC 2016, On harvest control rules for skipjack tuna in the IOTC area of competence, resolution 16/02, Indian Ocean Tuna Commission, Victoria, Seychelles.

Vincent, MT, Pilling, GM & Hampton, J 2019, Stock assessment of skipjack tuna in the western and central Pacific Ocean, working paper WCPFC-SC15-2019/SA-WP-05-Rev1, WCPFC Scientific Committee 15th regular session, Pohnpei, Federated States of Micronesia, 12 to 20 August 2019.

Zhou, S, Fuller, M & Smith, T 2009, Rapid quantitative risk assessment for fish species in seven Commonwealth fisheries, report to the Australian Fisheries Management Authority, Canberra.