Western Tuna and Billfish Fishery

Area fished, fishing methods and key species

The Western Tuna and Billfish Fishery (WTBF) operates in Australia's Exclusive Economic Zone and high seas of the Indian Ocean (Figure 24.1). In recent years, fishing effort has concentrated off south-west Western Australia, with occasional activity off South Australia. Domestic management arrangements for the WTBF reflect Australia's commitment to the Indian Ocean Tuna Commission (IOTC; see Chapter 20).

Key species caught in the WTBF are bigeye tuna (Thunnus obesus), yellowfin tuna (T. albacares), albacore (T. alalunga) and swordfish (Xiphias gladius). Striped marlin (Kajikia audax) is a minor component of the catch but remains an important species for management due to historically higher catches. The main fishing gear in the WTBF is pelagic longline, with low levels of minor-line fishing (Table 24.2).

Management methods

Under the management plan, output controls have been implemented in the fishery through individual transferable quotas (ITQs) for the 4 key commercial species (excluding striped marlin) (Table 24.2). Total allowable commercial catches (TACC) are set in accordance with Australia's domestic policies, and apply to the Australian Fishing Zone and the high-seas area of the IOTC area of competence. Previously, the WTBF was to use the harvest strategy framework developed for the Eastern Tuna and Billfish Fishery. However, in 2018, the Australian Fisheries Management Authority (AFMA) Commission decided to stop using this harvest strategy, due, in part, to its unnecessary complexity. Should fishing effort and catch increase significantly in the WTBF, generating sufficient data to develop stock indicators, then a harvest strategy will be developed. Until this time, the WTBF harvest levels will be set in line with outcomes of the IOTC, consistent with the Commonwealth Fisheries Harvest Strategy Policy (HSP) and guidelines (Department of Agriculture and Water Resources 2018a, b).

The biomass default limit reference point in the HSP is used to determine stock status in the WTBF. Under the HSP, the default limit reference point for biomass is 20% of the unfished biomass (0.2B₀), with values below this resulting in an overfished classification. In contrast, the IOTC currently determines a stock to be overfished if its biomass falls below the IOTC target reference point (typically biomass at maximum sustainable yield; B_MSY), even when biomass is above the IOTC interim limit reference points. This can result in the IOTC classifying a stock as overfished when the HSP defines it as not overfished. The IOTC is currently reviewing how it classifies and represents stock status.

Fishing activity

Effort in the WTBF was relatively low (<20 vessels) from the mid-1980s to the mid-1990s (Figure 24.2). Effort increased in the late 1990s, peaking at 50 active vessels in 2000, but then declined rapidly. Since 2005, fewer than 5 vessels have been active in the fishery each year.

Note: SFR Statutory fishing right.
Source: AFMA

Swordfish is the main target species in the WTBF, with annual catches peaking at more than 2,000 t in 2001 (Figure 24.3) and declining to a few hundred tonnes in recent years. Bigeye and yellowfin tuna are also valuable target species, although catches of these species have never been as high as for swordfish and have been more variable.

Source: AFMA

Table 24.2 Main features and statistics for the WTBF

Fishery statistics a	2019			2020
Stock	TACC (t) b	Catch (t)	GVP (2018–19)	TACC (t) b	Catch (t)	GVP (2019–20)
Striped marlin	125	1	Confidential	125	<1	Confidential
Swordfish	3,000	117	Confidential	3,000	96	Confidential
Albacore	n/a	16	Confidential	n/a	16	Confidential
Bigeye tuna	2,000	38	Confidential	2,000	31	Confidential
Yellowfin tuna	5,000	46	Confidential	5,000	18	Confidential
Total	10,125	218	Confidential	10,125	161	Confidential
Fishery-level statistics
Effort	Pelagic longline: 366,821 hooks Minor line: –			Pelagic longline: 231,085 hooks Minor line: –
Fishing permits	94 boat SFRs			94 boat SFRs
Active vessels	Pelagic longline: 2 Minor line: 2			Pelagic longline: 2 Minor line: 1
Observer coverage	12.8% c			12.1% c
Fishing methods	Pelagic longline (monofilament mainline), minor line (handline, rod and reel, troll and poling), purse seine
Primary landing ports	Fremantle and Geraldton (Western Australia)
Management methods	Input controls: limited entry, gear and area restrictions Output controls: TACCs, ITQs, byproduct restrictions
Primary markets	International: Japan, United States – fresh, frozen Domestic: fresh, frozen
Management plan	Western Tuna and Billfish Management Plan 2005 (amended 2016); SFRs issued 2010

a Fishery statistics are provided by calendar year to align with international reporting requirements. Value statistics are by financial year. b The TACC for each stock was first set in 2010, then revised in 2012, and was based on an approximation of the proportion of the total potential yield for the Indian Ocean that is available to the WTBF. c From 1 July 2015, e-monitoring became mandatory for all full-time pelagic longline vessels in the WTBF. At least 10% of video footage of all hauls is reviewed to verify the accuracy of logbooks, which are required to be completed for 100% of shots. Onboard observers are no longer used.
Notes: GVP Gross value of production. ITQ Individual transferable quota. n/a Not applicable. SFR Statutory fishing right. TACC Total allowable commercial catch. – Not available.

Striped marlin (Kajikia audax)

Line drawing: FAO

Stock structure

Mamoozadeh, McDowell & Graves (2020) evaluated genetic variation in striped marlin populations sampled from the eastern and western Indian Ocean, and across the Pacific Ocean. However, the sample size from the eastern Indian Ocean was small (8 fish) and the results were inconclusive. Therefore, striped marlin is currently considered to be a single biological stock for assessments in the Indian Ocean.

Catch history

Catches of striped marlin in the WTBF have been relatively low (<50 t) since the mid-1980s and very low (<5 t) since 2000, with less than 1 t taken in 2020 (Figure 24.4). Total international catches in the IOTC area of competence declined from around 6,000 t in 1995 to around 2,000 t in 2009 (Figure 24.5). Annual catches in 2019 were 2,660 t, which is below the estimated MSY (4,730 t).

Note: TACC Total allowable commercial catch; initial TACC for 19 months.
Source: AFMA

Source: IOTC

Stock assessment

A stock assessment in 2018 for the Indian Ocean–wide stock used 2 assessment models: JABBA, a Bayesian state-space production model, and Stock Synthesis 3 (SS3) (IOTC 2020c). Based on the SS3 model, the 2017 spawning biomass for the Indian Ocean–wide stock was estimated to be 13% (80% confidence interval [CI] 9–14%) of unfished (1950) biomass. Based on the JABBA model, the biomass is 33% (80% CI 18–54%) of the level that supports MSY (IOTC 2020c). Fishing mortality for the Indian Ocean–wide stock was estimated to be above F_MSY (JABBA: F₂₀₁₇/F_MSY = 199%; 95% CI 121–362%). Retrospective analysis for both the JABBA and SS3 models produced consistent stock status estimates, thus providing a degree of confidence in the predictive capabilities of the assessments.

Stock status determination

Both stock assessment models indicate that the Indian Ocean–wide stock has been heavily depleted and is below the Commonwealth's biomass limit reference point (0.2B₀). The stock is therefore classified as overfished. Despite relatively small domestic catches of striped marlin in the WTBF, fishing mortality for the Indian Ocean–wide stock was estimated to be well above F_MSY, so the stock is classified as subject to overfishing.

Swordfish (Xiphias gladius)

Line drawing: Gavin Ryan

Stock structure

Several studies, using both genetics and otolith microchemistry, have investigated the stock structure of swordfish in the Indian Ocean (Davies et al. 2020; Muths et al. 2013). Based on results of these studies, swordfish in the Indian Ocean is considered to be a single biological stock.

Catch history

Annual swordfish catch in the WTBF peaked at around 2,000 t in the early 2000s but has declined to below 350 t since 2005. In 2020, the annual catch was 96 t, a slight decrease from the 2019 catch of 117 t (Figure 24.6). Total international catches of swordfish in the IOTC area of competence peaked in 2004 at more than 40,000 t but declined to around 22,000 t in 2011 (Figure 24.7), likely because of the effects of piracy in the western Indian Ocean (IOTC 2020a). Annual catches in the IOTC area of competence have increased since 2011, reaching 32,482 t in 2019, which is slightly below the estimate of MSY (33,000 t).

Note: TACC Total allowable commercial catch; initial TACC for 19 months.
Source: AFMA

Source: IOTC

Stock assessment

In 2020, the Indian Ocean swordfish assessment was updated using SS3 with data up to 2018 (IOTC 2020c). The SS3 model was spatially disaggregated, sex explicit and age structured. The 2018 spawning biomass for the Indian Ocean–wide stock was estimated to be 42% (80% CI 36–47%) of unfished (1950) biomass and above the level that supports MSY (SB₂₀₁₈/SB_MSY = 175%; 80% CI 128–235%) (IOTC 2020c). Fishing mortality for the Indian Ocean–wide stock was estimated to be below F_MSY (F₂₀₁₈/F_MSY = 60%; 80% CI 40–83%).

Stock status determination

Assessments of the Indian Ocean–wide stock indicate that biomass is above the Commonwealth's biomass limit reference point (0.2B₀) and that swordfish fishing mortality is below F_MSY. As a result, the stock is classified as not overfished and not subject to overfishing.

Albacore (Thunnus alalunga)

Line drawing: FAO

Stock structure

A global genetic study of albacore found that the Atlantic Ocean and Indian Ocean populations were not genetically distinguishable, and found no evidence of genetic heterogeneity within the Indian Ocean (Montes et al. 2012). Similarly, a recent genetics and otolith microchemistry study of a relatively large sample suggests a single stock within the Indian Ocean (Davies et al. 2020). Therefore, albacore is assumed to be a single biological stock in the Indian Ocean for assessments.

Catch history

Historically, albacore catches in the WTBF have been low (Figure 24.8). Since 2004, annual catches have been below 30 t, and were approximately 16 t in 2020. Total international catches in the IOTC area of competence peaked at more than 43,000 t in 2010, and have fluctuated between 30,000 t and 42,000 t since 2011 (Figure 24.9). The catch in 2019 was 39,876 t, which is higher than the 2019 estimate of MSY (35,700 t).

Source: AFMA

Source: IOTC

Stock assessment

In 2019, SS3 was used to assess the Indian Ocean albacore stock and provide management advice (IOTC 2020c). The results from the SS3 model indicated that the current (2017) biomass for the Indian Ocean–wide stock was 26% (CI not available) of unfished (1950) biomass and above the level that supports MSY (SB₂₀₁₇/SB_MSY = 128%; 95% CI 57–207%). Fishing mortality for the Indian Ocean–wide stock was estimated to be above the level that supports MSY (F₂₀₁₇/F_MSY = 135%; 95% CI 59–217%) (IOTC 2020c). This is an increase since the last assessment in 2016 and is due to increased catches by several countries across the Indian Ocean since 2015.

Stock status determination

The assessment indicates that the spawning biomass is above the Commonwealth's biomass limit reference point (0.2B₀), and so the stock is classified as not overfished. Despite relatively small domestic catches of albacore in the WTBF, fishing mortality for the Indian Ocean–wide stock is above F_MSY, and so the stock is classified as subject to overfishing.

Bigeye tuna (Thunnus obesus)

Line drawing: FAO

Stock structure

Bigeye tuna in the Indian Ocean is considered to be a single biological stock for assessments based on genetic studies (Chiang et al. 2008; Davies et al. 2020) that indicated no evidence of intra-oceanic genetic differentiation within the Indian Ocean and tagging studies that have demonstrated large-scale movements of bigeye tuna within the Indian Ocean (IOTC 2014).

Catch history

Annual catches of bigeye tuna in the WTBF have not exceeded 200 t since 2004, with catches over the past 5 years below 100 t. Total international catches in the IOTC area of competence have declined from a peak of more than 160,000 t in 1999 to less than 100,000 t in recent years (Figure 24.11). Bigeye catch was 73,737 t in 2019, which is below the 2019 MSY estimate of 87,000 t.

Note: TACC Total allowable commercial catch; initial TACC for 19 months.
Source: AFMA

Source: IOTC

Stock assessment

In 2019, the 2016 Indian Ocean–wide stock assessment for bigeye tuna was updated using SS3 and JABBA (IOTC 2020c). The SS3 assessment was used to provide management advice. It consisted of 18 model configurations that were designed to account for the uncertainty in the stock–recruitment relationship, the influence of the tagging data and selectivity of longline fleets (IOTC 2020c). The assessment indicated that Indian Ocean spawning biomass was above 31% (80% CI 21–34%) of the initial unfished level. Current (2018) spawning stock biomass in the Indian Ocean was estimated to be above the level that would produce MSY (SB₂₀₁₈/SB_MSY = 122%; 80% CI 82–181%). Fishing mortality for the Indian Ocean–wide stock was above the level associated with MSY (F₂₀₁₅/F_MSY = 120%; 80% CI 70–205%), which is an increase since the last assessment in 2016 due to a significant increase in estimated purse-seine catches in 2018.

Stock status determination

The SS3 assessment indicates that bigeye tuna spawning stock biomass is above the Commonwealth's biomass limit reference point (0.2B₀). As a result, the Indian Ocean bigeye tuna stock is classified as not overfished. Despite relatively small domestic catches of bigeye tuna in the WTBF, fishing mortality for the Indian Ocean–wide stock is above the level that would produce F_MSY, so the stock is classified as subject to overfishing.

Yellowfin tuna (Thunnus albacares)

Line drawing: FAO

Stock structure

Preliminary analysis from a recent genetics and otolith microchemistry study found evidence for 2 distinct groupings of yellowfin tuna in the Indian Ocean, but the spatial delineation of these groups remains unclear (Davies et al. 2020). The stock structure of yellowfin tuna in the Indian Ocean remains uncertain, and the species is considered to be a single biological stock for assessments until the stock structure can be resolved.

Catch history

Since the early 2000s, declining effort in the WTBF has resulted in reduced catches of yellowfin tuna (Figure 24.12). Catches have not exceeded 100 t since 2004 (Figure 24.12). Total international catches in the IOTC area of competence peaked at more than 500,000 t in 2004, then declined for several years (2007 to 2011) because of the effects of piracy in the north-west Indian Ocean (IOTC 2020b). Catches in 2019 were 431,832 t, which is above the level of MSY (approximately 403,000 t).

Note: TACC Total allowable commercial catch; initial TACC for 19 months.
Source: AFMA

Source: IOTC

Stock assessment

In 2018, the 2016 Indian Ocean–wide yellowfin tuna assessment was updated using SS3; it incorporated catch data, size frequency data, tagging data and longline catch-per-unit-effort series (IOTC 2020c). Current spawning biomass for the stock was estimated to be 30% (80% CI 27–33%) of unfished biomass and below the level associated with MSY (SB₂₀₁₇/SB_MSY = 83%; 80% CI 74–97%). The 2017 level of fishing mortality for the Indian Ocean–wide stock was above the level that would achieve MSY (F₂₀₁₇/F_MSY = 120%; 80% CI 100–171%).

Stock status determination

The biomass is above the default limit reference point (0.2B₀), and, as a result, the stock is classified as not overfished. Despite relatively small domestic catches of yellowfin tuna in the WTBF, the assessments indicate that fishing mortality for the Indian Ocean–wide stock is above the level associated with MSY. As a result, the Indian Ocean yellowfin tuna stock is classified as subject to overfishing.

Key economic trends

Economic surveys of the WTBF have not been conducted recently because of the low level of fishing activity. Fewer than 5 longline vessels have operated in the fishery over the past decade and, since 2009−10, effort has reduced significantly, which is reflected in total catch volume decreasing by 63% and a persistently high proportion of uncaught TACCs. Declining effort and high latent effort both suggest that permit holders expect low profitability, and that the fishery achieves relatively low economic returns.

In recent years, relatively high-value species such as yellowfin and bigeye tunas have represented an increasing share of the fishery's total catch. The costs of fishing in the WTBF are generally higher than in the Eastern Tuna and Billfish Fishery, and effort in the WTBF is unlikely to improve without a significant decrease in fishing costs (and/or increase in prices for lower-value fish species).

Performance against economic objective

Current net economic returns (NER) are likely to be very low. Given the current market conditions it is likely that potential NER from the fishery are small. Therefore, the current management arrangements of ITQs and TACCs (set in accordance with the IOTC), while minimising the costs of management, appear to be a sensible strategy. If fishing effort does increase in the future, due to a change in the market conditions, then ITQs should provide an incentive for fishers to catch their share in the most efficient way.

The WTBF has been granted continued export approval under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), expiring on 11 November 2022. Conditions of export approval include a requirement to determine the impact of fishing on shark species and to make demonstrable progress in improving the status of shark bycatch in the WTBF, as well as working with the IOTC to improve the understanding of the status of stocks currently classified as overfished or uncertain.

AFMA's ecological risk assessment in 2009 examined 187 fish species in the WTBF (38 chondrichthyans and 149 teleosts), all of which were classified as being at low risk of potential overfishing based on the level 3 sustainability assessment for fishing effects analysis (Zhou, Smith & Fuller 2009). Although no shark species were identified as high risk, an increase in effort could move some species to a higher-risk category. A priority action identified in the WTBF ecological risk management report (AFMA 2010) is to monitor the catch of, and level of interaction with, sharks. Management of shark interactions in this fishery will be reviewed if the landed amount of any 1 shark species exceeds 50 t within a year. Trip limits on sharks apply, depending on species.

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). These are summarised below.

In 2020, 168 shortfin mako sharks (Isurus oxyrinchus) were hooked in the WTBF; all were released in an unknown condition. Sixteen porbeagles (Lamna nasus) were also released in unknown condition. Eleven leatherback turtles (Dermochelys coriacea) were also hooked and released alive, as was 1 loggerhead turtle (Caretta caretta). Two flesh-footed shearwaters (Ardenna carneipes) were released alive and 1 was dead. One unidentified shearwater was released alive and 1 was dead. Finally, 1 Australian sea lion (Neophoca cinerea) was released alive.

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 2010, Ecological risk management: report for the Western Tuna and Billfish Fishery, Australian Fisheries Management Authority, Canberra.

Chiang, H-C, Hsu, C-C, Wu, GC-C, Chang, S-K & Yang, H-Y 2008, 'Population structure of bigeye tuna (Thunnus obesus) in the Indian Ocean inferred from mitochondrial DNA', Fisheries Research, vol. 90, pp. 305–12.

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, final report submitted to the Indian Ocean Tuna Commission, Victoria, Seychelles.

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

——2018b, Guidelines for the implementation of the Commonwealth fisheries harvest strategy policy, Department of Agriculture and Water Resources, Canberra.

IOTC 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.

——2020a, Report of the 18th session of the IOTC Working Party on Billfish, online meeting, 2 to 4 September 2020, IOTC-2020-WPB18-R[E], Indian Ocean Tuna Commission, Victoria, Seychelles.

——2020b, Report of the 22nd session of the IOTC Working Party on Tropical Tunas, online meeting, 19 to 23 October 2020, IOTC-2020-WPTT22-R[E], Indian Ocean Tuna Commission, Victoria, Seychelles.

——2020c, Report of the 23rd session of the Scientific Committee, online meeting, 7 to 11 December 2020, IOTC-2020-SC-R[E], Indian Ocean Tuna Commission, Victoria, Seychelles.

Mamoozadeh, NR, Graves, JE & McDowell, JR 2020, 'Genome‐wide SNPs resolve spatiotemporal patterns of connectivity within striped marlin (Kajikia audax), a broadly distributed and highly migratory pelagic species', Evolutionary Applications, vol. 13, 677–98.

Montes, I, Iriondo, M, Manzano, C, Arrizabalaga, H, Jiménez, E, Angel Pardo, M, Goni, N, Davies, CA & Estonba, A 2012, 'Worldwide genetic structure of albacore Thunnus alalunga revealed by microsatellite DNA markers', Marine Ecology Progress Series, vol. 471, pp. 183–91.

Muths, D, LeCouls, S, Evano, H, Grewe, P & Bourjea, J 2013, 'Multi-genetic marker approach and spatio-temporal analysis suggest there is a single panmictic population of swordfish Xiphias gladius in the Indian Ocean', PLoS One, vol. 8, e63558.

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