Skipjack Tuna Fishery
Chapter 22: Skipjack Tuna Fishery
H Patterson and D Mobsby
FIGURE 22.1 Area fished in the Skipjack Tuna Fishery, 2007–08 to 2017–18
Note: The last effort in the fishery occurred in 2008–09.
Biological status a, b
|Fishing mortality||Biomass||Fishing mortality||Biomass|
|Indian Ocean skipjack tuna (Katsuwonus pelamis)||Not subject to overfishing||Not overfished||Not subject to overfishing||Not overfished|
No Australian vessels fished in 2018. Current estimates of fishing mortality in the Indian Ocean are less than the target reference point. Spawning biomass is above the limit reference point and at the target reference point.
|Western and central Pacific Ocean skipjack tuna (Katsuwonus pelamis)||Not subject to overfishing||Not overfished||Not subject to overfishing||Not overfished|
No Australian vessels fished in 2018. Current estimates of fishing mortality in the WCPO are below FMSY. Spawning biomass is above the limit reference point.
|Economic status||No Australian vessels fished in 2017 or 2018. Fishing is opportunistic, and highly dependent on availability and the domestic cannery market. Currently, no domestic cannery has active contracts for skipjack tuna.|
a Ocean-wide assessments and the default limit reference points from the Indian Ocean Tuna Commission are used as the basis for determining the status of Indian Ocean skipjack tuna. b Ocean-wide assessments and the limit reference point from the Western and Central Pacific Fisheries Commission are used as the basis for determining the status of Pacific Ocean skipjack tuna.
Notes: FMSY Fishing mortality at maximum sustainable yield. MSY Maximum sustainable yield. WCPO Western and central Pacific Ocean.
22.1 Description of the fishery
Two stocks of skipjack tuna (Katsuwonus pelamis) are thought to exist in Australian waters: one on the east coast that is part of a broader stock in the Pacific Ocean and one on the west coast that is part of a larger stock in the Indian Ocean. The two 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 two stocks are assessed separately. The ESTF and the WSTF extend through the same area as the Eastern Tuna and Billfish Fishery (ETBF; Chapter 21), and the Western Tuna and Billfish Fishery (WTBF; Chapter 24), respectively, with the exception of an area of the ETBF off northern Queensland (Figure 22.1). Australian waters are at the edge of the species’ range, with centres of abundance in the equatorial waters of the Indian and Pacific oceans. Availability of skipjack tuna in both the ESTF and the WSTF is highly variable. The Indian Ocean stock of skipjack tuna is managed under the jurisdiction of the Indian Ocean Tuna Commission (IOTC), whereas the stock found in the western and central Pacific Ocean (WCPO) is managed under the jurisdiction of the Western and Central Pacific Fisheries Commission (WCPFC).
Fishing methods and key species
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.
The 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, but have been defined in both the IOTC (on an interim basis) and the WCPFC. These reference points are consistent with those prescribed by 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 Conservation and Management Measure 2018-01. If the ESTF or WSTF become active again (see ‘Fishing effort’), 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. Catches of yellowfin tuna (Thunnus albacares) and bigeye tuna (T. obesus), which are often caught incidentally in purse-seine fisheries targeting skipjack, are limited by trip and season limits.
There has been no fishing effort in the STF since the 2008–09 fishing season. Variability in the availability of skipjack tuna in the Australian Fishing Zone and the prices received for product influence participation levels in the fishery.
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 885 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.
TABLE 22.2 Main features and statistics for the STF
Fishery statistics a
2016–17 fishing season
2017–18 fishing season
ESTF: 17; WSTF: 14
ESTF: 17; WSTF: 14
ESTF purse seine: 0
ESTF purse seine: 0
Purse seine (predominant), pole-and-line methods (when poling is used on its own, it is managed as a minor-line component of the ETBF and the WTBF)
Primary landing ports
None; previously Port Lincoln (South Australia) cannery, which closed in May 2010
Input controls: limited entry, gear (net size), area controls, transhipment controls
Domestic and international: currently none
Skipjack Tuna Fishery management arrangements 2009 (AFMA 2009)
a Fishery statistics are provided by fishing season, unless otherwise indicated. Fishing season is 1 July – 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. TAC Total allowable catch. WSTF Western Skipjack Tuna Fishery. WTBF Western Tuna and Billfish Fishery. – Not applicable.
22.2 Biological status
Indian Ocean skipjack tuna (Katsuwonus pelamis)
Line drawing: FAO
Skipjack tuna in the Indian Ocean is considered to be a single stock for stock assessment purposes. Tagging studies have shown large movements of skipjack tuna in the Indian Ocean and support the assumption of a single biological stock (IOTC 2014).
Total catch of skipjack tuna in the Indian Ocean increased slowly from the 1950s, reaching around 50,000 t in the 1970s. With the expansion of the purse-seine fleet in the early 1980s, catch increased rapidly to a peak of 610,000 t in 2006. Since the peak, purse-seine catch has declined, particularly in the areas off Somalia, Kenya and Tanzania, and around the Maldives. A similar decline has occurred in the catch taken by Maldivian pole-and-line vessels. These reduced catches may be partially explained by drops in effort related to the effects of piracy in the western Indian Ocean. Total catch in the IOTC area increased from 474,333 t in 2016 to 524,175 t in 2017 (Figure 22.2).
Historically, effort in the WSTF has been low. In 2005–06, catch was 446 t, before nearly doubling to 847 t in 2006–07 and 885 t in 2007–08. There has been no fishing in the WSTF since 2008–09.
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 stock biomass (SB) at or above the target reference point (0.4SB0), while avoiding the limit reference point (0.2SB0). The HCR requires a stock assessment to be conducted every three years. Estimates from the stock assessment of current SB, SB0 and the exploitation rate associated with maintaining the stock at 40% of SB0 are used to calculate the total annual catch limit. Application of the HCR provides a total annual catch limit for the following three years.
The Indian Ocean skipjack tuna stock assessment was updated in 2017 using Stock Synthesis 3. The updated assessment produced results that differed substantially from the previous assessments in 2011 and 2014 for several reasons, including:
- the correction of an error associated with selectivity for small fish
- the addition of tag–release mortality
- the inclusion of 1% effort creep per year since 1995 for European purse-seine catch-per-unit-effort (IOTC 2017).
The assessment estimated that the stock biomass is at the target reference point and above the limit reference point (SB2016/SB0 = 0.40; range 0.35–0.47). Catch (C) in 2016 (446,723 t) and the average catch over the past five years (2012–2016; 407,450 t) were lower than the estimated catch required to maintain the stock at the target biomass level (C40%SSB = 510,100 t; range 455,900–618,800 t; Figure 22.2).
The total annual catch limit for the Indian Ocean skipjack tuna stock calculated by applying the HCR was 470,029 t for the period 2018–2020. There is no allocation of this total annual catch limit among Member States of the IOTC unless the stock biomass estimated from the stock assessment falls below 0.40SB0.
Stock status determination
The results of the current assessment indicate that the spawning biomass is at the target reference point of 40% of unfished biomass and above the limit reference point of 20% of unfished biomass. As a result, the stock is classified as not overfished. The current (2017) level of catch, a proxy for fishing mortality, was above the level estimated to maintain the stock at the target biomass level. However, given that the average catch over the previous five years was well below this level, and the relatively high biomass, the current level of fishing mortality is unlikely to have reduced the stock below the limit reference point, and the stock is therefore classified as not subject to overfishing.
Western and central Pacific Ocean skipjack tuna (Katsuwonus pelamis)
Skipjack tuna in the WCPO is considered to be a single stock for stock assessment purposes (Rice et al. 2014).
Catch of skipjack tuna in the WCPO increased steadily throughout the 1980s as a result 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 11 years (Figure 22.3).
Historically, effort in the ESTF has been very low. Catch has only been registered once in the past 13 years, with 44 t caught in 2005–06.
The skipjack tuna stock assessment for the WCPO was updated in 2016 using MULTIFAN-CL software (McKechnie et al. 2016). The update incorporated three additional years of data, including a period of El Niño conditions, and the recommendations of the previous assessment (Rice et al. 2014). The outcome of the updated assessment is largely similar to that of the previous assessment. The base case in the assessment estimated that the latest (2015) spawning biomass was 58% of the level predicted to occur in the absence of fishing (SBlatest/SBF=0 = 0.58; range 0.39–0.68 across the base case and sensitivities) and well above the adopted limit reference point of 0.2SBF=0. Current fishing mortality (2011–2014 average) was estimated to be below the fishing mortality that will support maximum sustainable yield (MSY) (Fcurrent/FMSY = 0.45; range 0.38–0.64 across the base case and sensitivities). In 2015, the catch in the WCPFC area was 1,831,440 t (Figure 22.3); this is below the updated estimate of MSY (1,891,000 t). The 2015 catch was above the five-year average of 1,791,788 t.
Stock status determination
The results of the assessment indicate that the spawning biomass is relatively high and above the WCPFC 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.
22.3 Economic status
Key economic trends
Vessels have not been active in the STF since the 2008–09 fishing season; therefore, net economic returns (NER) are estimated to be slightly negative, reflecting the small cost of managing the fishery. Few vessels have fished in either the ESTF or the WSTF since 2003–04, suggesting that there is little economic incentive to fish. Opportunistic fishing was previously prominent in the STF, since the stock availability in Australian waters is highly variable from year to year. Historically, effort has largely depended on both fish availability and the existence of a domestic tuna canning market. Currently, there is no domestic cannery with active contracts for skipjack tuna.
The harvest strategy in place for the fishery is based on catch-level triggers that initiate management action and close monitoring of the fishery once catches exceed a certain level. Currently, 17 permits are issued in the ESTF and 14 in the WSTF. These are held by 15 companies, 6 of which hold one or more permits for both fisheries (AFMA 2019a, b). This implies that, if operational and market conditions were to change dramatically, fishing effort could be activated. It is unlikely that an increase in effort in the Australian skipjack tuna fisheries in the short term would negatively affect stocks and future NER flows, because the Australian catch is likely to be a relatively small proportion of the global skipjack tuna catch.
Performance against economic objective
The harvest of stocks that are internationally shared complicates both the selection of economic-based targets and the assessment of economic status against maximum economic yield (MEY). Assessment is particularly complicated when the Australian catch is a relatively small proportion of the total international catch. For the STF, reductions in any Australian catch in the fishery may not necessarily lead to an increase in stock and, therefore, profitability in the long term. Consequently, a BMEY target for the STF alone is not appropriate. Given these characteristics and no catch in the fishery since the 2008–09 fishing season, continuation of the low-cost management approach currently applied in the fishery is appropriate.
22.4 Environmental status
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. 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. Based on this assessment, which considered finfish and chondrichthyans, no species was considered to be 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 high risk in the level 2 ERA process (Daley et al. 2007). Mammals were not considered in the level 3 assessment. 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).
To date, no protected species interactions have been reported in the STF.
AFMA 2008, Skipjack tuna harvest strategy, Australian Fisheries Management Authority, Canberra.
—— 2009, Skipjack Tuna Fishery management arrangements 2009, AFMA, Canberra.
—— 2010, Ecological risk management report for the Skipjack Tuna Fishery, AFMA, Canberra.
—— 2019a, Western Skipjack Fishery permit holders—6 March 2019, Excel spreadsheet, AFMA, Canberra.
—— 2019b, Eastern Skipjack Fishery permit holders—6 March 2019, Excel spreadsheet, AFMA, 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 AFMA, Canberra.
Department of Agriculture and Water Resources 2018, Commonwealth Fisheries Harvest Strategy Policy, Department of Agriculture and Water Resources, Canberra.
IOTC 2014, Report of the sixteenth session of the Scientific Committee, Seychelles, 8–12 December 2014, IOTC-2014-SC17-R[E], Indian Ocean Tuna Commission, Victoria, Seychelles.
—— 2016, On harvest control rules for skipjack tuna in the IOTC area of competence, resolution 16/02, IOTC, Victoria, Seychelles.
—— 2017, Report of the twentieth session of the Scientific Committee, Seychelles, 30 November – 4 December 2017, IOTC-2017-SC20-R[E], IOTC, Victoria, Seychelles.
McKechnie, S, Hampton, J, Pilling, GM & Davies, N 2016, ‘Stock assessment of skipjack tuna in the western and central Pacific Ocean’, working paper WCPFC-SC12-2016/SA-WP-04, WCPFC Scientific Committee 12th regular session, Bali, Indonesia, 3–11 August 2016.
Rice, J, Harley, S, Davies, N & Hampton, J 2014, ‘Stock assessment of skipjack tuna in the western and central Pacific Ocean’, working paper WCPFC-SC10-2014/SA-WP-05, WCPFC Scientific Committee 10th regular session, Republic of the Marshall Islands, 6–14 August 2014.
Zhou, S, Fuller, M & Smith, T 2009, Rapid quantitative risk assessment for fish species in seven Commonwealth fisheries, report to AFMA, Canberra.