| Summary | Southern Bluefin Tuna (SBT; Thunnus maccoyii) is a migratory and valuable pelagic fish, and used as the fisheries proxy for this case. Juvenile Southern Bluefin Tuna are targeted in the Great Australian Bight by Australian purse seine fishing vessels, and towed to Port Lincoln where they are transferred to grow-out cages and fed intensively for 6–8 months before being harvested and exported to Japan. At the time of capture, these juvenile fish are predominantly in the two to three year age class, with small numbers of one and four year old fish (Phillips et al. 2009). More than 95% of Australia’s total catch of the species is taken by this method (TSSC 2010aw). The species has undergone very severe reduction in numbers as a result of heavy fishing pressure throughout its range. The Southern Bluefin Tuna is one of the most highly valued fish species for sashimi, especially in Japan. The SBT fishery is managed by the Australian Fisheries Management Authority (AFMA) under the Fisheries Management Act (1991) and the SBT Fishery Management Plan Amendment Plan (2004). |
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| Project | SESMAD |
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| Subtype | Natural Resource Unit |
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| Sector | Marine protected areas |
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| Biotic | Yes |
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| Explanation | |
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| Commons Aggregation | Population |
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| Explanation | A single spawning area, morphological uniformity, distribution of larvae and tag return data suggest that SBT area a single population (Caton 1991; Grewe et al. 1997; Farley and Davis 1998; Patterson et al. 2008) High vagility in relation to the geographical distribution also indicates limited potential for population differentiation (Caton1994). |
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| Commons Boundaries | Somewhat unclear boundaries (2) |
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| Explanation | SBT have a circumglobal distribution and are found in oceans of the southern hemisphere, from the tropics to the sub-Antarctic (30–50° S). A single spawning ground is known, in the north-eastern Indian Ocean between Java and Western Australia (7 to 20 degrees south). The juveniles are thought to ride the Leeuwin Current down the coast of Western Australia to spend summer months in south Western Australia and the Great Australian Bight region with an unknown proportion heading westwards towards South Africa (Honda et al. 2010; NSW DPI FSC n.d.; Farley and Davis 1998). Two to four year old SBT school near the surface during the summer months (December - April) in the coastal waters of the Great Australian Bight and spend winter months either in deeper, temperate waters or seasonally migrate between the south coast of Australia and the central Indian Ocean (CCSBT 2009). After 5 years of age, SBT move offshore to deeper waters, swimming between feeding grounds, areas of high productivity spread between New Zealand and South Africa (Farley et al. 2007). As mature adults they join the spawning migration to the tropics below Java. |
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| Commons Indicator | ["Status of species targeted by fisheries", "Status of highly migratory species"] |
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| Explanation | Status of species targeted by fisheries - Juvenile SBT (usually between 2-3 years old) are caught in the Great Australian Bight using purse seine methods, towed to Port Lincoln where they are transferred to grow-out cages, and then fed intensively for 6-8 months before being harvested and exported to Japan. More than 95% of Australia's total catch of this species is taken by this method (TSSC 2010). SBT are targeted throughout the rest of its global range by pelagic longliners that harvest fish from all age classes, from juveniles to adults over 12 years old (Phillips et al. 2009).
SBT are also a migratory species (although not used as a proxy for this in this case) - they have a circumglobal distribution and are found throughout oceans of the southern hemisphere, from the tropics to the sub-Antarctic (30–50° S).
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| Commons Unit Size | Medium (3) |
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| Explanation | SBT can reach 2.25m fork length and weigh >200 kg, although they more commonly reach 1.80m fork length and 100kg (Yearsely et al. 1999; BRS 2008). |
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| Environmental Medium | Oceanic |
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| Explanation | Oceans of the southern hemisphere between 30–50° S. |
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| Inter Annual Predictability | Moderate (2) |
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| Explanation | The availability or prevalence of SBT are predicted using scientific stock assessment methods. There is controversy over management actions and initiatives (e.g. implications of alternative interpretations of catch per unit effort) for bluefin tuna (Butterworth et al. 2003) but if a consistent methodology is followed relative magnitudes can be assessed.
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| Intra Annual Predictability | High (3) |
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| Explanation | The availability or prevalence of this commons is predictable within years - there is little seasonal variation in commons availability.
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| Commons Renewability | Renewable (1) |
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| Explanation | Renewable but vulnerable to exploitation and potentially slow to recover from fishing. |
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| Productivity | Poorly productive (1) |
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| Explanation | The SBT is long-lived and highly fecund but characteristics such as slow-growth, late onset of sexual maturity (at approximately 11-12 years of age), the presence of a single spawning ground, and highly migratory behaviour (exposing the stock to multiple fishing fleets) make it vulnerable to exploitation and potentially slow to recover from fishing (Collette and Nauen 1983; Pogonoski et al. 2002; BRS 2008; Gunn et al. 2008). |
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| Commons Accessibility | Very accessible (3) |
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| Explanation | Easily accessible for modern oceanic fishing fleets that target them. Two - four year old SBT school near the surface waters of the Great Australian Bight and are targeted by the South Australian purse seine fishery. |
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| Commons Heterogeneity | Moderate (2) |
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| Explanation | Low Commons Heterogeneity - Sexually mature SBT congregate in a single spawning ground located in the north-east Indian Ocean between Java and Western Australia (Caton, 1991; NSW DPI FSC n.d.; BRS 2008). Spawning occurs from August - April with a peak from October - February (Honda et al. 2010).
SBT are highly migratory, however they congregate in a single spawning ground therefore coded as moderate heterogeneity.
High Commons Heterogeneity - For the rest of year the tuna are distributed throughout the south-west and south-east Atlantic Ocean, eastern and western Indian Ocean and the south-west Pacific Ocean (Collette et al. 2011). Tuna are suggested to congregate at seamounts, lumps and reefs in the Great Australian Bight where prey species also congregate, and to move depending on water masses, such as influxes of nutrient rich sub-Antarctic waters, and sea temperatures (Fujioka et al. 2010) |
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| Commons Mobility | High (3) |
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| Explanation | SBT are highly migratory, occurring throughout the southern hemisphere oceans (Indian, Atlantic and Pacific Oceans) between 30–50° S, though the species is mainly found in the eastern Indian Ocean and in the south-west Pacific Ocean (BRS 2008, TSSC 2010). They make long distance migrations to a single spawning ground in the north-east Indian Ocean (BRS 2008). The species can travel up to 70 kilometres per hour while feeding (TSSC 2010). |
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| Commons Spatial Extent | 19395 |
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| Explanation | This is the approximate spatial extent of the GABMP (CW) (2012) that this environmental commons uses but SBT are highly migratory and are found throughout the southern hemisphere oceans between 30–50° S. |
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| Technical Substitute | No |
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| Explanation | |
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