Southern Bluefin Tuna Thunnus maccoyii

Heather Patterson^a

Table 1: Stock status determination for Southern Bluefin Tuna

Jurisdiction	Commonwealth
Stock	Global (CCSBT, SBTF)
Stock status
Stock status	Overfished
Indicators	Spawning stock biomass

CCSBT = Commission for the Conservation of Southern Bluefin Tuna; SBTF = Australian Southern Bluefin Tuna Fishery (Commonwealth)

Stock Structure

Southern Bluefin Tuna constitutes a single, highly migratory biological stock that spawns in the north-east Indian Ocean and migrates throughout the temperate, southern oceans, supporting a number of international fisheries¹.

Stock Status

Global biological stock

The most recent assessment (2011) estimates that the biomass of the Southern Bluefin Tuna biological stock is at 3–7 per cent of unfished spawning stock biomass². The biological stock is recruitment overfished at a global scale, and well below target reference levels chosen by the Commission for the Conservation of Southern Bluefin Tuna (CCSBT)^a+. Recent trends in recruitment appear more positive than in previous assessments. In addition, in 2011 the CCSBT has adopted a management procedure (i.e. harvest strategy) to guide the recovery of the biological stock to 20 per cent of unfished biomass by 2035, as well as a total allowable catch (TAC) setting process. However, measurable improvements in spawning stock biomass are yet to be detected.

On the basis of the evidence provided above, the biological stock is classified as an overfished stock.

Table 2: Southern Bluefin Tuna biology³

Longevity and maximum size	40+ years; 225 cm FL
Maturity (50%)	~11–12 years; 120–130 cm FL

FL = fork length

Figure 1: Distribution of reported commercial catch of Southern Bluefin Tuna in Australian fisheries, 2010

Main features and statistics for Southern Bluefin Tuna stocks/fisheries in Australia in 2010

Australian catch is mainly by commercial purse seine in the Great Australian Bight (96 per cent of total catch), which is transferred to aquaculture cages in South Australian waters. There is a small amount of longline catch (4 per cent). Recreationally, Southern Bluefin Tuna are targeted using rod and reel with bait and artificial lures.
Southern Bluefin Tuna commercial catch is regulated by a global TAC and individual country quotas set by the CCSBT. The CCSBT set the global TAC for 2010 at 9449 tonnes (t), out of which Australia had the largest quota (5265 t in 2010^a++).
In 2010, 6 purse-seine vessels and 18 longline vessels caught Southern Bluefin Tuna in Australia. The number of active vessels globally is not currently available but was approximately 1296 vessels in January 2010.
Total commercial Australian catch in the 2010 season (1 December 2009 – 30 November 2010) was 4091 t. The total global catch for 2010 (calendar year) was 9550 t. Total recreational and Indigenous catch in Australia is not known. However, a survey of recreational fishing in Victoria in 2011 estimated the retained catch at 240 t⁴.

Fig 2b) median spawning stock biomass from the 2011 stock assessment

Figure 2: a) Reported global catch of Southern Bluefin Tuna, 1951–52 to 2009–10 (fishing season)a+++;
b) median spawning stock biomass from the 2011 stock assessment²

Catch Explanation

Commercial catches were very high in the early years of the fishery, before declining steadily in the early 1950s. Catch in the Australian Southern Bluefin Tuna Fishery (Commonwealth) is strongly linked to the global quota. In 2009, the global quota was reduced because of the poor state of the biological stock. Australia's quota was reduced from 5265 t per year to 8030 t for two years (2010 and 2011), while the global quota was reduced from 11 810 t in 2009 to 9449 t for two years (2010 and 2011).

Effects of fishing on the marine environment

Southern Bluefin Tuna was listed as conservation dependent under the Environment Protection and Biodiversity Conservation Act 1999 in 2010.

A Ecological Risk Assessment on non-target species in the purse-seine fishery found that the risk to the sustainability of non-target species was low⁵.

Environmental effects on Southern Bluefin Tuna

Interannual variation in abundance of Southern Bluefin Tuna in the Great Australian Bight is well documented. This variation has not been directly linked to environmental variables⁷, although it is possible that environmental factors play a role.

^a Australian Bureau of Agricultural and Resource Economics and Sciences
^a+ www.ccsbt.org
^a++ This was the amount of southern bluefin tuna available for capture in 2010, based on a combined 8030 t TAC set by Australia for 2010 and 2011.
^a+++ Total global catches exceeded reported global catches over 1985–2005; some scientists estimate unreported catches to have surpassed 178 000 t over this period⁶.