FISH FACTS: Southern bluefin tuna, whats the catch?

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Southern bluefin tuna. Image: Jamie Crawford

SOUTHERN bluefin tuna (Thynnus maccoyii, or SBT) are one of Australia’s premier gamefish. But they are also an extremely high value fisheries product, due to their popularity as sashimi in Japan. Unlike other tunas such as yellowfin and skipjack which spawn at regular intervals over a wide part of their range, SBT are a long lived, late maturing species which spawns only in one location. Even then, it’s not clear what proportion of the spawning stock actually spawns each year, or even how often individual fish spawn.

Another factor which complicates management of SBT stocks is they are a highly migratory oceanic species, which range throughout the southern hemisphere mainly in temperate waters between 30 and 50 degrees south. Here they can live for at least 40 years, reach weights of over 200 kilograms, and grow to more than 2.3 metres in length. The only known spawning area for SBT is in the Indian Ocean off the coast of north western Australia near Java. Here, adult fish (around 1.6 metres long and no younger than 8 years old) aggregate to breed between September and April.

The larvae and juveniles are carried south down the west coast of Australia with the Leeuwin current into the Great Australian Bight. Growth is relatively rapid, but slower than for other tunas. Juvenile SBT reach on average around 50 centimetres long in their first year, 78 centimetres in the second and 100 centimetres in their third year of growth. SBT of all sizes are active feeders on pelagic squid and fishes, but adult fish are also known to dive to at least 500 metres in search of prey. They can tolerate a wide range of water temperatures because of their advanced circulatory system which keeps their core body temperature warmer than the surrounding water.

It’s this tricky combination of biology and high economic value which makes SBT vulnerable to overfishing. Indeed, due to decades of commercial overfishing (the global catch peaked at 81000 tonnes in 1961, mostly taken by the Japanese fleet), prior to 2010 the SBT stock was estimated as being only around 5 per cent of its original unfished biomass. However, since 2010, tough fisheries management measures have seen the stock rebuilding by several percent per year. This recovery has seen the resurgence of the recreational fishery for “school sized” SBT in areas offshore from WA, South Australia and Victoria, with fewer (but larger) fish being encountered more frequently in Tasmania and NSW. In 2020 the biomass estimate was 20 per cent, which bought SBT off the “threatened” lists to be classified as a “sustainably fished” (albeit still rebuilding) fishery in the “Status of Australian Fish Stocks Reports”. The eventual rebuilding goal, which now seems well in hand, is to achieve a target of 30 per cent of the original spawning stock biomass by 2035.

While the demise of SBT was due to commercial overfishing, in order to manage their recovery it is important to also manage recreational fishing effort. This is particularly the case when you consider how offshore recreational fishers have so much more fishing power today than was available in the 1980's and 1990’s, due to larger more efficient boats, GPS, better sounders, electronics and gear, and so on. However, in the absence of a national fishing license or registration system for the SBT recreational fishery, it has been difficult to estimate how many tonnes of SBT are taken by the recreational fishing sector each year in Australia. This problem festered for decades, but was recently addressed by Sean Tracey and the intrepid band of researchers at the Institute for Marine and Antarctic Studies, in Hobart. They utilised a range of off-site and on-site survey methods across five states including creel surveys, phone interviews and charter log books, to provide the first national assessment of the recreational SBT fishery in Australia. Their results were very interesting. The study estimated the national harvest of SBT for the 2018/19 fishing year was around 270 tonnes (95 per cent confidence intervals 239–301 tonnes) from around 16,410 fish (representing an average size of around 16 kg each). This estimate provided enough information to underpin the unprecedented allocation of 5 per cent of Australia’s SBT quota to the recreational sector. Other interesting details included the fact that private vessels were estimated to account for around 77 per cent of the harvest by weight, with the remainder being caught from charter vessels. An additional 5821 ± 572 SBT were released, equivalent to a release rate of 24 per cent. Previous studies of the release mortality of SBT indicated that 10–17 per cent of the released fish may die, representing a loss of between 9 and 15 tonnes of fish, which is less than the approximate 17.8 tonnes that are lost to depredation, mainly by seals.

So while the demise of SBT was not the fault of recreational anglers, the increased fishing power of anglers today means that recreational fishers can, and must, play an important role in the ongoing conservation and recovery of SBT stocks. This can be done by obeying bag limits, taking only what you need for a feed, and maximising the survival of released fish, as outlined in the Southern Bluefin Tuna Code of practice for recreational fishing.

For more information, check out the study at https://onlinelibrary.wiley.com/doi/abs/10.1111/fme.12528 and the code of practice for recreational fishing for SBT can be downloaded from https://www.imas.utas.edu.au/__data/assets/pdf_file/0007/799648/Appendix-10a-COP_DL.pdf.

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