Gustavo200897, Spanien
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Adult southern bluefin tuna lead an oceanic, pelagic lifestyle and are found at depths of at least 1,000 meters (Patterson et al. 2010).
Like other tuna species, the southern bluefin tuna is an opportunistic feeder that feeds on a variety of fish, crustaceans, cephalopods, salps, and other marine animals (Serventy 1956).
Thunnus maccoyii is a large, robust, and fast-swimming open-ocean predator with a highly developed circulatory system that maintains its body temperature above that of the surrounding seawater.
It is a black-blue tuna that turns silvery-white on the underside, with alternating rows of spots and lines, a yellow anal fin and yellow fin rays with black edges, as well as a yellowish to bluish first dorsal fin.
Thunnus maccoyii is highly sought after in global markets and is considered the world’s most expensive fresh seafood product; approximately 95% of the catch is consumed in the Japanese market for high-quality sashimi.
In addition, the tuna is farmed in aquaculture and is considered a particularly strong-fighting game fish.
Large southern bluefin tuna migrate north to spawn in a relatively small area off the northwest coast of Australia and south of Indonesia in the eastern tropical Indian Ocean (Nishikawa et al. 1985). Mature fish were caught in the spawning grounds every month except July, with stock density peaking in October and February (Farley and Davis 1998) when surface water temperatures exceeded 24 °C.
Once females begin spawning, they appear to spawn daily, similar to other tuna species.
In the spawning grounds, smaller fish (150–169 cm) are more heavily represented in catches in deeper waters, while larger fish (190–209 cm) and spawning fish are more heavily represented in catches in shallower waters (Davis and Farley 2001). Southern bluefin tuna are asynchronous, indeterminate spawners with an annual fecundity of 57 eggs per gram of body weight, similar to that of yellowfin tuna.
The fecundity of a 158 cm long female, whose ovaries weighed approximately 1.7 kg each, was estimated at about 14–15 million eggs (Thorogood 1986, Caton 1994, Farley and Davis 1998, Schaefer 2001).
It is not known whether all sexually mature fish spawn every year, every few years, or less frequently.
Larvae of the Southern Bluefin Tuna have so far only been caught between latitudes 7–20° S and longitudes 102–124° E from September to March (Ueyanagi 1969, Davis et al. 1990).
After the larval stage, the juveniles leave the spawning grounds, migrate south along the coast of Western Australia, and reach the southern coast at one year of age, where they spend the Australian summer (Hobday et al. 2016).
Most juveniles continue to spend the Australian summers in the Great Australian Bight until they are about five years old. During their stay in the Bight, telemetry data show that the fish spend most of the day in the upper 10 m and tend to migrate to deeper waters at night (Davis and Stanley 2002). In winter, they migrate across the Great Australian Bight and around Tasmania to 45°S, along the southeastern Australian coast to about 30°S off the northern coast of New South Wales, or as far east as New Zealand.
Fish older than five years are distributed widely across the southern oceans, from the Tasman Sea through the Indian Ocean to the western Atlantic.
The presence of juvenile fish aged 2–4 years on both sides of the Indian Ocean during the Australian summer months confirms that not all juveniles are restricted to the southern coastal waters of Australia (Farley et al. 2007, Chambers et al. 2017).
However, a large-scale study using archival tags (Basson et al. 2012) found that the number of juveniles migrating out of Australian waters is quite low (Collette and Graves 2019).
The use of radiocarbon dating suggests that a large percentage of fish with a total length of more than 180 cm are at least 20 years old and that southern bluefin tuna can reach an age of over 30 years (Kalish et al. 1996).
Age determination based on otoliths yielded ages of up to 41 years (Farley et al. 2007, Gunn et al. 2008, Shimose and Farley 2016).
Females predominate in catches of juvenile fish, but the sex ratio reverses in adult fish, with males outnumbering females. The onset of sexual maturity in females varies between 5 and 8 years, and the length at first sexual maturity ranges from 110–130 cm FL (Thorogood 1986). The size at 50% sexual maturity was estimated at 152 cm FL (Thorogood 1986, Caton 1994, Farley and Davis 1998, Schaefer 2001), and the average age at sexual maturity is 10–11 years, with no significant differences between the sexes (Gunn et al. 2008, Lin and Tzeng 2010, Collette and Graves 2019).
Previously, the generation length was very conservatively estimated at a minimum of 12 years based on an estimated age at first sexual maturity of 10 years and an estimated life expectancy of 40 years (CCSBT 2009, CCSBT 2010). However, most estimates regarding spawning appear to range from 10 to 15 years for 50% spawning, suggesting that 12 years is likely an underestimate of the generation length.
Even assuming 100% reproductive maturity at age 9 (knife-edge assumption), the generation length is likely to be at least 20 years given an estimated lifespan of at least 40 years (CCSBT 2013, Farley et al. 2014, A. Boustany, pers. communication, July 2020). Therefore, the generation time is estimated at 12 to 20 years, although 20 years may still be an underestimate.
Source: IUCN Red List & Fishes of Australia
Etymology:
The species is named after Professor Frederic McCoy, the first director of the National Museum of Victoria in Melbourne, Australia.
Synonyms:
Thunnus maccoyi (Castelnau, 1872) · unaccepted (misspelling)
Thunnus phillipsi Jordan & Evermann, 1926 · unaccepted
Thunnus thynnus maccoyii (Castelnau, 1872) · unaccepted
Thynnus maccoyii Castelnau, 1872 · unaccepted
Like other tuna species, the southern bluefin tuna is an opportunistic feeder that feeds on a variety of fish, crustaceans, cephalopods, salps, and other marine animals (Serventy 1956).
Thunnus maccoyii is a large, robust, and fast-swimming open-ocean predator with a highly developed circulatory system that maintains its body temperature above that of the surrounding seawater.
It is a black-blue tuna that turns silvery-white on the underside, with alternating rows of spots and lines, a yellow anal fin and yellow fin rays with black edges, as well as a yellowish to bluish first dorsal fin.
Thunnus maccoyii is highly sought after in global markets and is considered the world’s most expensive fresh seafood product; approximately 95% of the catch is consumed in the Japanese market for high-quality sashimi.
In addition, the tuna is farmed in aquaculture and is considered a particularly strong-fighting game fish.
Large southern bluefin tuna migrate north to spawn in a relatively small area off the northwest coast of Australia and south of Indonesia in the eastern tropical Indian Ocean (Nishikawa et al. 1985). Mature fish were caught in the spawning grounds every month except July, with stock density peaking in October and February (Farley and Davis 1998) when surface water temperatures exceeded 24 °C.
Once females begin spawning, they appear to spawn daily, similar to other tuna species.
In the spawning grounds, smaller fish (150–169 cm) are more heavily represented in catches in deeper waters, while larger fish (190–209 cm) and spawning fish are more heavily represented in catches in shallower waters (Davis and Farley 2001). Southern bluefin tuna are asynchronous, indeterminate spawners with an annual fecundity of 57 eggs per gram of body weight, similar to that of yellowfin tuna.
The fecundity of a 158 cm long female, whose ovaries weighed approximately 1.7 kg each, was estimated at about 14–15 million eggs (Thorogood 1986, Caton 1994, Farley and Davis 1998, Schaefer 2001).
It is not known whether all sexually mature fish spawn every year, every few years, or less frequently.
Larvae of the Southern Bluefin Tuna have so far only been caught between latitudes 7–20° S and longitudes 102–124° E from September to March (Ueyanagi 1969, Davis et al. 1990).
After the larval stage, the juveniles leave the spawning grounds, migrate south along the coast of Western Australia, and reach the southern coast at one year of age, where they spend the Australian summer (Hobday et al. 2016).
Most juveniles continue to spend the Australian summers in the Great Australian Bight until they are about five years old. During their stay in the Bight, telemetry data show that the fish spend most of the day in the upper 10 m and tend to migrate to deeper waters at night (Davis and Stanley 2002). In winter, they migrate across the Great Australian Bight and around Tasmania to 45°S, along the southeastern Australian coast to about 30°S off the northern coast of New South Wales, or as far east as New Zealand.
Fish older than five years are distributed widely across the southern oceans, from the Tasman Sea through the Indian Ocean to the western Atlantic.
The presence of juvenile fish aged 2–4 years on both sides of the Indian Ocean during the Australian summer months confirms that not all juveniles are restricted to the southern coastal waters of Australia (Farley et al. 2007, Chambers et al. 2017).
However, a large-scale study using archival tags (Basson et al. 2012) found that the number of juveniles migrating out of Australian waters is quite low (Collette and Graves 2019).
The use of radiocarbon dating suggests that a large percentage of fish with a total length of more than 180 cm are at least 20 years old and that southern bluefin tuna can reach an age of over 30 years (Kalish et al. 1996).
Age determination based on otoliths yielded ages of up to 41 years (Farley et al. 2007, Gunn et al. 2008, Shimose and Farley 2016).
Females predominate in catches of juvenile fish, but the sex ratio reverses in adult fish, with males outnumbering females. The onset of sexual maturity in females varies between 5 and 8 years, and the length at first sexual maturity ranges from 110–130 cm FL (Thorogood 1986). The size at 50% sexual maturity was estimated at 152 cm FL (Thorogood 1986, Caton 1994, Farley and Davis 1998, Schaefer 2001), and the average age at sexual maturity is 10–11 years, with no significant differences between the sexes (Gunn et al. 2008, Lin and Tzeng 2010, Collette and Graves 2019).
Previously, the generation length was very conservatively estimated at a minimum of 12 years based on an estimated age at first sexual maturity of 10 years and an estimated life expectancy of 40 years (CCSBT 2009, CCSBT 2010). However, most estimates regarding spawning appear to range from 10 to 15 years for 50% spawning, suggesting that 12 years is likely an underestimate of the generation length.
Even assuming 100% reproductive maturity at age 9 (knife-edge assumption), the generation length is likely to be at least 20 years given an estimated lifespan of at least 40 years (CCSBT 2013, Farley et al. 2014, A. Boustany, pers. communication, July 2020). Therefore, the generation time is estimated at 12 to 20 years, although 20 years may still be an underestimate.
Source: IUCN Red List & Fishes of Australia
Etymology:
The species is named after Professor Frederic McCoy, the first director of the National Museum of Victoria in Melbourne, Australia.
Synonyms:
Thunnus maccoyi (Castelnau, 1872) · unaccepted (misspelling)
Thunnus phillipsi Jordan & Evermann, 1926 · unaccepted
Thunnus thynnus maccoyii (Castelnau, 1872) · unaccepted
Thynnus maccoyii Castelnau, 1872 · unaccepted
