Southern Bluefin Tuna Bio-physiology
The southern bluefin tuna (T. maccoyii) taxonomic position is in the Class Osteichthyes, Subclass Actinopterygii, Order Perciformes, Suborder Scombroidei and Family Scombridae which includes all mackerels and tunas (Lagler et al., 1977). The specific name maccoyii was bestowed by Castelnau (1872) with the comment that “the flesh of this fish is not eaten, or at least is not esteemed as food”.
SBT are one of the largest bony fishes, living up to 40 years, growing to a length of 2.25 metres, and weighing over 200 kg (Patterson et al., 2009; Patterson et al., 2010). One specimen that washed up on a beach at Glenelg, South Australia in 1890 was reported to have weighed over 350 kg (Serventy, 1956).
SBT are a single, highly migratory stock (Patterson et al., 2009; Patterson et al., 2010) that is mainly found between the latitudes of 30° and 50°S (Collette and Nauen, 1983). The only known SBT spawning ground is located in the warm oceanic waters south of Java in the north-east Indian Ocean 10°-20°S, 105°-120°E, with the spawning season spanning September to April. Following spawning, the developing juveniles are transported in the Leeuwin Current along Australia’s north western shores to the south west tip of Australia and into the Great Australian Bight or west towards South Africa (Patterson et al., 2009; Campbell, 2001). There is some uncertainty about when SBT reach spawning age but the general view is that it is between 8 and 12 years with females producing several million eggs in a spawning period (Hayes, 1997). SBT are opportunistic feeders (Dickson, 1996), preying on fish, crustaceans, cephalopods, salps, and other marine animals (Young et al., 1996; Young et al., 1997; Itoh et al., 2011).
SBT have a range of distinguishing anatomical and physiological adaptations that assist with movement to minimise anterior resistance and maximise caudal thrust (Bushnell and Jones, 1994; Dewar and Graham, 1994; Brill, 1996; Fitzgibbon et al., 2008). These adaptations include a streamlined shape that is built for speed, maneuverability, drag reduction and efficiencies in locomotion (Magnuson, 1978; Dewar and Graham, 1994).
Their dorsal, pelvic and pectoral fins provide guidance but serve no role in propulsion. When moving at high speed (approx 70 km/h) (Wardle et. al., 1989) SBT use long propulsive beats of their tail and retract their fins into defined body grooves to minimise drag. Caudal keels along the top and bottom edges of the body act as spoilers to prevent turbulence (Attenborough, 1979; Altringham and Shadwick, 2001).
Tuna have been described as energy speculators because they “gamble” high rates of energy expenditure in nutrient poor pelagic environments for the capture of prey. This hunting approach depends on being able to capture and process food as efficiently as possible (Korsmeyer et al., 1996).
Similar to a number of other ocean roaming predators including lamnid sharks, billfishes, and the opah (Lampris guttatus) (Block et al., 1993; Block and Finnerty, 1994; Dickson and Graham, 2004; Runcie et al., 2009), some tuna are able to warm their viscera, myotomal muscles, eyes and brain above ambient water temperature, a characteristic known as regional endothermy (Carey et al., 1984; Graham and Dickson, 2000; Graham and Dickson, 2001; Dickson and Graham, 2004; Sepulveda et al., 2007).
Development of endothermy varies considerably within the Thunnini tribe and correlates strongly with tuna phylogeny (Block et al., 1993; Collette et al., 2001). However, only species within the subgenus Thunnus, including southern bluefin tuna (T. maccoyii), Atlantic bluefin (Thunnus thynnus), Pacific bluefin (Thunnus orientalis), albacore (Thunnus alalunga) and bigeye tuna (Thunnus obesus) are capable of visceral endothermy (Collette et al., 2001).
Regional endothermy enables tunas to expand their thermal niche from surface waters to depths greater than 400 m, and to migrate large distances (Block et al., 1993; Graham and Dickson, 2000; Dickson and Graham, 2004). It improves the performance of tuna, enhances rates of muscle contraction and power output (Altringham and Block 1997), cellular respiration (Stevens and Carey, 1981), vision, neural processing (Block and Carey, 1985), and digestion (Carey et al., 1984; Gunn et al., 2002).
Endothermy requires a source of heat and a mechanism to retain it. Tuna are believed to digest food more rapidly than other fish (Magnuson, 1978; Olsen and Boggs, 1986; Brill, 1996), and the heat source for endothermy is understood to be a by-product of their normal metabolic processes.
All fishes generate “waste” heat from metabolic processes including that associated with muscle contraction and SDA related processes (Graham and Dickson, 2001). However in most fishes waste heat is lost through convective heat transport through the gills.
In SBT, it is estimated that 35% of ingested energy may be attributed to standard metabolic processes (Fitzgibbon et al., 2008). In contrast, only 14% of ingested energy may be attributed to standard metabolic processes in freshwater fish such as largemouth bass (Micropterus salmoides) (Beamish, 1974), and up to 23% of ingested energy in grass carp (Ctenopharyngodon idella) (Carter and Brafield, 1992). Visceral temperatures and metabolic rates rise abruptly after feeding and then decline steadily, with the magnitude. A return to basal temperatures usually occurs between 36 h to 48 h after feeding (Gunn et al., 2002). It has been demonstrated that SBT maintain basal visceral temperatures 2-4 oC above ambient seawater temperature (Gunn et al., 2002).