Australian led research is helping improve technology to increase the success rate of breeding Bluefin tuna in captivity and enhance economic relations with Japan.
The popularity of Bluefin tuna for sashimi in Japan and the subsequent high market prices has led to serious overfishing, with the species currently considered as vulnerable to extinction.
It is possible to breed Bluefin tuna, but the success rate has thus far proved underwhelming. One of the key issues with this relative lack of success is due to water temperature and the ability of young tuna to regulate their body temperature.
To breed Bluefin tuna, sexually mature males and females are kept together in either sea cages or land-based tanks. When water temperatures become warm enough, around 25°C, the fish spawn by releasing sperm and eggs. The fertilised eggs are then maintained in a hatchery with regulated warm water temperatures.
When the eggs hatch, the fish larvae continue to develop in the hatchery until they metamorphose into young juvenile fish. The temperature in the hatchery is maintained to be warm (at about 25oC) because the larvae and young-of-the-year juveniles cannot tolerate cold water temperatures.
The reason for this is not completely known, but Associate Professor Kathy Shuller from Flinders University in South Australia believes it’s likely that the young fish have not yet developed endothermy, the capacity for body temperature regulation using internally generated heat, which means they cannot warm their bodies if the water temperature drops.
‘Wild Bluefin usually grow in warm water currents. At about six months, they develop the capacity for endothermy and this enables them to leave the warm water currents and migrate into cooler, more nutrient-rich and therefore prey-rich waters,’ says Kathy.
In captivity the young juveniles grow too big to be maintained in the hatchery and they have to be released into sea cages. This means that they can experience a drop in water temperature from the warmer, artificially-maintained temperature of the hatchery to the cooler, naturally-fluctuating temperature in the sea cages.
If these fish have not yet developed the capacity for endothermy, they will die if the water temperature in the sea cages falls too low. This is a particular problem in Australia where captive breeding of southern Bluefin tuna takes place at Arno Bay on the Eyre Peninsula of South Australia.
Research has shown that the minimum body size for the onset of endothermy in Pacific Bluefin tuna is approximately 55cm body length. However, the largest size of the captive-bred southern Bluefin tuna released into the ocean is approximately
20cm body length.
‘If we knew more about when juvenile tuna develop the capacity for endothermy, at what age and size, and if we knew what factors regulate the rate of development of endothermy in these fishes, for example declining water temperature, changing diet, increasing physical activity, then we could predict the minimum size at which they could be released into the ocean and survive or we could subject the young fish to treatments such as changing water temperature, diet, and physical activity that would advance the development of endothermy and thus make them less susceptible to the effects of cold temperatures at a younger age,” says Kathy.
Kathy worked with Dr Takashi Kitagawa at the Atmosphere and Ocean Research Institute of the University of Tokyo, including undertaking a field trip to Nakatosa (Kochi Prefecture). Kathy and selected postgraduate students from Tokyo University carried out biochemical and genetic analyses to map the development of endothermy in the Pacific bluefin tuna specimens collected in Nakatosa.
The potential benefits of this research are many. Over 90% of southern Bluefin tuna is exported to Japan. There is greater demand for the fish than can be supplied. Successful captive breeding will decrease pressure on wild stocks of fish, allow wild Bluefin to regenerate to sustainable levels and bring down the price of sashimi, while providing economic opportunities for the Australian aquaculture industry.
The project is supported by the Australia-Japan Foundation (AJF). The AJF is Australia’s oldest council supporting people-to-people links underpinning this key bilateral relationship.
Grant rounds open in February for projects deepening connections between Australia and Japan.