TUNAS are true hot rods of the sea. They are superbly adapted to swim fast via their streamlined torpedo shape and powerful fins, not only to hunt and capture prey, but to enable them to effortlessly cover large distances in the relative desert of the pelagic oceanic realm. This high activity level demands a high metabolic rate, which is serviced by other adaptations such as their need to “ram ventilate” their gills in order to obtain enough oxygen to stay alive, clever counter current blood exchange systems that can keep their blood warmer than the ambient water temperature, and a range of other performance enhancing features. All these features of tuna have been known for some time. What was not known, until recently that is, is how amazing and mind blowing the growth rates of juvenile tuna really are.
The Japanese (and more recently, Australians) have been working on aquaculture of tunas for some time now (since the 1970s in Japan), concentrating mainly on the highest value species used for sashimi like the southern bluefin tuna (SBT, Thynnus maccoyii) and the northern Pacific bluefin tuna (PBT, Thynnus orientalis). The initial emphasis in aquaculture was one of growing out wild caught juvenile fish to larger sizes, which is essentially a form of sea ranching. However, given the strict quotas that are used to limit commercial fishing for wild stocks of SBT, and the comparative rarity of the PBT, the holy grail of tuna aquaculture has been “closure of the lifecycle” which involves captive breeding and rearing of fertilized eggs into larvae and then juveniles of market size. While obtaining fertilised eggs from tuna broodstock has not been technically very hard, successful growout of the larval and juvenile fish has proven to be extremely difficult and it took over 30 years to achieve this in Japan. These difficulties arise due to their high performance and other adaptations to pelagic ocean life, many of which are incompatible with the land based tanks normally used for rearing fish larvae and small juveniles. The nutritional needs of larval and juvenile tuna are also not well established, meaning that the learning curve on how to keep larval and early juvenile tuna alive has been steep. But having these tiny tunas in tanks has allowed scientists to learn much more about them, and some of the growth rates achieved during the early life history of these species are eye opening.
Southern bluefin tuna in the wild are relatively slow growing (for a tuna), reaching 50 cm in their first year and maturity between 8 and 10 years old at 130 to 150 cm fork length, living beyond 40 years of age and growing to around 2.3 meters and 200 kg. At their only known natural spawning ground off north western Australia, 4 to 13 day old SBT 3.5 to 7.5 mm long gained around 0.3-0.4 mm length (around 5-8% of their body length) per day, with growth rates ultimately constrained by prey availability. It was found in the some patches of larval SBT, the tiny tuna ate virtually all the available zooplankton (mainly copepods) and then began to eat each other (SBT mortality rates in the wild were around 50% per day!). Similar growth rates have been achieved in aquaculture, again with food supply being a limiting factor.
In contrast, northern Pacific bluefin tuna are real hotrods. They reach maturity in less than half the time of SBT (3 to 5 years) at between 100 and 150 cm fork length and 50–60 kg. But unlike SBT, NBT keep growing rapidly after maturity to over 3 meters long and upwards of 500+ kg, within a lifespan thought to be only around 20 years. In aquaculture in Japan, larvae of PBT less than 10 mm long in their first week of life grow on average 4-6% in body length per day, and a whopping 27-31% per day in weight. In other words, larval northern Pacific bluefin tuna are doubling their body weight every 3 to 4 days! No wonder the main problem with rearing tunas is keeping enough food in the water for them!
