Bigeye tunas are excellent swimmers, with a number of remarkable adaptations which make them efficient predators of the ocean. Unlike many other fish, they cannot pump water over their gills, but instead, swim with their mouth open which forces water over their gills. While this is an efficient way of getting water over their gills, it means that if they stop swimming, they will suffocate. The gills of tuna cover a surface area up to 30 times larger than other fish, giving a large surface over which water can flow, enabling about half the oxygen present in the water to be absorbed. The hearts of tuna are also much larger than those of other fish; about ten times as large, relative to the size of the body. This, along with a high blood pressure, creates perfect conditions to rapidly transfer oxygen from the gills to other tissues (6).
As well as adaptations to enhance the amount of oxygen reaching the muscles, a unique system to regulate their body temperature enables tuna to maintain their body temperature above that of the ocean. This system is called the rete mirabile; a counter-current heat exchange system which prevents heat from being lost to the surroundings. This maximises the efficiency of the muscles and maintains good brain and eye function, allowing the bigeye tuna to forage in cold water (6).
Juvenile and small adult bigeye tunas school at the ocean’s surface, sometimes together with yellowfin or skipjack tuna, and often associated with floating objects (2), while adult bigeye tunas are found in deeper water (5). The bigeye tunas migrate between feeding grounds in temperate waters and their spawning grounds in tropical waters; however, they are on the move almost all the time as they search for areas of plentiful food (6). Bigeye tunas feed on a variety of fish, cephalopods and crustaceans; which it searches for in the daytime and nighttime. Bigeye tunas become prey themselves for larger billfish and toothed whales (2).
Mature bigeye tuna spawn at least twice a year; releasing between an incredible 2.9 million and 6.3 million eggs each time. In the eastern Pacific, spawning has been recorded between 10°N and l0° S throughout the year, peaking between April and September in the northern hemisphere and between January and March in the southern hemisphere (2).