Tuna has more than 50 varieties, most of which are large, the largest body length of 3.5 meters, weighing six or seven hundred kilograms; and the smallest varieties of only 3 kilograms, the size of the difference is very large. Tuna's reproductive capacity is relatively strong, a 50 kg female fish, about 5 million eggs per year, if these eggs can be fertilized and hatched out of young fish, and grow into 500 kg heavy fish, then, a female and a male fish will be able to produce 2.5 million tons of tuna, which is equivalent to the world's total catch of tuna. In fact, this is impossible. Because the vast majority of eggs do not survive to maturity, only a few are able to hatch out as juveniles. Even when they do hatch, most of them fall prey to other adult fish, seabirds and other marine animals, and tuna have a nasty habit of killing each other. Therefore, the survival rate of juvenile fish is extremely low, and it is estimated that only one or two in a million of these little creatures grow into large fish. But in addition to natural enemies, tuna also have protectors in the sea, whales and shark whales are its good friends, they often swim together. Tuna if the natural enemy, will rush close to the whale or shark whale, with the help of friends of the huge body to cover their own, there is a big tree on the back of a good attitude.