The body of the shad is spindle-shaped, and the middle of the body is very large. Once it is caught in the net, it is impossible to escape. Swordfish is slender and never flinches when swimming. It is caught by a finer net. Once caught in the net, you can't escape whether you are going forward or backward. When the puffer fish is frightened, it will expand its body, in order to make itself look bigger, so that predators can't eat it. This habit is completely ineffective for fishermen's prevention, and it can also be said that it is more convenient for fishermen to catch, just like a hedgehog blowing up its own thorns when it sees people not running, which is ineffective for people.
The main reason why these three kinds of fish are endangered is that they have the habit of seasonal migration. At that time, they all gathered together and were easily caught in nets, overfished and polluted by water. Moreover, due to the difficulty of artificial reproduction of migratory fish, there are fewer and fewer shad and swordfish. Although puffer fish is cultured artificially, its habits have changed, so its taste is much worse than that of wild fish.