There are various reasons why fish jump out of the water. However, the primary reason seems to be because they are threatened in some way. Some fish do this naturally, during their hunt for food, while many others because they are hunted.
The most interesting example of fish jumping out of water would be of the Asian Carp.
Why do they do this? Are they some kind of underwater army defending their territory?
The carps generally feed off the surface, and are easily startled with sounds of boats that pass by. As a defense mechanism, they jump vigorously, sometimes up to 4 meters. Humans, as usual, have taken advantage of this and have even gone as far as making it easier for them to shoot the innocent carps. The shooting of silver carp is taken as a sport, and humans use boats to force them to jump so that the shooting is easier and more fun.
Some fish, like the large-mouth bass are known to jump to escape the hold of the hook that’s pulling them.
Why did the Ayu cross the waterfall? To get to the other side of-course!
Some fish jump to overcome hurdles like a waterfall, during the time they migrate from one region to another. The Plecoglossus altivelis, commonly known as the Ayu, migrate downstream towards the sea during winter and go upstream during spring. At times, they encounter waterfalls (not too large). But these ain’t a problem for the Ayu. They simply hop to overcome them. Various factors affect the performance of this jumping, and it is observed that these fish generally tend to jump during the daytime. [Refer this paper for more info: https://www.jstage.jst.go.jp/art…]
The water’s dirty!
Fish need oxygen to live, like humans do. At times, when the oxygen level reduces in the water, the fish might jump out to other small water bodies. Bad pH level is another cause.
As an example, the killifishes (Cyprinodontiformes) are known to flop on the land to reach other sources of water, after they either leave their old home because of contamination, or if they are threatened by predators. They do this despite possessing no obvious morphological specializations to facilitate terrestrial locomotion.
Certain fish kept in water tanks jump out for the same reasons.
Old habits die hard.
Some fish are predators. They feed on small insects and these insects are generally found outside the water. The African butterfly fish (Pantodon buchholzi) have specialized eyes that help it find these insects. What’s more, their bodies allow them to jump and glide for sometime, easing the preying. They do the same for escaping predation as well. If you have one as a pet, you might want to buy a small lid to cover the tank so as to prevent it from leaping to death.
Another example is the marbled hatchetfish (Carnegiella marthae), which also prey on insects outside the water.
To conclude, most fish jump because they are either hunted, or are hunting. There are other reasons to why fish jump out of water too. The Manta-Ray jump in order to escape predators or to relieve themselves from parasites on their body. The flying fish has a streamlined body allowing them to glide far distances for faster locomotion and they jump to do so.
Well, we almost forgot about certain types of fish that jump for entertainment purposes (for themselves, as well as for us humans). No, I’m not talking about dolphins or Killer Whales (they are mammals and not fish, BTW).
These fish are only found on the app store or the play store, and jump only for entertainment purposes. 😉