Moving around the Volga you will quickly come across a big fish. But you can't kill it by shooting. You will have the opportunity to kill the fish only during a story mission.
Killing a fish will give you an achievement The Angler.
When you reach the campus on the eastern side of the map you can get rid of the fish. Your first task is to get to the other side of the building.
There you will come across a bell and a lever. This is the best place to save the game.
Killing a big fish is very easy. You only need to pull the lever at the right moment. But first call the fish with the bell.
When the fish comes close to you, pull the lever quickly. This way you can drop a lot of heavy elements on it.
In a few seconds the fish will jump on you, but it will release the last breath in front of your legs. This way you will get rid of this monster.