From PSU: “Within Metro Exodus The Volga level, there is a pesky, giant catfish that follows you around all the time. Believe it or not, but there is actually a way to kill this hulking monstrosity. Unfortunately, you will have to wait until late in the level to do so. Here is what you need to do in order to kill The Catfish. A written guide can be found below this video guide.”
Source: N4G PC Metro Exodus How To Kill The Catfish – Fisherman Trophy