There are a few different theories about why fishermen say "fish on" when they've hooked a fish. One theory is that it's simply an exciting announcement - after all, hooking a fish is the whole point of fishing! Another theory is that it's a way of alerting other fishermen nearby that there's someone with a bite. And finally, some believe that it's simply tradition - something that's been passed down from generation to generation of fishermen. Whatever the reason, one thing is for sure: "fish on" is definitely a phrase that every fisherman knows!