It would depend. You could say this when you are fishing, literally after you caught a fish. And you could say this when you feel you met someone who was a popular person to want to date, ''a catch''. Which even in a romantic sense, the meaning of "I got myself a catch" is still a idiomatic expression that stems from the fishing world.
If someone tells you, ''you're a catch'' it means that they find you to be attractive and someone who is very successful in life.