The title, like most of the poem, is pretty straightforward. It announces its subject without any mask or any bells and whistles. It gives us a real foothold right away. We know by the title and the first line that the poem will involve, in some capacity, a fish. No surprises.
The most interesting thing about the title is the tiny little article, "the." Think about how different the effect would be if the title was, "A Fish" or, "Fish." Bishop chose "The Fish" because it's a specific one, a significant fish, and as we continue to read the poem we discover why.