When you work in collage there's something appealing about a fish. It has an enigmatic expression on its face, and when you paste it into a scene there's that surrealistic fish-out-of-water disconnect. But more important, perhaps, is that a fish is easy to cut out, with smooth edges and few appendages (and if you have a picture of a fish with complicated serrations on its fins, you can always round them off with your scissors). And there are plenty of fish images available, especially if you're willing to cannibalize your old encyclopedias.
So I found that fish would frequently swim (fly?) into my collages.