My friends and I visit each others towns a lot in Animal Crossing: City Folk!
Ocean Sunfish (what I'm fishing up) appear to have a shark fin in the water. A big thing we do is go "shark hunting." Not only because they're worth a lot of bells (money), but because they're cute! :3 (I have two Hammerheads in my house!)
So when we spot a "shark," and fish it up to see it's only an Ocean Sunfish, we get very disappointed. XD

(Sorry for the poor quality of this photo!)