Nice idea, but the whale / shark is very hard to make out! Perhaps I'm just blind, but I had to really search carefully to see them amidst the fish and the background... Perhaps they could be some colour other than black?

Yes, they are meant to be difficult to pick out of the background at first - that is in keeping with their "shy" character (especially if one respons and the other is looking to eat it again!) and gives a bit of an advantage to the player who learns to spot them. I actually thought of naming one of them "Waldo", but that's another game. What do you think of highlighting them for a few frames - just enough to clue the players in?