I've seen a similar thing happen in New Zealand, A mullet fisherman took me out in his boat one day and that is what he looks for, if he see's a mullet jumping in the wake of his boat then he turns around and sets his nets in that area, he says "one mullet jumping can give away the whereabouts of a whole shoal"
Please don't get me started on invasive/noxious species being introduced and then spread by humans.