I guess you never seen the show "when sharks fly". It shows a ton of white sharks that use there stealth approach from the bottom to hit unsuspecting seals at the surface. I think most inland water sharks probally rely on sick and injured fish but the bigger examples like the white shark, tiger sharkmainly feed on living healthy food sources.
Even then, if not suffering a previous injury or wound that makes them a more probable target, those marine mammals are temerarious, and that's why they get picked off first. The white shark is nature's selection tool for weeding out character traits in sea lions.
I used to work at a public aquarium when I lived in California. When you ask aquarium employees whether the sharks eat the other fish, they're instructed to give you a line like this: "all the animals are well fed and cared for, and by feeding them more frequently than their natural predatory instincts require, we keep inter-species predation to a minimum." In reality, fish live longer than they should in aquariums. They begin to suffer from old-age diseases simply because of the good care they receive. There were numerous sharks our aquarium had to set free because they were too good at caring for the sick and aged.
Oh yeah, in case you're wondering, we never had any incidents with the divers who fed the sharks and maintained the exhibits...absolutely zero.
