We don't have many sharks here in Ontario, so we've never had to worry about using anything more than a stringer for the fish we're keeping. I have heard of pike or even snapping turtles taking a fish off a stringer, but it's not common enough to worry about.
I guess if we get the chance for some saltwater fishing then we'll need to think about what to do with the keepers.
Touring Kayak Paddles
Heel and Pegpads™