You might have noticed that some fish pulled from deep water can be released and will swim right back down, but others have trouble descending. That's because some fish (physostomous species like trout) can "burp" air as they're being pulled up, relieving the gas build-up in their swim bladders, while others (physoclistous species like walleyes) do not have a connection between the swim bladder and the gut, so their swim bladders expand, greatly increasing their buoyancy