when fishermen catch porcupine fish around here they (fish) blow up on the boat and the fishermen throw them back in, where they bob on the surface belly-up until the birds get them or they otherwise expire. I've seen them belch out the air from this position, but it doesn't seem common. I have been able to hold them by the tail (as Pufferpunk suggested) and shake the air out, after which they tend to swim away. Dokno what happens to them after that...
