Armed Killer Dolphins
Whatever are we to make of this:
It may be the oddest tale to emerge from the aftermath of Hurricane Katrina. Armed dolphins, trained by the US military to shoot terrorists and pinpoint spies underwater, may be missing in the Gulf of Mexico.
To answer your first question: toxic dart guns.
EDITED TO ADD (12/5): Snopes, a reliable source in these matters, claims this to be a hoax.