by Larry Powell   The "Grey Nurse" or "Sand Tiger," shark (  Carcharias taurus ), a coastal species on the ICU's Red List as  critically endangered. A public domain photo by Richard Ling.  Here's how sharks are "finned." After hauling them aboard their vessels, the fishermen cut off their fins, then toss them back into the ocean. Still alive, they sink to the bottom where they're either eaten by other predators or die of suffocation.    About 100 million sharks are believed to be taken by fishers each year, most of them for their fins alone.  It's an industry estimated to be worth US$400 million a year.  The blue shark (Prionaceglauca). Photo by Mark Conlin/NMFS. If one were to believe official trade records over the past twenty years, most fins traded on world markets have come from more abundant "pelagic" species (ones which live in the open ocean) like the blue shark (above).  The leopard shark (Stegostoma fasciatum). An ADV