Longline fishing is aptly named. Fishermen unroll a longline into the ocean up to 40 miles that is segmented with baited hooks and buoys. The crew let it float for a number of hours, hooking whatever swims along that has a taste for the bait.
Then the hooks are emptied of their catch as the line is rewound onto a big spool on the back of the fishing boat. Anything the fishermen want to sell is kept onboard, and anything caught incidentally, known as bycatch, is typically thrown over, dead or alive.
In the Gulf of Mexico, many fishermen use longlines to fish for swordfish and yellowfin tuna, but they also end up catching the threatened Atlantic bluefin tuna.