I just cut the gills. They bleed like a stuck pig. Try to just stun them witha good wack and don't kill them as their heart will pump a good amount of blood out. There is an artery in the tail, but it's harder to find and I think it kinda ruins the pictures at the end of the day.
What is the proper way to bleed a halibut for top quality meat?
... white side up when done. supposed to keep the worms in there gut i thinks..