Carrying forbidden food barrels? Are you sure they were forbidden ('v' over the dwarf, 'i' for its inventory, then 'enter' on the barrel to see if its forbidden, or curly braces around the barrel, {})? Where were they being taken to?
I'm pretty sure they were forbidden, since after mass designating them as forbidden I had to go back and unforbid the booze barrels.
If you want to get turtle bones quickly, then forbid all food. Go to the 'z' screen, then select 'stocks', then find all food items listed in the stocks menus (plants, meals, meat, prepared fish). Go down each list and 'f'orbid each item except for 'turtle'.
Oh, you can forbid things from the stock menu? That would have been a lot easier. Huh, and looking at the stock menu, I see that the turtles were all raw, which is why no one was eating them. I thought that they had been processed, but...
Make sure the turtle is prepared, as in not in the raw fish menu. Dwarfs will not eat raw turtle. To prepare it, then choose 'prepare raw fish' at a fishery and keep doing that until the turtle is picked up and prepared by a dwarf with the 'fish cleaning' labor.
I forgot about that part. Just like how in this game I'd forgotten to put my stonecrafter's jobs on "Repeat" until after the first dwarven caravan arrived and I realized I had only 12 trade items.

In my next game I'll have to make sure to catch some turtles right away. Or would turtles bought in the pre-embark supplies screen produce shells as well?