I think most fishermen that would actually take the time to cut off the turtles head would actually want the whole turtle for dinner. People all over the world eat turtle soup.
Some "pond" fishermen typically cut off turtles heads because they are seen as a nuisance. They kill all the small fish in ponds and destroy the ecosystems. I think there is a chance they didn't know the turtle was endangered.
As for taken just the skull; Wouldn't they want the shell if they were into collecting parts of animals for display or to sell? I think turtle shells can be expensive.
I don't think this was a malicous act. I think it was probably something like this.
60% Boat propeller
25% Pond fishermen that didn't know it was endagered
13% Idiot with a treble hook and a fishing pole
1% Henry VIII descendent
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