It's called ponassing and the central stick is indeed split and then clamped up the centre of the fish and tied at the top with split roots (or paracord
). The skewer sticks are to make the fish hang rigid as it cooks.
This shows more of those photos:
HERE
If you search BCUK forums I'm sure you'll find more details, though it's pretty easy to do and salmon or trout taste fantastic cooked this way.