I've seen them in the middle like that as well, the only thing I could think of was them all eating on a big school of shad. I have seen so many blue back harring in a school out there it looks like you could walk across the water. Btw you from or hunt in Delacroix?
There's a lot of shallow water out in the middle of the lake. I don't think they were spawning, though. I think they were getting hatching insects or very small bait. Seems like they do it regularly. It'd be much easier to shoot them that way in the day than going out at night. Might hafta tote the bow one of these days!
Theres times we find what bowfishermen call "floaters", there will be large amounts of gar in a small area seemingly floating in large schools for no reason. I don't have an explanation as to why they do this but it makes some awesome shooting lol