I've been after that little bugger for a while, intermittently, but still have had no luck getting him to drop. Good to know I'm not wasting my time though, at 450ish for fishing and focusing on the pools. I just need to have some patience!
Sorry for highjacking your thread for a moment OP!
They are little round accumulations of fish in the lakes and waters. If you don't have the pool tracking ability you can still find them quite easily just by visibility and they will have a tooltip on mouseover as well.