They release the fry shortly after hatching. This way they have the egg sack to feed on initially. The socks are such that the fry cannot pass.
If you want details I'm sure I can get them in fact I think they did an instructional video. They have several of these "walleye wagon" portable hatcheries and numerous chapter organizations so instruction was key.
For reference:
http://walleyesfortomorrow.org/https://www.facebook.com/WalleyesForTomorrow?ref=stream
