In a nutsack... The answer to your questions about why they are required to be released is...

The chinook in that river are not nearly as plentiful as they should be and the naturally spawned ones have done poorly as of late. The numbers of returning "native stock fish" are enough to sustain genetic diversity only because most of them, like the one you released, are born in hatcheries.

Without the hatchery native stock breeding program the summer chinook in that river would possibly be extinct.