Should fresh water fish (like crappie or bream) be frozen in a salt brine?
I grew up fishing and preserving the fish. We would only freeze and brine if we intended to smoke it later, we would would then re-brine it after it thawed (great way to make Northern Pike Jerky btw).
For smaller species like crappie, yellow perch and bream I wouldn't bother, as they freeze fast enough and consistent enough on their own. Key things to look out for would be ensure it's sealed tight, frozen flat (for easy storage and thawing), and descaled (if it's your preference) for some reason I am not sure why but descaling a previously frozen fish isn't terribly effective.
Good luck and tight lines.
Freezing it in brine will have no added effects, when you freeze the fish the cell walls are frozen solid making it impossible for the saline solution to soften the cell walls through the process of osmosis.
I have found brines most effective when the meat is defrosted and the brine room temprature. The colder the brine the longer the salting takes.