the practice of releasing a smaller "keeper sized" fish, & replacing it with a fish of greater size or weight....normally done when a "legal limit" of "keeper sized" fish has been boated, and the trip isn't over and the fish are still biting.
Of course, one may "cull" fish - BEFORE obtaining a legal limit !
Now, when there is no size or creel limit, or when you don't really want to bring a "legal limit" home to clean -- "culling" is generally thought of as just "throwing them back, to get bigger" :D
KK ... you've got me ROFLMAO on this one !! Mainly because I don't recall having the necessity to do any "culling" to my catches, very often. ...cp