that and some folks claim they are ok smoked. White Springs lose flavour once they darken up a bit. If the belly is at all grey put it back.
Sadly, many people won't do it, especially if it's a female. I have walked and waded the river quite a bit this long weekend, and I saw far too many female springs rotting on the bottom with their bellies cut open.
The hunger for roe as bait and ikura and caviar as a delicacy make some people throw their ethics out the window.
I agree with Ralph from firsthand experience that gray (and soft) belly is a good indicator that the fish should be put back. But a slightly darkened fish (especially with a copperish tinge = red spring) with a white belly, firm flesh, and no damage to the fins and scales is good table fare, even for sashimi if you process it right (freeze at -19C or colder for at least a week).
I hate it when people waste a fish only for the roe. There's a certain caviar-loving ethnic group at the Stave whose behaviour with chum female fish disgusts me so much that I don't bother to fish the Stave anymore.
I wish there was more enforcement in that regard.