I would say that unless you are very lucky and using a measure net this year, like most (except when wdfw/hatchery is getting rid of brood stock) you will not catch a rainbow 18 or over at Leech lake. And generally they will die over the winter.It is not helpful that they allow keeping a rainbow and otherwise general rules.
I have see some folks w fly gear enthusiastically taking their limits or more.
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And generally the state isn't a big fan of Eastern Brook trout (and they are very prolific) so the catch rate of the lake ebbs and flows based on the year.
Most fly fishing guys don't keep these rainbow as they are "put and take/ hatchery" fish. That being said even if most days every fly guy kept one it would still fish great as this year you would be very hard pressed to catch an 18+ rainbow.
This is just my opinion and observances.
And yes I will keep and cook a couple of brookies once a year.
Cheers and tight lines