I think you can fillet some fish in the field and not others. If there is a size limit or if you can no longer determine the species or its origin after filleting then it is not allowed. If there is no size limit on that species and you can still determin species and origin it should be ok. Crab have a size limit as do most kings, but some silvers have no size limit (depending on where they are caught).
I agree that is usually much easier to take care of you fish cleaning and filleting on the water, but I usually just gut them and wait till I get home for the rest to save the possibility of hasseling with the WDFW.
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bawddawg, no biscuit!