Gutting a fish before filleting is not strictly necessary, but it can help to make the process easier and result in a neater finished product. If you do choose to gut your fish beforehand, use a sharp knife to make a lengthwise cut along the belly from the vent (anus) to just below the head. Reach in with your fingers and pull out the entrails, taking care not to puncture any of the internal organs. Rinse the cavity well under cold running water and pat dry with paper towels before proceeding with filleting.