I've never needed more than a pocket knife and a fillet knife.  You can fillet a fish with a pocket knife if you have to (and I've had to).

For many years now though I've carried my fillet knife and some gallon ziplocks to store fillets in my tackle bag.  That way I can fillet wherever I catch them instead of filleting in camp or having to go back to retrieve the knife and then canoe to an appropriate filleting area