You have landed that sizable fish and now it is time to fillet it. Oh, wait, you mean you are going to try and fillet that fish with that standard pocket knife are you? Hmm. What a shame. You could have carried a fillet knife. Yes, such a creature does exist! And it is a good thing that reliable stainless steel fishing fillet knives are on the market because they are designed specifically for the purpose of cleanly filleting a fish with ease. There is nothing worse that attempting to fillet a fish with a knife whose blade is clearly not designed for filleting. Using an improper knife usually results is ugly, jagged cuts that turn a clean fillet into a butcher job. Yet, this is what many people do because they do not realize there are knives designed specifically for this purpose. As such, using a fillet knife is the far better option and is highly advised.
Good fillet knives are typically made of stainless steel and are very thin to cleanly slip through the delicate flesh and skin of a fish. The blades will range in length so get a couple of different lengths depending on the size of fish you typically are seeking out (large river salmon will need a longer blade than a smaller lake trout). Get a folding fillet knife if you feel this would help you carry it safely. Most other fillet knives are fixed blades so make sure it comes with a sturdy sheath for safety and to maintain the knives sharpness.