It seems that filleting a trout is one of those things that is not considered as an option by many fisherman.
I remember times back in the 1970's, whenever I first started trout fishing, whenever I brought up the subject of filleting trout, my friends would sort of look at me funny-like and have a look on their faces as if they were in the presence of a nimrod. Their response was that it was not done. That the trout was too small to mess with like that. Sure it's done with walleye, smallmouth bass, largemouth bass, pike, perch, etc., but not with trout.
I countered by saying that for many years I had been quickly filleting bluegills or other sunfish as caught and immediately put their filets on ice. Also, that the bluegills and other sunfish were almost always smaller than trout and if I could do it with them trout would be even easier. However, they were still skeptical.
I decided to do it anyway, in spite of what they thought. I figured that since I had done a lot of filleting of the other species and can totally fillet a fish in a couple of minutes that trout would be a snap. I was right.
Since that day of trout filleting I have never looked back.
During those times I filleted my trout catch the guys from camp would always watch me. Finally, one by one they would try it. Eventually they all became converts. Since that time many years ago every trout that was a keeper was a trout that was destined to be filleted.
Of course I am not the average trout fisherman such as those that throw their trout in a live well or holding net, rope stringer or clip stringer, many of those trout to die slowly from suffocating in the shallow, warm water.
If the trout is one that I plan on keeping I immediately eviserate it, remove the gills, thumb out the blood vein, wash it off and put it in a ziplock bag. It then goes under an icepack in my insulated coldsac. That trout stays 'fresh'.
With the trout already gutted and cleaned they are ready for filleting when I get home.
My technique therefore differs from the more standard filleting technique in that I do not use the 'carve around the rib cage method' at the beginning ( which is a method developed too avoid the necessity of gutting the fish before filleting ) as is done with the bass, perch, crappie or walleye filleting techniques.
The smaller / thinner bones of the normal sized trout ( 10 to 13 inches ) which are caught during trout season is more conducive to the technique of cutting directly through the rib cage bones - since the entrails are already gone anyway - going toward the tail all the while keeping the knife blade hugging the back bone.
A limit of trout of approximately 10 inches or longer ( lately, now that I am older, my limit of 'keepers' is two ). Yours may differ.
3. Kitchen Equipment
A long fillet board or a bigger wood cutting board,
A fillet knife,
A knife sharpener or 'steel',
a good pair of scissors,
I am right handed so the procedure shown is as if you were standing there, right behind me, while I am doing the filleting.
Make the first cut right behind the gill plate keeping the cut behind / under the fin that is there.
You do not want to cut off the head so you will need to be able to feel that the knife blade, after making the slight cut, has reached no farther than the side of the vertebrae.
The first cut is right behind the gill plate and under the pectoral fin.
After that first cut has been made it's time to turn the knifeblade so that you will be heading towards the tailfin.
As to make the turn with the knifeblade and start the cut you will arrive at the first rib bone real fast. This is where you will cut your way through each rib as you continue toward the tailfin making sure that the angle of the knifeblade is such that you remain in contact with the vertebrae as you cut.
You will 'feel' that you made it through the ribcage in short order. It goes reasonably easy on trout.
Continue cutting toward the tail, keeping contact with the vertebrae, until you almost are at the point where you would be cutting off the fillet if you continued.
Continue the cut through the rib cage toward the Pelvic fin.
My procedure is to stop cutting before the fillet is cut off that side of the fish and flip over the fillet while the skin is still attached. It makes it easier to fillet off the skin this way - my opinion - based on thousands of fillets.
However, some people cut off the side and contend with the de-skinning later. You will eventually decide on which method is more preferable for you.