This method works for cod fish but I suspect it will work for other bullet shaped fish.
Tools required - A sharp knife, preferably a fish knife but a Swiss Army Knife will also do.

Pierce the skin from anus to gills and gut the fish. Discard or retain the entrails depending on whether you have some arcana or composting to perform.

Remove the tail just where the flesh ends.

Remove the dorsal fin structure by cutting a few millimeters below it.

Place the fish top side up on the cutting board. Break the back bone by inserting your knife just behind the head where the body cavity is largest.

The next move requires a small amount of forcefulness.
After breaking the backbone the head should hinge quite well. Holding the fish round its fleshy middle onto the cutting board , grab the head with your other hand, bend it completely backward and pull strongly towards the tail of the fish . As long as you dont twist the fish then the skin should start to peel away from the flesh. Dont panic the ripping sound is normal.
Keep peeling backwards until the skin is completely removed. You should now have a skinless but not boneless chunk of fish.

Carry on preparation until you feel you have an edible fish.

See Also: How to Clean a Fish.

After much debate, this "How to clean a Trout" writeup is being addended to Cleaning Fish The technique is much the same, however you are allowed some luxuries when cleaning a rainbow trout that you are not afforded when cleaning the generic fish.

    Supplies
  • A sharp knife. (Small, pocket sized is usually best)
  • Two buckets (or bins) of water. Stream water works fine.
  • Your victim

Most rainbow trout come from a stream of some sort. so naturally the first step to this process is to simply rinse the fish. Give it its last rites, because chances are (if you've kept it fresh) its still alive. Open your pocket knife and holding the fish parallel to the ground, belly side down (firm hold) strike the back of it's head (about where the neck would be if it had one) with the non-bladed side of your knife. This takes a decent snap of the wrist, and until you are a master you should probably execute this maneuvre with lockjaw pliers. However, it looks simply more badass to do it with the same knife you're about to clean the fish with.

Chances are, if you're human, you've dropped the fish in the dirt (or stream) by this point. If it's the swift part of the stream let it go man, its gone. However if the fish is recoverable, rinse it again (in the bin reserved for "dirty water"). At this point, the fish should not be flopping at all. insert your forefinger in one gill and your thumb in the other, with the back of the fish facing the ground. Squeeze the midsection of the fish, chances are you'll be able to squeeze some poop out of it. Rinse again (always in the dirty water until otherwise noted.)

Now, with your knife perpendicular to the line of the fish, cut a 45 degree angle cut on one side of the anus, about a centimeter deep. This length may vary with the length of the fish, so you're going to have to use your judgement. Next, another 45 degree (but in the opposite direction, and on the other side of the anus) cut, effectively removing the fish's anus. Throw this wedge away. Its inedible.

This next step is where the most fish gets ruined. This is because the cut you are making passes closely to the gall bladder and if ruptured, the gall bladder will make the fish taste utterly horrible. (You can tell you've screwed it up if you see a bunch of green ink everywhere. Make a shallow cut, beginning in the anus-wedge, and proceed between the fins and all the way up the fish. This cut should end right before the "chin" of the fish. Next, (at least visualize this) place your thumb and your forefinger of your right hand underneath your own jawbone and press up. Your fingers (if done correctly) should sink into your chin a bit, and you should feel like you're gripping your toungue. Now imagine that you are the fish. you want to insert your knife on one side of this meaty part, and weave it out the other. the blade should face towards the direction the fish would swim (or the fore of the fish.) pull with your knife so you have basically cut out it's toungue.

In the next step, you should take care not to cut yourself on your own hook (if you failed to remove it) or the fish's teeth which are actually just spines located on it's tongue. place your thumb in the slice you just cut out, grasping it's tongue and grip tightly. With your other hand, grab the fish's jawbone. Pull your hands in opposite directions. If you've completed cleaning the trout correctly to this point, the guts will cleanly come out of the fish. Rinse. Toss the guts.

Next, you should see a large pocket of blood along the spine of the fish. This is removed by inserting your thumb near the back, and using a motion like you're playing marbles scraping the blood out through the mouth of the fish. Get this good and clean. Rinse.

Depending on preference, you can stop here. I prefer to lever the head off using my knife. Placing the knife at the base of the neck where you struck earlier, tilt the head back until the spine snaps. Then saw with your knife until it is cleanly off. As well, at the back of the fish, there should be some tiny specks in small rivulets of bone. These are cleaned with the point of the knfe. When finished (and by this time, you can start rinsing in the clean water) The insides of the fish should be completely white.

Take note that due to the way trout is cooked, there is no need to skin or bone it. The fins will come off after cooking as well.

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