There are a few things you can do depending on where you go.
I normally would make a soup – clean the fish, cut in chunks and cook in my pot/mug. Ideally I would bring some spices and veggies to put into the soup to make it even better, but that's only when i'm certain that i will catch fish. Otherwise it's just salt and pepper.
If you have a small frying pen, then frying is a great way to get some awesome flavors. A packet of oil from Subway comes handy. Or i bring a small container with oil.
If open fire is allowed in the area – then clean the fish and put a small stick through the mouth and into the flesh then slowly roast it above fire. If you have some extra foil – wrap in foil and put on coals from a fire.
If you are spending a lot of time in one spot from 4 days to a week (more of a camping trip than hiking) AND there are no big wild animals around such as bears, mountain lions, etc – salt the fish well for a day and put it on a string to dry in the sun. A bug net around the fish may be needed to keep flies out. At the end you will get fish jerky or more properly named "TARANKA"
I've seen some precipices here that show a bit of water on the bottom of a pot along with sticks and pine needles with fish on top for some sort of steaming and smoking.