Grilling fish is an excellent cooking technique for fillets, steaks, and whole fish of the more oily fish such as salmon, trout, striped bass, and cod. It is similar to broiling, except that the heat comes from underneath, rather than from above. And a lot of the fish recipes can be easily used for both methods.
Delicate fish such as catfish fillets can be used as well. However, they tend to break apart and fall through the grill. So it's best to use to use a small-grid grill basket or aluminum foil with some holes punctured in it to allow air flow.
Both methods rely on the intense radiated heat to cook the food quickly and give a nicely browned appearance. The grill itself gives the food the characteristic grill marks as it transfers heat to it. The outer parts of the fish are nicely seared while the meat on the inside will remain moist and tender.

The design of the grill and the fuel source all play in the factors of grilling fish. It can be electric, gas, charcoal, or wood, which will determine the intensity of heat at the surface of the grill. And of course, the heat determines how long it will take to cook the fish, and how much of the smokey flavor will be infused in it. A stove top grill or a grill pan is also excellent to use.
The grilling surface should have bars spaced no wider than 3/4 of an inch apart. The grill should be clean. You can rub the grill with oil to help create a non-stick surface. Better yet, most people instead prefer to lightly coat the fish itself with oil, butter, or mayonnaise before placing it on the grill.
Fish will most likely stick to a cold grill. Completely preheating the grill for about 10 minutes before grilling fish on it will ensure that it won't stick. It will quickly sear and firm up on the outside, and become flaky and moist inside.

A good way to test the hotness of the grill is by placing your hand over the heat and to count until it's too hot for you to keep your hand there. If you can hold your hand over it for a second or less then it's considered to be "very hot", 2-3 seconds is "hot", and 4-5 seconds is "moderately hot."
Fish take very little time to cook so timing is crucial. You can very easily overcook it, which will draw out its moisture and become firm throughout. Figure about 8-10 minutes per 1 inch of thickness, or 4-5 minutes per ½ inch of thickness.
Another way to add flavor is with wood chips. Hickory, mesquite, and apple wood chips are just a few of the many that are on the market today. After soaking them in water for at least half an hour, place them in an aluminum pouch with holes punctured in it to allow the smoke to escape. You can add half dry chips in with the wet ones. The dry chips starts the smoking process off first, and the wet chips will allow it to smoke longer.

If the fish is directly on the grill, carefully use a long spatula to get underneath much of it as possible, and gently turn it over.
If it's sticking then just wait for another 30 seconds to a minute to allow it to finish cooking on that side. It should then be a little easier to release from the grill.
Cook the second side long enough for it to brown. Remove the fish from the grill about a minute or two before you think that it may be done, as it will continue cooking using the heat generated from within it.
There are bamboo, wooden, and variations of metal skewers. It is recommended to soak the wooden skewers in water for at least half an hour before using them on the grill so that they don't burn.
Allow the skewers to hang out over the side of the grill, or place a strip of foil underneath the sections of the skewers where you'll be handling them. This way they won't burn or feel hot to the touch.
Planking is another technique used which
creates a moist subtle flavor that is remarkable and not overpowering.
This method can be used in the oven or on the grill. The fish is placed
on a lightly oiled wooden plank, usually fresh aromatic types of cedar
or almond wood. After preheating the grill to a very hot temperature,
place the planked fish on the grill. Once the fish is about halfway
cooked, turn the fish over and close the lid. Allow it to finish
cooking this way until it's done and serve immediately.

Happy Grilling and Cooking Fish!
Good luck freshwater fishing! Enjoy!