Fluorocarbon line can be used just for leaders (common) or for the main line - depending on a few factors.
Fluorocarbon – Line vs. Leader
Nearly-invisible fluorocarbon fishing material is manufactured in two different methods, and for two different purposes. It is made as line to put on a fishing reel, and as leader to use at the end of your fishing line. They are not manufactured by the same process, and they should only be used as the manufacturer label instructs them to be used.
Some of the older fluorocarbon line made for fishing reels are strong, but are stiff and retain the shape of the spool especially when cold. Recently, some manufacturers are making a new type of fluorocarbon fishing line that is more flexible and less stiff on the reel, making casting much easier with fluorocarbon line.
Fluorocarbon fishing line material has much less memory than the leader material. Currently, the line is only available up to 25 pound test due to the fact that manufacturers have not developed a way to compensate for the memory or the production cost.
Fluorocarbon line should be used for line only on your reels and not for leader because it is softer, more supple, thinner and less abrasion-resistant then the fluorocarbon leader.
Fluorocarbon Line – Pros and Cons
Fluorocarbon fishing line is very useful in clear water because it is nearly invisible. The super-clarity comes from having a refractive index that is approximately equal to that of water so that light goes through fluorocarbon fishing line as easily as it goes through water. Water has a refractive index of 1.30 and fluorocarbon is 1.42, which is very close on the refractive scale.
When manufactured, fluorocarbon is comprised of a carbon base combined with other materials. At first glance it resembles monofilament, although the similarities end there. Most monofilament lines stretch substantially whereas fluorocarbon has hardly any "give or stretch."
PROS and CONS of Using Fluorocarbon
Fluorocarbon is nearly invisible in water, making it ideal for fishing in very clear water, when pursuing fish with excellent eyesight, etc.
Fluorocarbon is fairly dense and sinks easily, which can help get the bait to the bottom quickly.
Fluorocarbon is great for instantaneous hook sets as it does not stretch.
Fluorocarbon is superior in abrasion-resistance to all other fishing lines, taking abuse that could never be tolerated by monofilament or braided lines. However, any sort of nicks and scratches make fluorocarbon visible at those points. Most anglers cut that nick out, or otherwise replace that section of line.
It has a built-in resistance to elemental factors such as sunlight and chemicals, so it does not break down as quickly as monofilament or braided line.
Fluorocarbon's density and heaviness is not ideal for use with Top-water lures since the heavy line can sink, which in turn will make it more difficult to present and work your lures. Poppers, Walk-the-Dog and prop baits are intended to float high on the surface. Using heavy fluorocarbon line sometimes forces these top-water baits down under the surface.
Fluorocarbon line is slippery and knots can potentially work themselves loose. So properly drawing your knot is important. Also, an extra wrap or two will help eliminate knot failures. (Read the Ask a Captain the end of this article on the best knots for tying fluorocarbon leader material to monofilament line).
Fluorocarbon is expensive.
It is hard to see above the water, which makes it easy to lose track of your line.
Fluorocarbon Leader Material -- It is NOT Line
The abrasion resistance combined with its super clarity makes fluorocarbon a superior choice as leader material. The fluorocarbon leader is processed differently than fluorocarbon fishing line. It is thicker, harder and even more abrasion-resistant. Fluorocarbon leader is great to use with braided fishing line, since braided fishing line is very visible to fish. As leader material, it is available in a wide variety of test values, ranging from 10 pound to over 100.
Fluorocarbon leader is great for instantaneous hook sets as it does not stretch. When fishing braided line with live baitfish, it is recommended to use a substantial length of fluorocarbon leader – at least 30 inches or more. The braided line is visible to fish, so using the extra long fluorocarbon leader will keep the baitfish away from your braided line. When using artificial lures, a shorter fluorocarbon leader can be used of anywhere from 4-to-20 inches depedning on applications.
There are certain instances where anglers can save a little money and substitute to a monofilament leader (as monofilament leader is significantly less expensive). If the fish are in a heavy feeding mode, sometimes they appear oblivious to the leader. If this is the case, try a length of monofilament leader instead of fluorocarbon leader. Also, if fishing in muddy, dirty or low-visibility water conditions, you can get away with a monofilament leader. If you aren't getting bites, switch to back to fluorocarbon.
If your fluorocarbon leader gets scratched or nicked, it now becomes more visible defeating the purpose of being invisible. So it needs to be changed. That nick or scratch also weakens its breaking strength by a considerable percentage. Fluorocarbon leader should be used only as leader because it is thicker and more abrasion resistant.
Many fly anglers rely on fluorocarbon as leader material. In certain conditions, some fly anglers choose to fish a fluorocarbon leader on their floating fly line as opposed to switching to a full sinking line. When tying your fluorocarbon leader to your main line, it is a good idea to use a line to leader knot rather then a swivel. A swivel is much more visible then your line to leader knot. (see article below on leader knots).
The science behind the manufacturing of Fluorocarbon is an engineering marvel. To make a fishing line that has the same refractive index as water itself, thereby making it nearly invisible to fish, is quite an achievment.
While Fluorocarbon line and leaders cost a bit more than traditional fishing lines and leaders, the payback comes in spades. If you take a friend (or client) to the Florida Keys to pursue the elusive bonefish, you will be fishing in very clear water. If you use anything but Fluorocarbon for your line and/or leader, you will likely be eating at Sloppy Joe's and paying $30 for a bonefish dinner.
Many brands of Fluorocarbon are on the market, but we have had good experience with Shinsei Japanese Fluorocarbon material by P-Line, and their prices are good also.
The Online Fisherman