Anglers and crew that primarily target billfish really only have three choices when it comes to fishing line. Each one comes with special attributes that shine at certain times, under certain conditions. Yet not one of them will do everything I want my fishing line to do in all situations. Sometimes I even need to use all three of them at once!
The lines I'm talking about here are mono, Dacron and Spectra.
Nylon monofilament is by far the most widely used fishing line in the world today. Dozens of companies make this plastic, single-stranded line in more breaking strains for a wider variety of uses than all the other types of fishing line combined.
Nylon, the man-made raw material from which mono is created, started out as a six-carbon-chain structure, but today's family of nylons includes molecules with many more carbons and is further complicated by the addition of sophisticated groups of chemicals. The unfortunate upshot of all this is that anglers can no longer really trust the label on a spool of monofilament line - especially when it comes to breaking strength.
In past years, any knowledgeable tackle-shop owner could measure the diameter of a piece of mono with a micrometer and tell you quite accurately what its breaking strain would be. Today, because of the varying chemistry, and to some degree by changing the temperature and draw rate, a thin line can actually be significantly stronger than a thicker line. (See "History of Line" at www.marlinmag.com/historyofline.)
For most of my big-game sport fishing, I actually prefer a mono with a thicker diameter. Thicker line suffers less from small amounts of chaffing than thin line and will usually outlast a thinner line in long, tough fights under heavy drag. Heavy drag can also cause thin line to dig down into itself and cause a break, especially if the line underneath wasn't packed on tight. This often happens when a fish changes direction and allows an angler to recover a couple of hundred yards of line quickly under low pressure. (Although, mono is the least likely of all the line types to bury itself.)
Unfortunately, the good old thick mono is fast becoming very hard to find, so I'm hoarding my spools for use as top shots to help me get the most use out of my finite supply.
In the cutthroat world of marketing, the truth often suffers. In my mind, claims like "the strongest 30-pound line you can buy!" are false and misleading advertising. The manufacturer of the line knows full well that the diameter of the line may equal what was once the standard for 30-pound, but it's not really 30-pound-test. It probably breaks closer to 50 than 30 because of the changes and improvements made in the raw materials and the manufacturing process.
If a spool of mono's label says "IGFA Class," it means that the manufacturer tried to keep the true breaking strength at, or just below, the labeled strength. No one, however, makes any guarantees. Luckily, it's pretty easy to test the real breaking strain of mono. A lot of tackle shops and fishing clubs have testing machines, and most will allow anyone to come in and find out the true strength of their line.
Other attributes such as knot strength, abrasion resistance, ability to withstand UV and the extent to which the line will stretch without breaking (more than one-third of its length on many types) are also variables that can be controlled by changes in the line's manufacturing process. Spool labels often lay claim to low stretch, good knot strength or high abrasion resistance.
With all that said, the better line companies constantly test and rework their offerings, and today's monofilament nylon lines represent the best, least expensive fishing lines ever made.
Every morning my crew and I check our mono lines from the connection at the swivel, up past the point where we put it in the rigger clip. If there is no discernible chafe and no milky, opaque change in its original clarity, we continue to use the line. A mere fading of the line's color is just a change in the dye and doesn't affect the line's performance.
Consequently, we don't get the surprise breaks with mono that occasionally occur when we use Dacron braid. I suspect that the mono's shock-absorbing qualities keep it from being damaged internally while towing big baits or lures.
During a day's fishing, our lines frequently contact one another or get briefly tangled around rod tip or guides (or badly designed and positioned zinc anodes). But it's always easy to check mono and know whether it is OK to use or if the line needs changing.
The stretch in mono also keeps violent yanks from tearing out hooks snagged in soft tissue and not lodged in or behind bone.
Monofilament fluorocarbon is much more expensive than nylon mono, and while highly regarded by many anglers and crew as a leader material, it's not commonly used as a fishing line. The main attraction is its near invisibility in water - due to its index of refraction being closer to that of water than nylon. (Braided lines do not allow the passage of light through the line.)
I've found that fluorocarbon leader, even in lengths as short as four or five feet, increases bites when using live bait, when bottomfishing, or when chumming or chunking for tuna or other sharp-eyed species like snapper. However, I'm not convinced of its value when trolling, especially when pulling lures at faster speeds compared to those usually used with dead baits or when kite fishing, where the leader is mostly out of the water anyway.
Under pressure, both nylon and fluorocarbon monofilament line exhibit a large amount of stretch. The accumulated pressure of hundreds of turns of stretched line around a reel trying to return to its original length puts tons of pressure on the spool's spindle and side plates. This can increase the drag, "lock up" the reel and even spread the spool and ruin the reel. (Even new, top-dollar fly reels should not have mono backing under the fly line.)