What we call yellowtail fishing is anchored chum fishing on or near the reef using a free line technique. Frozen chum melts out of the chum bag and drifts with the current. The free line method is to drop your bait in the water by the chum and let it drift with the chum. So rule 1 is if the bail is closed or you are holding your line to feel the bite you are not free lining. As we say in the South, "I don't care how you do it up North, you ain't up North." So the two basics of yellowtail fishing on the reef is chum and free lining.
Now things can change depending who you are fishing with. I tend to be frugal. That is another word for cheap. Chum costs money, so I prefer to catch the most fish with the least amount of chum since I can only keep so many fish. A commercial snapper fisherman wants to pay the rent so since they don't have to stop at ten fish per person they use lots and lots of chum. It's a profit thing.
To be a frugal snapper fisherman, shallower water is where you can catch more fish with less chum. Shallow water generally means smaller fish, but not always. Commercial snapper fishing with lots of chum is normally done in deeper water where the fish are generally bigger, but not always. It is the not always that makes thing interesting or hard on the wallet.
30 to 70 feet is the frugal fishing depth with 70 to 100 the commercial depth. This is of course general. I know quite a few commercial guys that fish the 45 to 60 range looking for 16 to 20 inch fish. Not flags by any means, but there is a good market for them. I tend to prefer the 14 to 18 inch fish in the shallower water for the first part of my bag limit then try to be a hero after some of the pressure is off. If I have a regular crew that knows the ropes, then we mix it up a bit.
Trained Fish: After fishing the same spots for years, the fish seem to recognize me and pop up to say hello before I even start chumming. Unlike most honey holes where you want to guard your numbers with your life, some yellowtail spots are better shared to keep the fish happy or trained if you prefer. So if you stop into a tackle shop on vacation they will likely give you some decent numbers for a yellowtail spot. This can lead to a little predicament since there is likely going to be a boat on the spot when you get there. Since they are chum fishing, don't get close to their chum slick. If you do a single finger wave is likely. Sometimes they take it real personal. Most of the yellow tail spots though are not little structure areas, they are large coral fields. So several boats can fish the area if they work together by not getting too close or anchoring up or down current of each other. On these trained spots you can limit out in less than two hours most of the time. Giving other boats about 200 yard space is generally good manners in Marathon for 'tailing. In the Tortugas, two miles is a good gap. I don't like fishing up other people's butts nor do I like them fishing up mine, but it is tolerated for yellowtail fishing on "public" numbers. Public is important. If you know that everyone and his brother knows the numbers, you are welcome to join the fun. If you just want to steal some numbers, things can get pretty testy. So use your best judgement.
Patience Pays: For yellowtailing it is better to let the chum do its job and get the fish up before putting a free line in the water. In shallow water that may be just a few minutes or in deep water that may be 20 minutes. When the time to catch begins, try to work like a team. Sometimes one or two lines in the water at a time is more productive. Some times three or four lines started at the same time is more productive. Five or six lines at a time with four or five of them getting tangled is generally least productive. As a fishing consultant on your boat or rental boat, my job is to teach, and people learn from their mistakes so I am perfectly happy to let you learn from your mistakes if you prefer to not listen to me.
Tackle: I definitely recommend a spinning rod for yellow tail. The line comes off the spool much smoother with less effort. You can use a baitcaster, but it requires a lot more attention and tends to be backlashed when a big mackerel crashes the yellowtail party. Hook size ranges from size 4 for a slow bite to 2/0 for a killer flag bite. Bait size varies with the hook size. Leader material is not required if you have new or clean clear monofilament line. In very clear water with spooky fish, flourocarbon can make a big difference, so I always carry 12 pound flouro. Most of the time no weight is required. Small pinch weights, size seven and smaller are useful if your line does want to flow off the reel right. Some people prefer to use small 1/8 ounce jigs or smaller with yellow a popular color. If something works for you, stick with it until someone else starts showing you up. I normally will use a barrel swivel about five feet from the hook if I need weight. It also helps cut down on line twist where there aren't enough mackerel in the area.
Snapper Don't Like Wire: Speaking of mackerel. If you get tired of them cutting your hooks off, you can use a light wire leader, number 2 to number 4 about 6 inches long to catch these guys. You will probably find that since you are ready for mackerel, that you start catching yellowtails. Fish love to make a liar out of people, so don't believe everything you hear, sometimes wire works like magic.
Flag Fishing: Flag yellowtail are fish over 20 inches or close to three pounds. For their size, yellowtail are pretty good scrappers. So flags can be a challenge. With a good flag bite you will need fifteen pound line minimum with twenty a good idea unless loosing fish is not big deal to you. Flags take off for the bottom as soon as they feel the hook so you will need a heavier drag setting than normal, about 50% of line test. The heavier line and the heavier drag setting means larger or stronger hooks. Size 2 to 2/0 depending on things.
Bait: Fresh is best. Yellowtail are not all that finicky. They will eat nearly anything. Some baits are better though. Fresh ballyhoo fillets are great. You can cut them into just about any size you like. You can get all fancy and stuff a little chum in a roll of fillet. You can also catch fresh ballyhoo most to the time you have a chum slick going. Silver side minnows are also a great bait. They are in the right size range, the right shape and they smell like fish. Since they are small fish, you don't have to slice them up. Just stick one or two on a hook and get to fishing. Fresh pilchard are a great bait alive or dead. You will probably have to cut some down to size, but if you have the two to three inch pilchards, just stick 'em on the hook. In the winter, it is a good idea to have live shrimp. Not that you need them for yellowtail, they just can increase the variety of fish that take your line for a ride. Squid is the bait of last resort in my opinion, messy and smelly, but it does work. Size is the thing with the baits. Try 1 1/2" to 2 inch strips and adjust as needed.
Chum: There are quite a few "secret" recipes. I know one guy that cooks elbow macaroni for his chum bag. Others have a blend of ground bait fish, menhaden normally, rolled oats, Purina Shrimp Chow, dry dog food, dry cat food and/or anything they may have read about in a glossy magazine. Ground baitfish, menhaden or sardine, chum works fine with some oats on the side not a bad idea.
Chum Bag: Small mesh bay, medium mesh channel and landing net deep. With a small mesh bag a 7 pound block of chum will last nearly three hours. With a medium mesh a 7 pound block will last about an hour. With a landing net mesh bag, a seven pound block of chum is nearly a waste of time, get the 25 pound block. At a buck a pound average, you can figure out how long you want to fish and how much you are willing to pay for snapper.
The Bite: If you are free lining properly you may never feel a bite. You will just see the line start to peal off faster or sometimes slow down. When it does, just close your bail and reel. Nothing to it. Notice, that I did not mention a hook set because it is not only not required it is not a good idea most of the time. Save the Bassmaster's hook sets for bass fishing. Anytime you close your bail, you may as well reel in and check your bait. The 'tails will almost always bite while the line is drifting with the chum. They only bite with the bail set to make me look like a liar.
Starting The Drift: No casting is generally required, just drop the line in the water and let it go. If you have birds or trash fish bothering you, casting may help. Some like to shake the bag at the start of the drift. Fishing shallow with a medium mesh bag that is not a bad idea. A little splash of oats works on a slow day. Watch the fish to see what works best. Deep water fishing you should have a steady chum flow, so no shaking required.
Conditions: Wind with the current is best. If the chum is flowing out of the back of the boat things are looking good. If the chum is flowing up the anchor line, things will be slow. If the chum is falling straight to the bottom, no current, things can be interesting. I catch most of my hogfish and larger porgies with no current, but most of the time the 'tail bite is very slow. Cloudy water normally means a better bite with shallower better than normal. Crystal clear water normally means a slower bite with deeper water better.
One of the great things about yellowtail fishing in the Keys is all the other critters that may show up in the slick. Have a rod ready to pitch a big bait to a big fish. Having a bottom rod down with a big live bait is also a good idea. The main thing is to take advantage of the opportunities that present themselves.