There are many sinking-line choices facing the beginning saltwater fly fisher. As our sport gains popularity, manufacturers are eagerly vying for our dollars with an ever-increasing, and often bewildering array of new lines. Unless your name is Bill Gates, you can't hope to have one of each, and even if you did, you would still be faced with the choice of which lines to take on any given day.
If you know what species you plan to target, there is no reason to pack more than two sinking lines. Many species such as halibut, lingcod and rockfish are found only near the bottom. For the deepwater bottom huggers you will want the fastest sinking line you can get your hands on. Many other species including bonito, tuna, dorado and coho salmon tend to feed near the surface. When targeting near surface feeders choose a line with a sink rate of 1 to 3 inches per second.
If the fish are at an intermediate depth Ñ say 6 to 20 feet Ñ you can reach them with either a slow- or a fast-sinking line. To reach intermediate depths you need to count down as your line sinks. For example, if you want to fish in the 15-foot range, all that's necessary is to know the sink rate of your line and the ability to do some basic math. For example, a line that sinks at a rate of 6 inches per second will reach 15 feet in approximately 30 seconds.
Remember that several independent tests have found that some lines do not sink as rapidly as advertised. When in doubt, give it a few extra seconds to reach the strike zone.
Remember also that wind and current can move your boat, which has the effect of lifting the line in the water column. The same is true for a rapid retrieve. As you strip in line, your fly will move toward the surface. For all these reasons it is often best to err on the side of a more rapidly sinking line. Why wait 30 seconds for a slow-sinking line to reach the strike zone when a fast-sinking line can get you there in half the time?