When do I use an intermediate fly line? Do I even need an intermediate fly line? Many times a floating line will get the job done in shallower waters, but fly fishing with an intermediate line can offer several advantages here as well.
Intermediate lines are designed to sink very slowly, typically about 1–1.5 inches-per-second (ips) or so. Designed in two different styles, there's full intermediate where the entire line sinks, and intermediate sink tip where only a certain amount of the end of the line sinks while the rest floats. Obviously, not every fly line is perfect in all aspects, so let's run through a few of the pros and cons of intermediate fly lines:
Keeping a Fly Deeper....but Not Too Deep - Intermediate lines sink very slowly which makes them great for fishing shallower waters. During the retrieve with a floating line, the fly tends to rise up in the direction of the line (the surface), especially on a brisk retrieve. Of course, the degree to which this happens also depends on the fly style/weight, too. Regardless, even in shallower waters, this might cause the fly to ride just a bit too high in the water column for an effective presentation.
Since the intermediate line sinks slowly, it helps keep the fly subsurface without plunging too quickly into the depths and scraping bottom. This makes it really effective for fishing over shallow weedbeds and such. An intermediate can get down a ways if you're willing to wait it out, but I like them best when fishing in about the top 8 feet of the water column.
Wave Action - Adding to the point above, wind chop or wave action can really mess up a retrieve. With a floating line and light fly, the entire line can get tossed around easily and the fly may ride so high in the water due to the wind/waves that it can actually break the surface at times during the retrieve. Whether sink-tip or full, an intermediate line works really well for staying below the chop and keeping the fly submerged.
More Stealth - Intermediate lines are typically clear or at least a very muted color for some or all of their length. This makes them less visible and less likely to spook fish.
Shorter Leaders - Due to the fact that intermediate lines are typically pretty low-key in color means you can use a shorter, simpler leader with less chance of freaking the fish out. Since the line sinks as well, the shorter leader will also allow the end of the fly line pull the fly down and keep it there more efficiently.
Easy to Cast - I find intermediate lines to be easy to cast. Any line or portion of a line that sinks has a thinner diameter than a comparable floating line, so they also slice into a headwind better and deliver the fly with more authority.
Fishing...the Surface? - An intermediate line for fishing the surface? Disclaimer: I've never tried this. However, some folks have mentioned fishing their topwater streamers (particularly patterns like big poppers, Gurglers, and Crease Flies) with an intermediate line. I guess the line gives these flies and others a more unique action and also causes them to go subsurface for parts of the retrieve depending on fly, speed, and casting distance. For this reason, an intermediate might be an interesting choice if you want to just bring one line to primarily fish subsurface but maybe want to dabble with topwater as well.
Mending - A full intermediate line can't be mended like a floating line, so this could be a headache if fishing in an area with current. However, using a sink-tip line that has a short intermediate tip can offer good line control since the majority of the line floats and can be mended/manipulated easier. The downside of that is, depending on how you're fishing and the exact length of the sink tip, it might not get or keep the fly down as effectively as a full-length intermediate line.
Loose Line Sinks - As you strip intermediate line, the loose line you pull in will sink. If fishing from shore or aboard a boat this means little to you, but if wading the added line drag in the water can be an annoyance when time to re-cast. Again, a sink-tip line can alleviate some of this headache since most of the line floats allowing for improved line management.
Going too Deep - An intermediate line may cause the fly to go too deep for where you're fishing, especially when casting long and stripping slowly with a full intermediate. Sometimes, it's better to just use a floating line and a weighted fly when fishing slow in shallow water. A longer leader with a thinner diameter will also help keep the fly down, while fluorocarbon is also a good choice since it sinks quicker than monofilament.
Sight Fishing - For me, sight fishing can be tough using a full intermediate line, especially one that's crystal clear. I find it hard to track the whereabouts of my fly since the line sinks and blends into the water so well. Using a short intermediate sink-tip line can make this slightly less frustrating since the easy-to-see floating portion helps you to trace the fly's approximate location a little better.
Line Pickup - Since a floating line is on the surface, it's easier to pick up more line from the water when it comes time to make another cast. With an intermediate line (especially a full-length one), all of that subsurface line creates increased resistance. When compared to a floater, this means you'll have to strip in more line before you can cast again without overloading the rod.