Fly selection when working the spring spawning run can be critical. Hatches are very sporadic and light when they do occur and the pickings are slim, as there just isn't the diversity of emerging insects during early spring as you find later on in the season. Caddis and midge nymphs are always a safe bet to fish through out the year as they are always present. With early spring, however, I often think of the other March Madness, the stonefly hatch.
Staging fish aren't always interested in feeding, as they have one thing on their minds, spawning. And rarely do I find suckers feeding on the surface due to their subterminal mouths. So I fish low and slow. I like fishing tungsten weighted stonefly and drake nymph patterns like my Thunderbird, Boreal Bomber, and Fresh Pimp nymphs and bounce them along the bottom. Every so often a fish may take an emerging fly on the rise, at the end of the drift, but this is not typical during the prespawn phase. If you need more weight to reach bottom, throw on some extra lead. Suckers will hold tight to the bottom and will only feed if a fly is easy pickings. I like to utilize tightline nymphing during this time of year, as I can better detect the subtle takes. Bouncing the fly on the bottom using your rod tip will also add to the detection as well as help give your fly the appearance of a dislodged nymph tumbling along the bottom of the stream bed.
This isn't bobber fishing or fishing for stockers, this is a numbers game. Getting your fly in the zone is crucial, as are the repeated runs and drifts of your nymph. Fish may spook and get lined, but they'll quickly roll back into their hold in the current seam, jockeying for precious spawning gravel real estate. It is to your advantage that you can keep drifting your fly through the zone in the hopes of an eventual hook up.
Your ROI (return on investment) may be low but if your put your time in, you will rack up the numbers of caught fish.