If you see spawning fish, cast the fly so it will sink to the bottom downstream from the happy couple. It should be fished dead drift near the bottom during spawning seasons. If you see spawning salmon, cast so the fly lands just behind them and drifts down stream from the spawning pair. Many fly anglers won't use an egg fly because it looks too much like bait. I think that is a silly reason not to use a fly.
Egg flies imitate a naturally-occurring food that wild trout, steelhead, and salmon eat with gusto.
It is generally helpful to use a color of fly that matches the overall color of the riverbed. Some rivers are generally brown like weak coffee without the milk and these conditions call for a dark brown or black fly such as the Munro killer, Thunder and lightening or Stoats tail. Other rivers that flow off bare rock or limestone are often crystal clear. During early spring and late autumn when the rivers are in full flow after a recent storm try some of the brighter flies. Use garish yellow and orange flies for cold days like Ally's Shrimp, General Practitioner, Dunkeld, Garry Dog, Adrian's Deer Hair Prawn, Adrian's Orange Shrimp and Silver Doctor.