This behaviour is seasonal and not restricted to winter. In winter and early spring, the shallows heat up sooner than deeper water and warmer water means more prey-activity. Gammarus (around all year), sculpins, sticklebacks, shrimp etc. are active and draw in the trout. Trout actually remain active in the shallows for a long time, but for a shorter and shorter period the water the water gets. Which means getting up very early and bu midsummer, fishing right through the night.
Summer fishing can be excellent on deeper water, but as the sun sets and rises again, you can *very* often find feeding fish close to shore, especially in the mornings. Just as light is about to break, a small gammarus or shrimp can be quite effective, and I usually begin by fishing them very high in the water. As more and more light pokes though, prey go deeper and deeper, and as the sun rises fully on a summer morning, I’ve very often had good success with a gammarus fished right over the bottom on deeper water.
But back to the shallows. Fishing shallows of course presents the challenge of keeping your fly free from the bottom. You can do this with a fast retrace, but a fast retrieve isn’t always the most effective in cold water. That’s when a “hover fly” often comes in handy as I wrote about some weeks ago.
You can incorporate foam into most fly designs, but especially with gammarus and shrimps it’s obvious to add a shell back of foam.
Hovering flies aren’t just for winter - for me they are a vital addition to the fly box year round. What they all have in common is that they are small. In the shallows, sea trout have no trouble spotting small prey.
Have a great weekend!