Trout seem more easily spooked in the winter. A trout’s metabolism makes them want to seek out pools near the bank that are warmed by the sun, so they are more vulnerable. Also due to the slower metabolism, the fish may be sluggish. They often will bite gingerly, hook-ups will be primarily in the tongue. As trout may often go after several small insects in one mouthful. So rigorous setting of the hook is not required. Winter is the time to use subsurface flies. Although, flies will hatch and trout will rise to them under the right conditions during the cold weather months, drifting nymphs under a strike indicator is the most dependable and productive winter fly fishing technique to use this time of the year. Pheasant tails, golden hares ear, stone fly, midge, weighted lures, especially the goldheads are also a popular fly throughout the cold winter months, I would suggest you take quite a few different colours if this is your chosen method. Gold head trout flies can be fished on any line, from a full floater to the fastest of sinkers.
Downsizing your fly patterns is recommended. Sizes 12 to 20 works best since like the fish, the bugs have a slow metabolism in winter also, and they haven’t grown to their full size yet. Downsizing your leader and tippet is also recommended. Many times the water will be more clear in the winter. Dropping down a size or two will help prevent leary fish from being spooked.
Mild winter days, specifically afternoons, can provide some shirtsleeve fishing opportunities. Coastal regions that rarely get snow or ice, need to be checked more for river levels. Three or four days of steady rain can make a river unfishable. It is also rarely enjoyable to fish in well below freezing temperatures. Even if you can dress warmly enough, the ice on the guides make casting nearly impossible, freezing fingertips and the slickness of entering the river is extremely dangerous. Extreme care should be taken when wading in general, one slight misstep, can send you into the river, dangerous all year, this is multiplied by the cold air temperatures in the winter. Also light is much less, making it difficult to see the river bottom while wading. So step very cautiously. And always fish with a partner.