Spring is often the best time of year to go fly fishing because many species are very active when the water first starts warming up. Many species of freshwater fish respond to the warming water to find places to spawn. In addition, spring is when they start feeding more actively. Once the ice has disappeared, and the water begins to warm up, most fish start feeding heavily.
Going after spawning fish is legal in many areas, but should check where it is allowed, since since it makes for some of the fastest action of the year. Trout will certainly feed actively at this time, as will perch and other species. And bream get more active as the water temperature rises.
If you enjoy saltwater fishing, springtime brings many species of saltwater fish into rivers to spawn. However, spring seems to come later in salt water since there is more water to warm. Spring is the beginning of the migration north by many species that spend the winter in warmer tropical waters. So be at the ready if you happen to live on a migration path. As with freshwater fishing, it may also be illegal in some regions to go after spawning saltwater fish, so make sure the species you seek on your fly fishing trip are not in a protected area