Sprat are small, herringlike fish. They look almost exactly like herring asnd can grow almost as big (up to arounf 6-6 inches I think), but when fishing for mackeral, the schooling of sprat we observe are rarely more than 6-7cm long and that also goes for the ones we find in the stomach content of the mackerel.
Small, silverlike, sometimes with a light greenish back, but often just silver-silver-silver. And of course, a predominant eye. I believe imitation is an important weapon, or skill, and so, a sprat imitation is an important addition to the fly box.
The fish imitation *every* saltwater flyfisherman knows is the legendary Surf Candy by equally legendary Bob Popovics, and perhaps, a Surf Candy in a few different sinxes really is all the fish imitation anyone will ever need. But the Surf Candy is of course a sand eel imitation, and they sand eel look quite different. Sprat are shorter, taller, slight more "round bodied" and more silver.
For this type/shape of fish, I prefer Morten Valeur's "STF Fish". Morten is a well known flyfisher and flytier in Denmark and Scandinavia with several good and proven patterns to his name. I think the STF Fish is the most well known and certainly one of the more effective. It's not a fast tie as such, although if you do the prepration for say 10 at a time, the don't take long. For imitating sprat, I tie it in all white and with plenty of flash. STF is a synthetic dubbing that readily accepts a marker, so sometimes I'll tint the back olive, but mostly I just leave it white and flashy.
There's a tying instruction video here.
Sprat are present in coastal waters from around April (where the come in to spawn) and until winter sets in where they leave for deeper, warmer water. I believe that some inlets/fjords are habitats to resident populations, but I assume that they still seek deeper, warmer water suring the winter.
Have a great weekend!
PS. All major, Danish salmon waters are still closed due to temperature and extremely low water levels, so if you're planning on coming for salmon, don't :-).