The Crease Fly is a type of fly that was originated for saltwater by Capt. Joe Blados of Greenport, NY. It was created to target striped bass and bluefish but has proved to be effective on many types of saltwater species and freshwater species as well. I was recently searching for flies for dorados and the Crease Fly was one that was recommended.
A common mistake is thinking Crease Flies are poppers, but that was not their original purpose. They can be fished in this way, but won’t have as much splash as an actual popper. The Crease Fly is intended to be fished sub-surface on a steady retrieve preferably against a current.
Below is a good video describing how to tie a Crease Fly.