USE FOAM DRY FLIES
Where scum gathers in sheltered bays, so do many terrestrials at the surface, making dry fly tactics a favoured method. It’s a paradox I know, but despite an oily slick, many conventional dry fly patterns (CDC flies in particular) will quickly become swamped. In such areas, it’s best to use foam-based terrestrial patterns like a Foam Beetle or FAPT (Foam All Purpose Terrestrial).
With a foam thorax, the FAPT continues floating in the most adverse of conditions.