Blue Duns and surface film

Okay. The actual behaivior is that the mayflies hatch (most at the
surface) from the nymph, become the "dun" and fly away. This is the
behaivior you described with the bug coming to the surface in a
bubble. Later the spinners, or egg laying adults drop onto the
water and release their eggs into the water, often, they just stay on
the water and die. They then slip beneath the surface film.
So, with these two situations, you'll have 4 major feeding actions and
their tale signs.
The first is when the mayflies are hatching from the nymph. The
mayfly shoots to the surface and tries to fly away as quickly as
possible. The trout will rocket to the surface in pursuit. This
makes a big splashy rise as the fish feed on the emergers.
When the dun hatches out at the surface, sometimes, due to damp
weather or wind, the dun will get stuck on the surface. The rise on
this one is the fish half heartedly chasing the bug to the surface and
then realizing it can just sit at the surface and munch. Not really
splashy, but a very determined rise.
The third one is with the spinners hitting the surface, before they
drop below the film. The fish will porpoise and sip the bugs off the
surface. Normally very gentle.
The final one is then spinners that have slipped below the surface.
This one you may miss. If you see the spinners dropping onto the
water, use a spinner pattern with splayed out wings as a sub-surface
fly. Fish it like a nymph. You'll nail tons of fish.
Frank Reid