Blood worms should be a staple in any anglers fly box.
Works well for Yellowfish as well as Trout in both rivers and lakes.
There are many variations to this fly, this one with the extra movement of the marabou tail works exceptionally well for trout in our lakes.
Fish upstream nymphing in rivers (don’t be afraid to fish it deep) and below a strike indicator in lake.
A great pattern often paired up with a nymph on a tandem rig.