A purposed made fly to imitate the small perch fry as they grow from egg till small fry.
Easy tied and can be fished on a floating or intermediate line dependent on depth where the fry shoaling.
There are many successful fry patterns out there,this little pattern can be deadly when the fry reach a sizeĀ to interest the rainbow trout.
You may have to be intuitive with your retrieves ,study the small shoals will let you see how they dart about in the water.