Slight Leech Olive - Leech. A Olive Slight Leech can be a formidable tool in the early fall months when the trout are feeding heavily upon young leeches. The simplistic design of the Slight Leech resembles that of a young leech making it a tasty treat for the hungry trout. This fly is fished best with a longer leader of 15-16 feet, left to sink just slightly and brought in extremely slowly. Follow those simple guides and you are sure to bring in one mighty trout.