The Pliva is a river in Bosnia and Herzegovina noted for its clean water and excellent fly fishing. That area has hosted several World Fly Fishing Championships, but definately in 2016.
The Pliva is very clear and most of the water comes from inside a mountain in the form of two springs. The Pliva is a difficult river to fish and has Greyling, Rainbow and Brown Trout, and several other fish. One of the most successful flies for the Pliva is the fly shown below.
The top photo is shown in regular light and the second below is with a Ultraviolet light shown on the body. Fish apparently can detect UV so many of these styles of flies have UV sensitive materials in them.
We tied this Pliva Perdigon style of fly in our Rocky River Trout Unlimited fly tying class in January of 2018
This is the same fly, but viewed from a different to take advantage of the UV.