I too have fished the lake. 15' is pretty shallow for kokanee but sometimes they can be found there. My guess would be that they are large holdover trout, I have seen some monster blips on the radar down about 150 + feet but never bother, I always go about 30' and run for kokes. If you find a good concentration of fish in a single depth use downriggers with either pop-gear or small spoons and plugs such as needlefish mepps and Rapalas if they are trout they should bite..... if not.... Don't know what to tell ya!