The BTS is a fly that we have been using for many years. It imitates salmon fry very effectively in the spring, and can be extremely deadly at times. We seldom fish in the spring without this fly in our box! It is effective fished as a nymph or on the slow swing and has application anywhere salmon or steelhead fry are present. This fly is tied by Aqua Flies, a great commercial fly tying company. It is tied to Kevin Feenstra's specs.