The Royal Wulff murders / Keith McCafferty.

Enhanced descriptions from Syndetics:

The first novel in the clever and fast-paced Sean Stranahan Mystery Series.

When a fishing guide reels in the body of a young man on the Madison, the Holy Grail of Montana trout rivers, Sheriff Martha Ettinger suspects foul play. It's not just the stick jammed into the man's eye that draws her attention; it's the Royal Wulff trout fly stuck in his bloated lower lip. Following her instincts, Ettinger soon finds herself crossing paths with Montana newcomer Sean Stranahan.

Fly fisher, painter, and has-been private detective, Stranahan left a failed marriage and lackluster career to drive to Montana, where he lives in an art studio decorated with fly-tying feathers and mouse droppings. With more luck catching fish than clients, Stranahan is completely captivated when Southern siren Velvet Lafayette walks into his life, intent on hiring his services to find her missing brother. The clues lead Stranahan and Ettinger back to Montana's Big Business: fly fishing. Where there's money, there's bound to be crime.