Meniscus Profiling of Fly Fishing Lures by Optical Diffraction

Fly fishing is a method of fishing using hand-tied lures called "flies" that mimic local prey like insects or crayfish. In this study, we focus on fly fishing in calm waters using "dry" flies, which are absent of any gel coating, and float on the water surface by virtue of their construction of low-density materials. Despite an 1800-year history of fly fishing, little is known how these flies look like to fish. We introduce an optical method using the diffraction of transmitted underwater riverbed patterns to quantify the meniscus shape around floating objects. We use this method to describe the meniscus shape around a commercial fly lure, a live cockroach, and a desiccated house fly. We find that fresh insects are denser than water, creating convex menisci while dead insects and fly lures are buoyant, creating concave menisci. On the water surface, the meniscus provides a larger visual signature than the insect itself, conveying information such as body density, weight distribution, motion of legs, and body perimeter. We speculate that the meniscus helps fool the fish into striking.