Flexible paddle sheds new light on speed: a novel method for the remote measurement of swim speed in aquatic animals

Abstract

Speed is a key determinant of energy expenditure in free-living animals, and particularlyin marine vertebrates, where power requirements for swimming increase as a cubed function ofthe speed. However, current devices used to measure swim speed in free-living animals have limitations,including excessive drag, low resolution, high stall speed (ca. 0.3 m s–1), cost, biofouling andsusceptibility to damage. We present a speed sensor system that utilises the reflectance of infraredlight against a flexible paddle that bends in relation to the flow of water over the study animal. In laboratorytrials, this performed well across a range of speeds (0.1 to 1.75 m s–1), and had a stall speed of0.1 m s–1. The advantages of this present paddle system are that it is impervious to the presence ofmatter in the water column, is inexpensive and easily replaceable. Furthermore, the system is able torecord speed data at an unparalleled resolution, limited solely by sampling frequency. Data fromdeployments of devices on free-living imperial cormorants Phalacrocorax atriceps identified changesin speed within and between swim strokes, and also showed that greater speed was generated perkick as the buoyancy decreased with depth. As such, the flexible paddle system holds promise for themeasurement of speed in free-living, aquatic animals.