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 particularly in marine vertebrates, where power requirements for swimming increase as a cubed function of the 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 and susceptibility to damage. We present a speed sensor system that utilises the reflectance of infrared light against a flexible paddle that bends in relation to the flow of water over the study animal. In labaratory trials, this performed well across a range of speeds (0.1 to 1.75 m s-1), and had a stall speed of 0.1 m s-1. The advantages of this present paddle system are that, it is impervious to the presence of matter in the water column, is inexpensive and easily replaceable. Furthermope, the system is able to record speed data at an unparalleled resolution, limited solely by sampling frequency. Data from deployments of devices on free-living imperial cormorants Phalacrocorax atriceps identified changes in speed within and between swim strokes, and also showed that greater speed was generated per kick as the buoyancy decreased with depth. As such, the flexible paddle system holds promise for the measurement of speed in free-living, aquatic animals.