Surf’s up! The energetic costs of labriform swimming in unsteady flows
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Swimming represents the primary means through which fishes interact with their environment, and factors influencing swimming performance can profoundly affect their distribution and survivorship. Traditional measures of swimming performance are estimated under laboratory conditions using steady water flow. However, these experiments potentially underestimate the actual cost of swimming under unsteady water flows that characterize natural systems. Using a swimming respirometer and video recordings, we swam Cymatogaster aggregata using a standard Ucrit swimming trial under one of three flow conditions with the same mean water velocity at each speed increment: steady flow (control), low amplitude water velocity fluctuations ( A=0.5BLs-1) and high amplitude water velocity fluctuations (A=1BLs-1). We found that unsteady flows increase the metabolic cost of swimming, but only when high flows push a swimming fish beyond the threshold for exclusively aerobic metabolism (beyond Uburst). Furthermore, unsteady flows at the highest amplitude treatment decreased the maximum mean velocity (Ucrit) and mean gait transition velocity (Upc) achieved by individuals compared to the control and low amplitude unsteady flows. However, lower costs of swimming than predicted in low amplitude flows below Uburst suggest that fish are able to take advantage of the cyclical wave patterns and economize energy expenditure. Mean pectoral fin beat frequency did not differ significantly among treatments. This is the first study exploring the costs of swimming under unsteady flow in a marine labriform swimmer.