Fluctuating hydrodynamic forces increase fish locomotor variability in turbulence

Abstract

Turbulence has been shown to alter fish locomotion and increase the energetic cost of swimming, yet the mechanisms underlying these effects remain unclear. To address this, we studied giant danios (Devario aequipinnatus) swimming at multiple speeds in both laminar and turbulent flows. Within this turbulent regime, mean tail-beat kinematics were largely unchanged relative to laminar conditions, but cycle-to-cycle variability increased, with larger effects occurring at higher speeds, where the velocity fluctuations are stronger. Turbulence also increased the frequency of asymmetric tail beats, particularly under stronger fluctuations, providing further evidence of increased locomotor variability. A quasi-steady hydrodynamic model suggests that velocity fluctuations in this flow regime produce relatively small changes in mean thrust and drag, whereas thrust variance scales with the square of velocity fluctuation magnitude. These variable forces likely prompt fish to adjust locomotor output to maintain position, which could explain the observed increase in kinematic variability. We further show that mechanical swimming power can be increased through correlations between fluctuating hydrodynamic forces and tail velocity. Together, these results suggest a mechanistic link between turbulent velocity fluctuations, locomotor variability, and increased energy expenditure.