Swimming Speed of Zebrafish in Jet-Stirred Turbulence

Turbulence is heterogenous in riverine habitats. The ability of individual zebrafish (Danio rerio) to regulate swimming under turbulent conditions has not been investigated. We examined age- and sex-specific behavioural responses using a controlled jet-stirred tank that generated turbulence levels spanning turbulence kinetic energy (TKE) values from 3.5 & times; 10-& sup3; to 2.4 & times; 10-2 m2 s-2 (where 3.9 & times; 10-& sup3; to 8.6 & times; 10-& sup3; m2/s2 are used for fish swimming observations), comparable to natural streams. Twenty-two zebrafish were tested individually across three turbulence conditions and still water. Three-dimensional swimming trajectories were reconstructed with stereoscopic imaging, and swimming speeds were quantified. We found that live zebrafish exhibited up to two orders of magnitude higher instantaneous swimming speeds, compared to dead controls, with median values of 7.4-11.5 cm/s that remained stable across turbulence levels, indicating strong behavioural self-regulation. However, these speeds were higher than commonly reported cruising speeds in the literature, which suggest that zebrafish may expend greater effort when swimming in near-zero-mean-flow turbulence. Nonparametric statistical tests revealed significant differences between sex and age groups but weaker differences within groups across different TKE values. Two-year-old females consistently swam faster than one-year-old females, while males showed intermediate swimming speeds. These findings suggest that zebrafish combine individual resilience with group-level diversity in locomotor responses, providing insight into how small-bodied fishes may adapt to anthropogenic flow modifications and climate-driven changes in hydrodynamics.