Biomechanics of fish swimming and leaping under waterfalls: a realistic field, image-based biophysical model with...

Worldwide river fragmentation by infrastructures is altering essential ecological processes including fish migrations. Unlike laboratory approaches, field methods and biophysical models have the potential to provide realistic representations of interacting fish-obstacle systems, furthering insights in behavioural and biomechanics science, and allowing better bioinspired engineering. We developed a new field, image-based method that integrates a biophysical mechanistic model to describe the swimming and leaping biomechanics of wild populations of fish in the non-lab ecological context where their reproductive migration takes place. A weir obstacle in natural riverine conditions where fish freely migrate upstream to their breeding grounds was filmed. A biophysical model including the relevant biomechanical and hydraulic forces and their interactions was parametrised and calibrated with the spatial coordinates of fish trajectories. The method was validated with independent empirical data under field conditions. The distribution of fish initial velocities and angle of emergence of the sample of filmed leaps were reliably quantified in field conditions. The distribution of burst swimming velocities underwater was differentiated from that of the initial leaping velocities associated with the thrust of hydraulic forces; fish behaviour while emerging from water was described. Fish approximated the optimum angle to negotiate the waterfall but did not reach the minimum velocity needed to negotiate the obstacle. The method demonstrated the ability to provide realistic, accurate and precise ecological data on field-based fish interactions with challenge zones during upstream reproductive migrations. The method is cost-effective as it is based on general purpose digital cameras, image analysis, and modelling equations in spreadsheets; all inexpensive and readily available. This new approach can be directly applied to solve scientific problems and bioengineering challenges in any freshwater ecosystem that has natural or artificial obstacles and migratory fish with leaping behaviour.