In Situ Thrust Measurement of Fish During Locomotion; Test Case: Sharks

Abstract

We present a novel method of measuring thrust of aquatic animals using in situ video data of swimming motions. To demonstrate its utility, the method was applied to several large elasmobranch species, which are typically highly challenging to measure. Using motion tracking software, we analyzed video footage of wild and captive sharks to track their instantaneous position and speed. In order to estimate the force output, we used the tail/body motion based on the swimming modes of the fish to calculate the water displaced by this motion during locomotion. Using Newton 3rd law, we have calculated the instantaneous force exerted by the water on the shark. The force output, that is thrust was calculated by averaging the instantaneous force over the tailbeat cycle. The thrust, for each fish was converted into a nondimensional parameter defined as: scaled thrust, allowing comparisons independent of size based on prior knowledge of the fish length and mass. This scaled thrust was analyzed for various swimming modes and caudal fin morphology to correlate to behavioral features through principal component analysis (PCA) we demonstrate the coupling between morphological traits and hydrodynamic forces. For the species studied the ratio of the upper to lower lobe of the caudal fin (CLAR) emerged as a strong predictor of scaled thrust, accounting for more than 80% of the observed variation. Our findings for the species studied indicate that coastal pelagic species exhibited lower scaled thrust values than benthic species, suggesting that benthic species may be less efficient, expending more energy to remain aloft or compensate for drag relative to generating forward motion. We propose that the unique ecological niches of these species drive behavioral changes that result in morphological adaptations to optimize performance.