Flow field-induced drag forces and swimming behavior of three benthic fish species

Joschka Wiegleb,Philipp E Hirsch,Bernd Egger,Frank Seidel,Patricia Burkhardt-Holm

doi:10.1016/j.limno.2020.125812

Joschka Wiegleb, Philipp E Hirsch + Show 3 more

Open Access

https://doi.org/10.1016/j.limno.2020.125812

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Abstract

Modern ethohydraulics is the study of the behavioral responses of swimming fish to flow fields. However, the exact drag forces experienced by fish remain poorly studied; this information is required to obtain a better understanding of the behavioral responses of fish and their current resistance strategies. We measured near-ground frontal drag forces on preserved individuals of three benthic fish species, round goby (Neogobius melanstomus), gudgeon (Gobio gobio) and bullhead (Cottus gobio), in a flow channel. The forces were compared to acoustic Doppler velocity (ADV) measurements and fish tracking data based on video observations of live fish in the flow channel. Overall, we observed drag coefficients (CD) of ∼10−3 at Reynolds numbers ∼105. The frontal drag forces acting on preserved fish with non-spread fins ranged from -1.96 mN*g-1 (force per fish wet weight, velocity 0.55 m*s-1) to 11.01 mN*g-1 (velocity 0.85 m*s-1). Spreading the fins strongly increased the drag forces for bullhead and round goby. In contrast, the drag forces were similar for gudgeon with spread fins and all fish with non-spread fins. Video tracking revealed no clear relationship between the position of the fish in the flow field and the forces experienced by the preserved fish at these positions. Collectively, these results suggest that i) the differences in frontal drag forces between species are small in homogenous flow, ii) individuals chose their position in the flow field based on factors other than the drag forces experienced, and iii) whether fins are spread or non-spread is an essential quality that modulates species-specific differences. The methodology and results of this study will enable integration of flow measurements, fish behavior and force measurements and inform ethohydraulics research. More advanced force measurements will lead to a detailed understanding of the current resistance strategies of benthic fish and improve the design of fish passes.

Highlights

The successful passage of river barriers via fish passes is a global conservation goal and research topic (Katopodis and Williams, 2012; Williams et al, 2012)
In comparison to the strong effect of fin position on the frontal drag forces experienced by round goby and bullhead, the kind of species had a rather small effect on the forces experienced at both fin position treatments
Due to the small impact of the pectoral fins on the frontal drag forces experienced by gudgeon and their strong swimming capabilities, it is possible that the pectoral fins of gudgeon mainly exert a piloting function

Summary

Introduction

The successful passage of river barriers via fish passes is a global conservation goal and research topic (Katopodis and Williams, 2012; Williams et al, 2012). Body shape and pectoral fin usage are suggested to be highly relevant to the current resistance of benthic fish. Carlson and Lauder (2010) described the strategies used by benthic fish to resist currents with specialized body postures leading to negative lift forces. Flow field observations based on digital particle imaging close to the pectoral fins of Mottled sculpin (Cottus bairdi) revealed that the pectoral fins sig nificantly altered the downstream flow field. These effects of the pec toral fins were suggested to increase negative lift forces and thereby increase station holding capability (Coombs et al, 2007)

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