Rethinking swimming performance tests for bottom-dwelling fish: the case of European glass eel (Anguilla anguilla)

P Vezza,P S Kemp,W Bertoldi,F Libardoni,T Tsuzaki,C Manes

doi:10.1038/s41598-020-72957-w

Abstract

Systematic experiments on European eel (Anguilla anguilla) in their juvenile, early life stage (glass eel), were conducted to provide new insights on the fish swimming performance and propose a framework of analysis to design swimming-performance experiments for bottom-dwelling fish. In particular, we coupled experimental and computational fluid dynamics techniques to: (i) accommodate glass eel burst-and-coast swimming mode and estimate the active swimming time (tac), not considering coast and drift periods, (ii) estimate near-bottom velocities (Ub) experienced by the fish, rather than using bulk averages (U), (iii) investigate water temperature (T) influence on swimming ability, and (iv) identify a functional relation between Ub, tac and T. Results showed that burst-and-coast swimming mode was increasingly adopted by glass eel, especially when U was higher than 0.3 ms-1. Using U rather than Ub led to an overestimation of the fish swimming performance from 18 to 32%, on average. Under the range of temperatures analyzed (from 8 to 18 °C), tac was strongly influenced and positively related to T. As a final result, we propose a general formula to link near-bottom velocity, water temperature and active swimming time which can be useful in ecological engineering applications and reads as {rm{U}}_{rm{b}}=0.174cdot left({{rm{t}}_{rm{ac}}}^{-0.36}cdot {rm{T}}^{0.77}right).

Highlights

Systematic experiments on European eel (Anguilla anguilla) in their juvenile, early life stage, were conducted to provide new insights on the fish swimming performance and propose a framework of analysis to design swimming-performance experiments for bottom-dwelling fish
To quantify the swimming performance of glass eel, our study reports swimming curves that were derived by processing data obtained from flume experiments and Computational Fluid Dynamics (CFD) simulations
The large majority (95%) of the tested eel swam for less than 30 min. Considering this upper time-threshold, values of 30 min were observed only in a few treatments, when water temperature was equal to 15 °C and 18 °C, not influencing the median value of swimming times used in the analysis

Summary

Introduction

Systematic experiments on European eel (Anguilla anguilla) in their juvenile, early life stage (glass eel), were conducted to provide new insights on the fish swimming performance and propose a framework of analysis to design swimming-performance experiments for bottom-dwelling fish. Burst swimming is used by fish to flee from predators or, during migration, to pass obstacles, such as man-made river-infrastructure, which cause abrupt increases in flow velocity. The swimming performance of several fish species has been quantified and compared using so-called swimming-curves ( called fatigue-curves) relating the time a fish can swim continuously against a stream and the velocity of the stream itself These curves have been obtained from experiments conducted using either constant[2] or incremental[1] velocities while the fish is constrained in a swim chamber (i.e. a pressurized flow without free-surface), and forced to swim against the moving water. Fish swimming tests, carried out with incrementallyincreased flow velocity, usually differ from one another in defining both incremental velocity steps (e.g., from 0.5 to 1 fish body length/s) and the prescribed duration for each increment (from 10 to 60 min[1])

Methods

Results

Conclusion