Effects of obstacles and flow velocity on locomotory behavior in juvenile, silver carp,Hypophthalmichthys molitrix

Abstract

We currently have only a minimal understanding of energy‐saving strategies of fish in unsteady flows. In this study, we found that obstacle shape can affect swimming ability of fish: our results suggested that a half cylinder improved critical swimming speed of silver carp (Hypophthalmichthys molitrix) compared with swimming downstream of a full cylinder, square tube or for free‐flow conditions. We also discovered how silver carp alter their locomotory behavior in response to turbulent flow caused by varying flow velocities around obstacles. At a flow velocity of 1 BL/s (body length per second), the fish spent similar percentages of time between holding station and moving forward behind the half cylinder: their motions resembled freestream swimming behavior. The greatest percentage of time spent downstream of the half cylinder occurred at a flow velocity of 5 BL/s. Fish were unstable and displayed irregular body motions with large lateral displacement at 5 BL/s. However, the fish held station for a long time and moved forward steadily behind the half cylinder at 3 BL/s. In holding station behind the half cylinder, fish maintained constant tail‐beat frequency and tail‐ beat amplitude when flow velocity was increased from 1 to 3 BL/s. In moving forward behind the half cylinder, fish increased tail‐beat frequency and tail‐beat amplitude while maintaining constant ground swimming speed, swimming acceleration and ground stride when flow velocity was increased from 3 to 5 BL/s. Moreover, fish positioned their snouts in a small area approximately 1–3 BL downstream from the half cylinder at 3 BL/s. These behaviors support the hypothesis that turbulence behind a half cylinder was a suitable location for fish to conserve energy at 3 BL/s. Our results indicate that the combined effect of obstacles and flow velocity influence fish locomotory behavior, and some combinations may be beneficial to fish energy expenditure swimming in unsteady flow.

Swimming behaviour of silver carp (Hypophthalmichthys molitrix) in response to turbulent flow induced by aD‐cylinder

“…Within the models, all fixed effects are categorical variables with the following levels: BL, small fish (1) and large fish (2); CD, free stream (1), 1.9 cm cylinder (2), 2.5 cm cylinder (3), 3.2 cm cylinder (4) and 5.0 cm cylinder (5); MBT, non-FRB swimming (1) and FRB (2). Previous study (Ke et al, 2021) has shown that silver carp's motions resembled free-stream swimming behaviour at low flow velocity (less than <3 BL/s). To better understand how U rcrit is affected by FRB, we selected intermediate to high velocity (40-90 cm s -1 ).…”

“…Kinematics parameters such as tail-beat frequency and amplitude were calculated from 10 midlines corresponding to a complete tail-beat cycle. Tail-beat frequency refers to the number of tail beat cycles recorded per second, whereas a beat cycle refers to a single bout of movement that begins when a fish tail is at its highest point and ends when it returns to that point (Ke et al, 2021). Tail-beat amplitude refers to half of the peak-to-peak amplitude recorded in the tail beat cycle.…”

“…There are abundant and diverse obstacles in nature-like fishways, including square column, full cylinder, half-cylinder or boulders. A previous study has shown that the critical swimming speed of silver carp is optimal in the presence of a half-cylinder, compared with other obstacle shapes (Ke et al, 2021). Liao revealed that rainbow trout (Oncorhynchus mykiss) adopted novel body kinematics behind a cylinder, termed the Karman gait, to reduce the energy cost of locomotion at particular flow speeds (Liao, 2004;Liao et al, 2003a,b).…”