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.

中文翻译：

---

障碍物和流速对幼鱼，silver鱼，下眼鲷的运动行为的影响

目前，我们对不稳定流量中鱼类的节能策略只有很少的了解。在这项研究中，我们发现障碍物的形状会影响鱼的游泳能力：我们的结果表明，半圆柱可以提高of鱼的临界游泳速度（Hypophthalmichthys molitrix）与全缸，方管下游或自由流动条件下的游泳相比。我们还发现silver鱼如何响应障碍物周围流速变化引起的湍流而改变其运动行为。在流速为1 BL / s（每秒体长）的情况下，鱼在停水站和向半圆柱体后方前进之间花费的时间百分比相近：它们的运动类似于自由游泳的行为。在半缸下游花费的最大时间百分比发生在5 BL / s的流速下。鱼是不稳定的，表现出不规则的身体运动，横向位移大，速度为5 BL / s。但是，鱼保持了很长时间，并以3 BL / s的速度平稳地向前移动到半圆柱体后面。在半圆筒后面的固定台上 当流速从1 BL / s增加到3 BL / s时，鱼的尾拍频率和尾拍幅度保持恒定。在半圆柱体后面向前移动时，当流速从3 BL / s增加到5 BL / s时，鱼会增加尾部拍频和尾部拍幅，同时保持恒定的地面游泳速度，游泳加速和地面步幅。此外，鱼类将口鼻部以3 BL / s的速度放置在半圆筒下游的大约1-3 BL的小区域内。这些行为支持以下假设：半圆柱后方的湍流是鱼类以3 BL / s节省能量的合适位置。我们的结果表明，障碍物和流速的综合影响会影响鱼的运动行为，并且某些组合可能有利于鱼在不稳定流量中游动的能量消耗。