Objectives As one of the “four major Chinese carps” in the Yangtze River, unlike general mechanical movement, grass carp (Ctenopharyngodon idellus) is a kind of organism with locomotor vigor, which can control its movement process through its own muscle nerves, making it difficult to study the swimming dynamics of grass carp. In addition to guiding fishery production, the study of the swimming behavior of grass carp is also of great importance in the construction of ecological waterways. Methods Numerous studies have proved that the shallow ridge-deep pool riverbed topography created by damming and other engineering projects forms a slow-flowing area, which has been proven to be a suitable habitat for fish.The flow augmentation method was used to study the swimming locomotor characteristics of juvenile grass carp in a deep pool with variable velocity currents. The hydrodynamic environment of a continuous shallow deep pool was created by topographic design modifications.The flume test was carried out by Particle Image Velocimetry (PIV) to study the variation of flow structure in deep pools and to analyze the flow characteristics under different flow conditions in terms of two dimensions: flow velocity characteristics and flow structure.Behavioral characteristics of grass carp, including the active area and tail swing frequency of grass carp, were observed in the experiments to reveal the relationship between the movement process of grass carp and the response of the hydrodynamic environment under variable-speed currents in deep pools.The swimming movement of grass carp is essentially a process in which the state of movement of an object changes under the action of an external force. The grass carp will be subjected to the force exerted on it by the surrounding water body in the process of swimming, which will make the grass carp undergo a stress reaction and produce thrust through muscle contraction to counteract the water.Based on Newton’s second law, the concept of current-induced force (Fw) was proposed, and the force on the juvenile grass carp in the variable-speed current of a deep pool was analyzed by treating the grass carp as a general object that can generate its own motion thrust. The total energy dissipation coefficient (Cps) during the movement of the juvenile grass carp was calculated to evaluate the energy consumption of the juvenile grass carp during swimming movement. Results and Discussions 1) The flow velocity of the water in the deep pool is significantly smaller than the shallow flow velocity, and due to the hindering effect of the dam, the water flowing from upstream to downstream undergoes reflux, forming flat-axis vortex structures of different sizes. As the dam body is inundated by the water flow, the area of reflux is getting smaller and smaller, gradually moving in the downstream direction of the deep pool. When the flow rate is greater than 1.5 L/s, the vortex region starts to appear in the deep pool, and the area of the vortex region increases with the flow rate and extends downstream from the upstream of the deep pool gradually. 2) As the flow velocity increased, the grass carp showed an obvious clustering effect. The complex flow environment of the deep pool provided a good habitat and activity site for grass carp, which preferred to move in an area with a large water depth, low flow velocity, and turbulent flow pattern.There is a correlation between the area of grass carp larval activity and deep pools.The percentage of the activity area of grass carp decreased from 80.99% to 28.54% in the experiment, which not only reflected the clustering effect among fish groups but also reflected the suitable environmental preference of grass carp, and the suitable habitat area of grass carp in the deep pool gradually decreased with the increase of flow. 3) The turbulent water flow in the deep pool always caused the grass carp to exhibit intermittent and sudden tail wagging phenomena, and the overall fluctuation of tail wagging frequency was large. However, with the increase in flow velocity at the inlet, the frequency of tail wagging gradually increased, but the range of frequency fluctuations in the frequency of the grass carp tail swing gradually decreased. In the low-flow environment, the test fish were subjected to less flow coercion, and the movement of the fish was influenced by other subjective factors (e.g., hunger, fatigue, etc.), which resulted in different wagging frequencies and different degrees of activity for different test fish, and therefore the wagging frequency of grass carp fluctuated more at this time. When the flow rate increases, the fish usually increase the frequency of the tail swing to obtain power to maintain their own stability or upstream.4) As long as it is in motion, it is inevitable to produce energy dissipation. The main movement of fish organisms is swimming, and tail swinging is the main energy dissipation behavior during swimming. Based on Newton’s second law and force analysis on grass carp, the total energy dissipation coefficient of grass carp larvae in variable-speed deep pool currents was calculated.The total energy dissipation factor for grass carp movement was lowest when water flowed over the dam and at a flow rate of 1LB/s. When the flow rate was less than 1LB/s, the total energy dissipation coefficient of grass carp decreased with the increase in flow rate; when the flow rate was more than 1LB/s, the total energy dissipation coefficient of grass carp increased with the increase in flow rate. Conclusions The above study quantifies the movement process of grass carp larvae, fills a gap in the research on the movement characteristics of grass carp under variable-speed water flow, and provides guiding suggestions for fish habitat creation projects as well as the design of fishways. © 2025 Sichuan University. All rights reserved.
