An improved method of local coloring in vertical-slot fishways to enhance the fish migration effect of grass carp.

Vertical-slot fishway (VSF) has been used in many water conservancy projects to restore the river connectivity. A high-quality fishway project should facilitate fish to discovering the exit and passing through, avoiding to long stay in the fishway and delay the migration. Current research on fishway engineering has not yielded an expected passing ratio of fish migration, and it is therefore of great significance to further study the assisting effect of VSF in fish migration. To begin with, we preliminarily determined the attractive and repelling colors of grass carps based on their swimming behavior in a static water pool configured with local colors. Combined with the migration route of the grass carp in a VSF pool without local coloring, four local coloring cases were designed. Based on the camera results of the four experimental local coloring cases, a comparative analysis was conducted with the blank control group frame by frame. This was followed by the statistics of the number of successfully migrated grass carps and their total completion time. On that basis, the assisting effect of VSF in fish migration under the four cases was evaluated in terms of the reduction rate of migration route length, the reduction rate of completion time, and the improvement rate of passing ratio. The research outcomes indicated that green and blue act as attractive colors while yellow and red serve as repelling colors for grass carp. Adding colors to the training wall and dividing wall in the VSF pool, the migration route of grass carp was appropriately adjusted, alongside a shortened completion time and an improved passing ratio. Of the four local coloring cases, the recommended case showed a significant effect on migration route, with more concentrated moving trajectories and shortened route length. Typically, the migration route length decreased by 26%, and the frequency of fish long staying at the junction between the training wall and dividing wall was markedly reduced, as well as the frequency of fish swimming along the water flow from upstream to downstream. The completion time was shortened by 26%, and the passing ratio was enhanced by 44%. The approach of combining local coloring with fish behavior and fishway hydraulics in the pool surpassed the method that optimizes the fishway design only from the fishway hydraulics. The improved method greatly shortened the migration route length, reduced the completion time, and significantly improved the passing ratio of fish passage objects in the VSF. The present research mainly focuses on using model experiments to evaluate the local coloring cases. In the future studies, we will configure local colors to the sidewalls of on-site fishways using environmentally friendly paint or colored organic glass panels. With the monitoring results of the completion time and passing ratio of fish passage objects, the recommended case can be further verified and optimized, thereby providing a more reasonable and feasible local coloring case for assisting fish migration in the VSF project.

Experimental study on the passage behavior of juvenile Schizothorax prenanti by configuring local colors in the vertical slot fishways.

The vertical slot fishway is one of the most widely used fishways in the world. To solve the problems of long passage time and low passage rate of fish passing objects in the conventional vertical slot fishway, this study proposed a method of configuring local colors in the internal sidewall of the fishway pond to improve the vertical slot fishway based on the approach-avoidance effect of fishes on different colors. Taking Schizothorax prenanti as the research object, comparative fish passing experiments were carried out to analyze the effects of configuring local colors on the passage behavior. It has been found that green and yellow are the approach and repellent colors for Schizothorax prenanti, respectively. By configuring local colors of the guide wall and partition wall at the upstream and downstream of the pond, the passage track of Schizothorax prenanti was significantly changed, which in turn affected their passage time and passage rate. The configuration of local green to the guide wall and partition wall on the upstream negative side, and local yellow to the sidewall of the guide wall on the upstream negative side, the guide wall on the downstream positive side, and the partition wall on the downstream positive side can shorten the overall passage time of Schizothorax prenanti by 9 % and increase the passage rate by 23 %. Compared with the scheme that completely depends on the flow conditions to change the structure of the pond, the method of configuring local colors in the pond is an improved method combining fish ethology and hydraulics. The scheme of local color configuration significantly shortens the passage time and passage rate of fish passing objects in the vertical slot fishway, and can be applied to the fishway design and the improvement of operation effect, which has a certain practical application value.

Schizothorax prenanti swimming behavior in response to different flow patterns in vertical slot fishways with different slot positions.

Fish swimming behavior is a critical parameter for the design of vertical slot fishways. Although Schizothorax prenanti is a significant reproductive potamodromous migratory fish species in the upper reaches of the rivers in Southwest China, its swimming behavior has not been extensively researched. Therefore, in this study, a comparative experiment was conducted in vertical slot fishways to analyze the behavior of Schizothorax prenanti in response to different flow patterns, with respect to different slot positions. The experimental model was designed with a length scale of 1:4, and a method for selecting the appropriate fish size in the scaled physical model was proposed. Based on these experiments, it was found that the typical upstream trajectories of Schizothorax prenanti are traceable to the sidewall of the pool, which are characterized by low velocity and turbulent kinetic energy (TKE). The hydraulic variables exhibited an asymmetric distribution within the vertical slot, and the fish were found to pass through the area with the lowest velocity and TKE. A flow pattern with a guide wall length-to-pool width ratio of P/B = 0.25, in which Schizothorax prenanti can immediately find the sidewall, is suitable for fish migration. Therefore, this is the recommended value for the construction of effective vertical-slot fishway structures.