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.
