Fish body geometry is highly variable across species, affecting the fluid-body interactions fish rely on for habitat choice, feeding, predator avoidance and spawning. We hypothesize that fish body geometry may substantially influence the velocity experienced by fish swimming. To test this hypothesis, we built nine full-scale physical prototypes of common freshwater fish species. The prototypes were placed in a large laboratory flume and upstream time-averaged velocity profiles were measured with increasing distance from the anterior-most location of each body. The measurements revealed that the body geometry can have a significant influence on the velocity profile, reducing the flow field at a distance of one body length upstream of the fish. Furthermore, it was found that the upstream velocity profiles from the nine fish species investigated in this study can be normalized to a single fit curve based on the freestream velocity and fish body length under subcritical flow conditions. These findings are significant, because they show that conventional point velocity measurements overlook the reducing effect of the fish body on the upstream flow field, creating a systematically biased representation of the velocity experienced by fish in subcritical flowing waters. This bias is illustrated by velocity field maps created with and without the presence of the physical models for three different fish species. Finally, we provide an example of how point velocity measurements can be recalculated to provide upstream velocity field maps closer to “the fish’s perspective”.

鱼体几何形状因物种而异，影响了鱼类依赖于栖息地选择、觅食、避开捕食者和产卵的流体-体相互作用。我们假设鱼体几何形状可能会显着影响鱼游动的速度。为了验证这一假设，我们构建了九种常见淡水鱼类的全尺寸物理原型。原型被放置在一个大型实验室水槽中，随着与每个物体最前面位置的距离增加，测量上游时间平均速度剖面。测量结果表明，身体几何形状可以对速度剖面产生显着影响，从而减少鱼上游一个身体长度距离处的流场。此外，发现本研究中研究的九种鱼类的上游速度剖面可以根据亚临界流动条件下的自由流速度和鱼体长度归一化为单一拟合曲线。这些发现意义重大，因为它们表明传统的点速度测量忽略了鱼体对上游流场的降低影响，从而对鱼在亚临界流动水中所经历的速度产生了系统性的偏差表示。这种偏差通​​过速度场图来说明，无论是否存在三种不同鱼类的物理模型。最后，我们提供了一个示例，说明如何重新计算点速度测量值以提供更接近“鱼的视角”的上游速度场图。