Experimental Study on Hydrodynamic Preferences and Collective Behavior of Fish Groups During Upstream Migration in Non-Uniform Flow

Rivers are the arteries of the natural world, carrying water, nutrients, and life across the landscape. For many fish species, this flow is not just a medium to swim through but a vital highway for migration, allowing them to travel from their feeding grounds to specific spawning sites to reproduce. However, the construction of thousands of dams has severed these connections, turning free-flowing rivers into a series of isolated pools. To fix this, engineers build fish passage facilities, essentially underwater ladders or channels designed to help fish bypass the barriers. Yet, these structures often fail. The problem is not always that the fish cannot find the entrance, but that they cannot navigate the complex, churning water once they are inside. While we know that individual fish can sense water speed and turbulence with their lateral lines—a sensory system running along their sides that detects movement and pressure—we have long struggled to understand how groups of fish move together through these chaotic environments. Do they stick together? Do they change their formation when the water gets rough? And how does the shape of the water flow influence their ability to swim upstream?

A team of researchers from Sichuan University and the China Three Gorges Corporation set out to answer these questions by recreating a challenging river environment in a controlled laboratory setting. They built a large, open-channel flume, a long tank designed to mimic a river, and filled it with water flowing at different speeds across its width. Instead of a uniform current, they created a non-uniform flow field with three distinct zones: a slow lane, a medium lane, and a fast lane, separated by sharp transitions where the water speed changed abruptly. Into this system, they released groups of ten Schizothorax prenanti, a species of fish native to the region that migrates upstream to reproduce. By filming the fish from above and tracking their movements with high-speed cameras, the researchers could observe exactly how the group reacted to the changing water conditions, measuring their speed, their spacing, and their collective direction.

The results revealed a clear and consistent strategy. The fish groups overwhelmingly preferred to stay in the slowest parts of the water. They actively avoided the high-speed zones and the turbulent areas where the water speed changed rapidly from one side to the other. When the researchers analyzed the water conditions the fish actually used compared to the conditions available to them, the preference was stark: the fish chose areas with lower flow speeds, weaker turbulence, and less chaotic water movement. They did not simply drift where the water took them; they made deliberate choices to stay in the calmer, more predictable currents. This preference for low-hydrodynamic-load areas suggests that the fish are conserving energy and seeking stability, avoiding the physical stress of fighting against powerful, churning water.

However, the story of how the fish moved was more complex than simply hugging the slow lane. The researchers observed two distinct routes the groups took to get upstream. In one common route, the fish stayed close to the wall in the slow zone for most of the journey, only moving into the faster zones near the very end. This path was efficient, and the fish completed the passage relatively quickly. In the second route, the fish left the safety of the slow zone much earlier, repeatedly crossing back and forth between the slow and medium-speed lanes. This zigzagging behavior was far less efficient; the groups took significantly longer to reach the end, and their movement was less organized. This finding highlights that while all the fish shared a preference for calm water, the way they coordinated their group movement to navigate the transition zones made a huge difference in their success.

When the fish did move through these difficult transition zones, their group behavior changed. In the calm, slow water, the fish moved in a highly organized, unified school, all swimming in the same direction with their bodies aligned. This "schooling" behavior was the dominant mode of travel. But as they approached the boundaries where the water speed changed, the group often broke this perfect alignment. They would turn sharply or scatter slightly, a behavior the researchers identified as "turning" or "swarming." These moments of disorganization were not signs of confusion but rather necessary adjustments. When different parts of the group experienced different water speeds, the fish had to constantly reposition themselves to stay together while still making progress upstream. Once they crossed the turbulent boundary and found a new stable current, they would quickly re-form their tight, unified school.

The study also uncovered a fascinating relationship between how close the fish stayed to one another and the water conditions they faced. The fish did not simply huddle together when the water got rough, nor did they spread out evenly. Instead, they found a "sweet spot" of spacing. When the water was moderately calm, the fish maintained a specific distance from their nearest neighbor, roughly between 0.2 and 0.3 meters, which corresponded to the lowest levels of water stress. If the water became very turbulent, the fish would sometimes huddle closer together to maintain contact, but they would also sometimes spread out if the differences in water speed across the group became too great to manage. This non-linear response shows that the fish are dynamically adjusting their formation to balance the need for group cohesion with the physical reality of the water around them.

Perhaps the most striking finding concerned the fish's swimming speed. As the groups moved from the slow zone into the faster zones, their swimming speed increased dramatically. In the slowest water, they swam at an average of 0.458 meters per second. As they entered the faster currents, their speed rose to over 1.0 meter per second, and in the most intense zone, they reached a maximum burst speed of 2.06 meters per second. This was significantly faster than what has been recorded for this species in smaller, uniform laboratory tanks. The researchers noted that the fish only spent a tiny fraction of their time, less than one percent, in these high-speed zones. This suggests that the fish are capable of powerful, short bursts of speed to cross difficult patches of water, but they rely on the continuity of low-speed corridors to conserve their energy for the long journey.

These findings offer a new perspective on how to design fish passage facilities. The research suggests that simply providing a low-speed area is not enough; the design must ensure that these calm areas are connected in a continuous corridor. If the path forces fish to constantly cross between fast and slow water, or if the high-speed sections are too long, the fish will struggle to maintain their group structure and expend too much energy. The study indicates that successful passage depends on the spatial continuity of favorable flow conditions and the ability of the fish to transition smoothly between different zones without breaking their collective organization. By understanding that fish groups are not just collections of individuals but coordinated units that react to the flow as a whole, engineers can create passages that align with the natural behaviors of migrating fish, helping to restore the vital connections in our dammed rivers.