Abstract: Abstract: Sea cages have been widely used in fish industry in the recent years. Traditional method to transport fish is to lift the fish container, which is energy-intensive and could lead to fish losses. Energy-efficient fish pumps have been developed as a replacement in aquaculture to transport fish aimed to alleviate fish losses. Based on its operating principle, traditional fish pump can be classified into three types: Impeller fish pump, pressure/vacuum (P/V) fish pump, and jet fish pump. The impeller fish pump has specially designed high-speed rotating blades, which are efficient to transport fish but could hit the fish and result in casualties. The P/V fish pump is more friendly to the fish, but its discontinuous operation in suction and discharge is inefficient in energy. In contrast, the annular jet pump works by transferring momentum from a high-velocity primary stream to a secondary stream, improving its overall performance compared to other two fish pumps due to its non-rotation and continuous operation. Based on high speed photography, this paper presents an experimental study on impact of the primary flow rate on locomotion of Carassius auratus, Megalobrama amblycephala and Ctenopharyngodon idella in a jet fish pump under five operating conditions. The experimental results showed that with primary flow rate increasing, both percentage of backward-moving fish and the average transit time of the fish decreased, while the posture change rate and the average collision force between the fish and the wall increased. At low primary flow rate, the percentage of backward-moving fish was above 85%, the average transit time of the fish was more than 300 ms, the posture change rate was less than 6% and the average collision force between the fish and the wall was between 1 and 3 N. At the high primary flow rate, the percentage of backward-moving fish was between 50% and 85%, the average transit time of the fish was between 125 and 175 ms, the posture change rate was between 9% and 18% and the average collision force of the fish was between 5 and 7 N. Among the three types of fishes, Ctenopharyngodon idella had the longest transit time and highest posture change rate in the pump; they also suffered the biggest collision force with the cage wall in most operating conditions. Our results alluded that the jet fish pump can be optimized by intensifying the mixture of the primary flow and the secondary flow in the suction chamber and the throat to increase the posture change rate of the fish. This can reduce the transit time of the fish and increase the transport performance of the pump. In summary, the main contribution of our work is elucidation of the influence of the primary flow rate on locomotion of fish in jet fish pump. It provides guidelines to optimization of jet fish pump with locomotion of fish in consideration.