Does swimming at the bottom serve as a hydraulic advantage for benthic fish Neogobius melanostomus Pallas (1814) in flowing water?

Benthic fish, such as the round goby (Neogobius melanostomus Pallas, 1814) tend to swim near the bottom, especially at increased water velocities. To test whether these fish have a hydraulic advantage from swimming near the bottom and how the substrate affects the forces experienced, we measured the hydraulic forces experienced by preserved fish in a flow channel. The fish were tested 5.0 mm above the bottom at smooth and rough surface, and in the water column (10.0 cm elevation) above smooth and rough surface at 0.95 m/s water velocity. No significant effect among the mean hydraulic forces was observed between both fish positions, whereas the mean hydraulic forces in the water column were significantly higher (P<0.05) above the rough surface (mean 0.077 N±0.025 s.d.) than above the smooth surface (mean 0.068 N±0.021 s.d.). A convolutional neural network (CNN) predicted the column smooth treatment was the most characteristic force data time series (mean F1=0.88±0.03 s.d.). We conclude that the body posture and body movements of the fish are more relevant for the hydraulic forces experienced by the fish than the vertical position in the water column. Further factors explaining the affinity to swimming near the bottom are discussed.