Effects of Test-Section Length and Cross-Sectional Blockage on the Swimming Performance of Juvenile Grass Carp: Implications for Standardized Testing and Fish-Passage Design

Closed-flume spatial constraints may bias swimming-performance data used in fish-passage design. Two independent experiments were conducted using different size classes of juvenile grass carp (Ctenopharyngodon idella). Fish with a mean body length (BL) of 7.92 ± 0.61 cm were used to assess five test-section lengths (8, 6, 4, 3.5, and 3 BL), whereas fish with a mean BL of 15.76 ± 0.97 cm were used to assess three blockage ratios (1.88%, 9.26%, and 15.31%). Incremental-velocity tests quantified critical swimming speed and behavior, and three-dimensional simulations examined the local flow field under the 9.26% blockage. Relative critical swimming speed was similar at 4–8 BL (9.71–9.95 BL/s) but decreased by 10.96% from 4 to 3.5 BL. Increasing blockage from 1.88% to 9.26% and 15.31% reduced it from 6.99 to 5.34 and 5.24 BL/s, respectively. Behavioral responses also depended on spatial constraint: shorter sections reduced the space available for forward movement and postural adjustment, whereas higher blockage restricted lateral body and tail movements. As velocity increased, fish intensified tail beating; under higher blockage, compensation relied increasingly on tail-beat frequency because amplitude adjustment was constrained, with greater variation among individuals. Simulations showed that local streamwise velocity near the maximum fish cross-section was 20.8–25.0% above inlet velocity. At least 4 BL is recommended for fish comparable to those in the length experiment. Below 5% is suggested as a precautionary blockage target.