Influence of Time Step on Fish Critical Swimming Speed 

Critical swimming speed (Ucrit) of fish is a fundamental metric for evaluating fish swimming performance. It provides an estimate of the maximum aerobic swimming speed of a fish, and is widely used to inform fish passage design, and habitat assessment. Ucrit is typically determined using a respirometer test through incremental velocity tests where flow velocity increases periodically until fish gets fatigue. In incremental tests, the choice of time step, &#916;t is critical and varies significantly across studies, ranging from 15 to 200 minutes. This study investigates the influence of &#916;t on Ucrit for a Labeo rohita (a species of carp in the family Cyprinidae, native to south Asia). In this study, &#916;t = 15, 30, and 60 minutes are considered. Juvenile Labeo rohita (body length: 6.8&#8211;12 cm, weight: 15&#8211;30 g) were tested in a custom-built 10L respirometer at IIT Madras, India.Ucrit values in incremental velocity tests were 4.58 &#177; 0.18 BL/s, 4.21 &#177; 0.13 BL/s, and 3.72 &#177; 0.10 BL/s at &#916;t = 15, 30, and 60 minutes respectively. Additionally, fixed velocity tests conducted at flow velocity of 4.5 BL/s and 3.5 BL/s (< Ucrit), showed that fatigue times (30 and 80 minutes) exceeding the predicted maxima based on Ucrit (15 and 60 minutes). These findings suggest that Ucrit is sensitive to the chosen time step and further experiments with varying &#916;t could be used to determine optimal value. This will enhance understanding of the trade-offs between shortening time steps and biological interpretation of results.Keywords:&#160; Critical swimming speed, respirometry, time step, incremental velocity, fixed velocity Labeo rohita