Abstract: To investigate the effects of velocity increment and duration on the critical swimming speed (U_crit) of juvenile darkbarbel catfish (Pelte obag vachelli), the U_crit was determined with different velocity increment and duration at water temperature 25 ℃. The fish were first tested at the duration of 5, 10, 15, 30, and 60 min at a velocity increment of 15% U_crit (10 body length), and then, tested at a velocity increment of 5%, 10%, 15%, 20%, and 30% U_crit with a duration of 20 min. The absolute swimming speed (U_a) of experimental fish decreased from （47.4±0.7） cm·s^-1 at 5 min to （39.1±1.5） cm·s^-1 at 60 min (P<005), but the U_a in 15 min group was not significantly different from that in 30 min group. The U_a increased from （44.1±0.6） cm·s^-1 in 5% U_crit increment group to （47.0±0.4） cm·s^-1 in 10% U_crit increment group (P<005). There were no significant differences in U_a among the groups of 10%, 15%, and 20% U_crit, but the U_a in 30% U_crit increment group was significantly lower than that in other groups (P<0.05). It was suggested that the values of critical swimming speed were profoundly affected by the velocity increment and duration, and the proper duration and velocity increment for darkbarbel catfish were 15-30 min and 10%-20% U_crit, respectively.

Key words: Quinclorac, Flooded paddy soil, Microbic populations, Aerobic heterotrophic bacteria, Anaerobic bacteria