Castro-Santos, T.*; Haro, A.: Optimal sprint speeds of fish traversing velocity barriers: further thoughts on burst-swimming data.

Migrating fish traversing velocity barriers are often forced to swim at speeds greater than U_crit. Since prolonged and burst-swimming are powered by inefficient anaerobic metabolic processes, models predicting optimal swim speeds based on maximized energetic efficiency are inappropriate in this situation. Instead of efficiency, fish need to maximize distance of ascent, both to maximize probability of success and to minimize the number of ascent attempts with their associated metabolic costs, recovery time, etc. From the standard fatigue-time equation (lnE = a + bU; E = endurance, U = swim speed), we show that there exists an optimal ground speed for maximizing traversed distance against any flow velocity greater than U_crit, and this speed equals –1/b. Six migratory fish species allowed to sprint volitionally against fixed flow velocities of 1.5 – 4.5 m*s^-1 chose similar ground speeds regardless of flow velocity, supporting the hypothesis that sprinting speeds are modulated to maximize distance of ascent.