Endurance swimming performance and physiology of juvenile Green Sturgeon (Acipenser medirostris) at different temperatures.

Sturgeon are threatened by anthropogenic changes to river systems, including entrainment or impingement at water diversions (i.e. the unwanted passage of fish through a water intake or physical contact with a barrier screen, likely caused by high intake velocities). Though there are no universally accepted protocols to determine water diversion risk, previous studies on sturgeon suggest that laboratory evaluations of swimming performance are an effective way to describe susceptibility to entrainment or impingement. The swimming performance of juvenile Green Sturgeon (~5 cm fork length), Acipenser medirostris, was quantified for fish acclimated to 13 and 18°C for 2 weeks using fixed water velocity endurance tests. Water velocities ranged from 25 to 55 cm s^-1, and time-to-fatigue was measured at 5 cm s^-1 increments. Green Sturgeon were quicker to exhaust at the lower acclimation temperature (13°C) compared to fish acclimated to 18°C, for example at 40 cm s^-1 13°C acclimated fish impinged ~7.7 times faster than 18°C acclimated fish and ~41.3 times quicker at water velocities of 45 cm s^-1. Whole-body cortisol grouped by time-to-fatigue (i.e. sustained swimming: time-to-fatigue >200 min, prolonged swimming: time-to-fatigue between 5 and 200 min, rapid swimming: time-to-fatigue <5 min, and non-swimming: control fish) was highest following the swimming experiment for fish utilizing prolonged swimming strategies regardless of temperature exposure. Furthermore, whole body lactate was elevated in fish utilizing prolonged and rapid swimming strategies compared to sustained and control non-swimming fish. Taken together, when swimming to exhaustion, these results suggest that Green Sturgeon were upregulating stress markers and relying on anaerobic metabolism, although both the above trends were driven by 18°C acclimated fish. The time-to-fatigue data suggest that the risk of entrainment was reduced to zero at water speeds ≤ 29.4 cm s^-1 for 18°C and ≤ 22.6 cm s^-1 for 13°C acclimated fish.