Close Encounters with a Fish Screen III: Behavior, Performance, Physiological Stress Responses, and Recovery of Adult Delta Smelt Exposed to Two‐Vector Flows near a Fish Screen

Abstract

AbstractThe threatened delta smelt Hypomesus transpacificus is vulnerable to more than 2,000 water diversions distributed throughout the Sacramento–San Joaquin Delta system of California. Although fish screen installation should decrease direct losses, the physiological stress associated with exposure to two‐vector flows near a fish screen may increase delta smelt mortality. We measured the swimming performance, screen contact rate, and stress responses of adult delta smelt to handling and simulated screen exposure at 12°C. Results from handling stress and flow stress experiments showed that fish collection, crowding, and transport elicited significant stress responses in delta smelt, as did exposure to the two‐vector flows and a fish screen. However, delta smelt recovered from the cumulative handling stress within 2 h. Combinations of high approach velocity (10 or 15 cm/s) and high sweeping velocity (31 or 62 cm/s) increased the primary stress response (plasma cortisol concentrations) and the number of moribund fish. Fish screens designed to protect the delta smelt population could minimize physiological stress by decreasing approach velocities in areas with high sweeping velocities.