Beyond the hatchery: testing managed wetland impoundments as semi-natural rearing habitat for endangered Delta Smelt.

Conservation of the endangered Delta Smelt (Hypomesus transpacificus), endemic to the San Francisco Estuary (CA, USA), increasingly relies on captive propagation. Yet fish reared under standard hatchery conditions may not develop the behaviours and physiological traits necessary for survival following release. Semi-natural rearing environments that expose fish to natural environmental variability while maintaining protection from predators and other risks can address this challenge. We evaluated the potential of managed wetland impoundments equipped with floating wetland as semi-natural rearing habitats for Delta Smelt before supplementation. Three field deployments were conducted on Bouldin Island between winter 2022 and spring 2023. We monitored survival, growth, glycogen reserves, cortisol concentrations, environmental conditions and zooplankton communities. Survival was high during winter and early spring but declined later in spring as temperatures approached ca. 20-23°C and repeated low-oxygen events occurred. Growth responses varied among deployments and treatments. Fish exhibited depletion of glycogen reserves during field exposure, consistent with increased energetic demand or adjustment to natural foraging conditions. Cortisol concentrations increased over time, indicating rising physiological challenge as environmental conditions became more demanding. Floating wetlands slightly reduced diurnal fluctuations in temperature and dissolved oxygen but did not improve fish performance. Zooplankton communities were dominated by suboptimal prey taxa, suggesting prey quality may have constrained energetic condition. These results suggest that managed impoundments can function as transitional habitats that help prepare hatchery-reared Delta Smelt for natural conditions before release. However, their effectiveness will depend on seasonal timing, prey availability and water-quality conditions.