UC Davis

Salmon are socio-ecologically important fishes distributed across the Pacific rim, and multiple ecologically significant units are declining in the Sacramento-San Joaquin River Delta of California. Available data suggests that spring-run smolt survival from the upper river in San Joaquin River restoration area (>270 rkm) to the California Delta (<160 rkm) is consistently low. Additionally, limited information from previous acoustic telemetry studies suggest that Franks Tract, a 1429 ha flooded island, is a potential mortality hotspot for outmigrating smolts. During Winter/Spring 2020, we conducted a series of targeted acoustic telemetry and tethering experiments to better understand salmon entrainment into Franks Tract as well as overall survival patterns. Telemetry studies investigating broad scale emigration patterns involved surgical tagging and release of 796 juvenile spring-run Chinook Salmon (Oncorhynchus tshawytscha) with JSATs acoustic transmitters. Tagged fish were distributed between three releases: an upper (n = 350) and a lower river (n = 348) release in the San Joaquin River, and an additional 98 smolts released directly into Franks Tract. The third release was conducted to investigate survival and emigration potential from the area. Provisional telemetry results suggest poor outmigration survival from all three release groups (<1% to ocean entry), but at least some (n = 2) fish released in Franks Tract did survive to Pacific Ocean entry (Benicia Bridge, 52 rkm). We also conducted a series of tethering experiments to contextualize relative predation risk of smolts within different submerged aquatic vegetation (SAV) coverage, water depth, and tidal movement in Franks Tract. Tethering results suggest that relative predation risk was moderately high over the deployment time (10.5% over 2 hours). Further, high- versus low-vegetation coverage had no effect on relative predation risk, nor did water depth. Tidal movement did have an effect on relative predation risk, with a hazard ratio for predation of 4.96 during ebb tides. Combined, our data supports the hypothesis that Franks Tract is a mortality hotspot for juvenile salmon, at least during the period studied here. Future telemetry and tethering studies may be essential for disentangling the ecology of declining salmon runs and investigating effectiveness of various management activities aimed at increasing juvenile survivorship, especially within tidal lake habitats.