Quantifying component mortality estimates of out-migrating juvenile steelhead (Oncorhynchus mykiss)

Low juvenile outmigration survival rates (<5%) have been linked with low productivity and abundance of federally threatened California Central Valley (CCV) steelhead (Oncorhynchus mykiss), but no studies have quantified the specific sources of mortality at this life-stage. Here, we use empirical data to estimate predation versus total juvenile outmigration mortality rates. We tagged 300 juvenile steelhead with Predation Detection Acoustic Tags (PDATs) in the San Joaquin River from March to May 2024, detected them with passive and manual telemetry, and categorized PDAT detections into three observations: (1) alive, (2) predation, (3) unobserved. We then analyzed the capture histories using multistate mark-recapture models, incorporating fish length, temperature, discharge, and turbidity as covariates. Survival in our study area was 0.623 [0.502 - 0.720 posterior credible interval (CrI)]; mortality, primarily from fish predation, was 0.257 [0.188 - 0.339 CrI], and other sources contributed 0.120 mortality [0.060 - 0.208 CrI]. Predation and unknown mortality decreased as steelhead length increased and predation mortality increased with higher discharge. Our study confirms that fish predation is the primary mortality source for CCV steelhead, providing critical information to inform management within this system.