The Central Valley Project Improvement Act (CVPIA) has led to the implementation of a
Decision Support Model (DSM) to assist in the prioritization of CVPIA restoration actions. The fall-run Chinook salmon DSM depends on a coarse-resolution salmon life-cycle model to predict the population benefits of different restoration actions and scenarios. One critical element of the life-cycle model is how to incorporate predation mortality during the juvenile rearing and outmigration portion of the salmon life-cycle in the Sacramento-San Joaquin Delta. Of particular importance to potential restoration activities, is the predation mortality that occurs in proximity to, and as a result of contact points between predator and prey fishes.
Sacramento River winter-run Chinook salmon (Oncorhynchus tshawytscha) are a genetically distinct Evolutionary Significant Unit (ESU) with a unique life history and are listed as endangered at both state and federal levels. Predation of juvenile winter-run by piscivorous fishes is considered to be an important stressor that may reduce the population size of this ESU. Notably, the presence of artificial illumination at night (ALAN) has been shown to aggregate and slow out-migrating salmonids and increase predation by piscivores, and may be an important contact point affecting winter-run Chinook salmon, especially given the vast majority of winter-run Chinook salmon are born and rear within the city limits of Redding, CA. Perhaps the most significant source of ALAN in this region of the Sacramento River is the iconic Sundial Bridge. This bridge is illuminated at night and regional biologists have long been concerned on its potential impacts on winter-run Chinook salmon, as mediated through predation by Rainbow Trout. We therefore performed a field-based experiment to better inform this management concern.
To assess the impacts of Sundial Bridge ALAN on fishes, our first objective was to determine whether variable ALAN intensities altered the relative Rainbow Trout density (RTD) during the night (≥90 min past sunset). To measure the temporal response of RTD to ALAN, we quantified RTD using sonar cameras during the shift from day to night and throughout the night under different ALAN treatments. During our final week of sampling, we also assessed diel patterns of RTD, further elucidating the relationships of RTD with time of day. Finally, we assessed whether Sundial Bridge ALAN impacted the relative predation risk of salmonid fry. We found that ALAN significantly increased RTD on river right when any amount of ALAN was present, potentially leading to competition and fitness consequences; RTD did not change significantly in response to ALAN on river left. Although RTD generally increased in response to ALAN, salmonid fry predation was almost nonexistent. Therefore, while ALAN may decrease out-migrant survival in other waterways, there was no evidence at our study site. These are the datasets and scripts used in this published analysis (citation below).
Nelson, T. R., C. J. Michel, M. P. Gary, B. M. Lehman, N. J. Demetras, P. N. Dudley, J. J. Hammen, and M. J. Horn. 2022. Riverine fish density, predator–prey interactions, and their relationships with artificial light at night. Ecosphere 13:e4261. DOI: 10.1002/ecs2.4261
https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.4261
