Abstract
Historically, Chinook Salmon in the California Central Valley reared in the vast wetlands of the Sacramento–San Joaquin Delta. However, more than 95% of floodplain, riparian, and wetland habitats in the Delta have become degraded because of anthropogenic factors such as pollution, introduced species, water diversions, and levees. Despite pronounced habitat loss, previous work using otolith reconstructions has revealed that some juvenile salmon continue to successfully rear for extended periods in the Delta. However, the extent to which the Delta functions to promote salmon growth relative to other habitats remains unknown. In this study, we integrated otolith microstructure (daily increment count and width) and strontium isotope (87Sr/86Sr) records to fill this critical knowledge gap by comparing the growth of natural-origin fall-run Chinook Salmon from the American River that reared in the Delta with those that remained in their natal stream. Using generalized additive models, we compared daily otolith growth rates among rearing habitats (Delta vs. American River) and years (2014 to 2018), encompassing a range of hydrologic conditions. We found that juvenile Chinook Salmon grew faster in the Delta in some years (2016), but slower in the Delta during drought conditions (2014 to 2015). The habitat that featured faster growth rates varied within and among years, suggesting the importance of maintaining a habitat mosaic for juvenile salmonids, particularly in a dynamic environment such as the California Central Valley. Linking otolith chemistry with daily growth increments provides a valuable approach to explore the mechanisms governing interannual variability in growth across habitat types, and a useful tool to quantify the effects of large-scale restoration efforts on native fishes.
Highlights
The stability and resiliency of salmon populations depends partially on a juvenile salmon’s ability to access and utilize suitable freshwater rearing habitats during emigration to the ocean
We focused on juvenile fall run originating from the American River because the 87Sr/86Sr signature of the American River is distinguishable from the other tributaries in the California Central Valley, mainstem Sacramento River, and the Delta (Ingram and Weber 1999; Barnett-Johnson et al 2008; Phillis et al 2018)
The average daily growth between 2014 and 2018 varied between 0.47 ± 0.08 to 0.63 ± 0.08 mm in the American River, and 0.49 ± 0.09 to 0.57 ± 0.19 mm in the Delta, which is consistent with other growth studies conducted in the California Central Valley (Table 2)
Summary
The stability and resiliency of salmon populations depends partially on a juvenile salmon’s ability to access and utilize suitable freshwater rearing habitats during emigration to the ocean. Juvenile salmon often utilize a diverse mosaic of habitat types off the main migratory corridor—such as shallow-water wetlands, tidal marshes, and floodplains—which can provide increased feeding opportunities (Sommer et al 2001; Williams 2006; Jeffres et al 2008; Limm and Marchetti 2009). This habitat diversity increases growth opportunities for juvenile salmon in freshwater habitats (Brennan et al 2019), which is critical for age class success because early growth during freshwater residence can significantly affect marine survival (Woodson et al 2013). To maintain diverse habitats and enhance salmon populations, it is critical to understand the temporal and spatial variation in habitat availability and use by juvenile salmon, and how these modified freshwater habitats function to promote salmon growth
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