Assessment of Juvenile Chinook Salmon Rearing Habitat Potential Prior to Species Reintroduction

Successful Pacific salmon reintroduction into rivers where they have been extirpated requires understanding the range of habitats and environmental conditions that currently exist and their ability to support target species. Chinook Salmon Oncorhynchus tshawytscha were extirpated from the San Joaquin River, California, over 60 years ago and are targeted for reintroduction into the system. To assess the remaining habitat along the longitudinal lower San Joaquin River gradient, juvenile Chinook Salmon were reared within an Alluvial Site, a Transitional Site, and two lowland sites (Lowland Cinnamon Slough and Lowland Eastside Bypass) during two rearing periods (early and late). The highest Chinook Salmon growth rates were observed during the early rearing period within the lowland sites when water temperatures were moderate and prey densities were greatest. However, high-water events precluded a spatial comparison of growth with that of fish reared at the Alluvial and Transitional sites during the early period. In the late rearing period, the lowest and highest growth rates were observed at the Lowland Eastside Bypass (lowest prey abundance) and Lowland Cinnamon Slough (highest prey abundance), respectively. Growth rates at the Alluvial and Transitional sites were intermediate between those at the two lowland sites. Main-channel (Alluvial and Transitional) growth rates were generally lower than those within the lowland sites, suggesting that main-channel restoration may require targeted food web enhancement. Our results indicate that lowland floodplain sites can provide quality rearing habitat and growth benefits early in the rearing season. As temperatures increase, salmon can continue to grow if sufficient prey is available. Managers considering measures for improving floodplain access to provide juvenile Chinook Salmon rearing habitat should consider interactions between environmental conditions and prey density-and how these factors vary along the river continuum-to determine when these habitats can provide the greatest growth benefit.