Shallow Tidal Freshwater Habitats of the Columbia River: Spatial and Temporal Variability of Fish Communities and Density, Size, and Genetic Stock Composition of Juvenile Chinook Salmon

Abstract

We investigated the spatial and temporal variability of the fish community and the density, size, and genetic stock composition of juvenile Chinook Salmon Oncorhynchus tshawytscha in shallow tidal freshwater habitats of the Columbia River. We sought to address data gaps related to juvenile Chinook Salmon in this portion of the Columbia River and thereby inform habitat restoration efforts. We examined fish communities, juvenile salmon life history characteristics, and relationships between salmon density and habitat conditions by using beach seine data collected over a 63‐month period (2007−2012) from two tidal freshwater areas: the Sandy River delta (river kilometer [rkm] 188–202) and the lower river reach (rkm 110–141). We found few differences in the fish community across the two study areas. Fish community patterns were largely attributable to seasonal changes as opposed to spatial gradients and habitat types. Juvenile Chinook Salmon were the most common salmon species in our catches; this species was the only salmonid encountered during all four seasons. Chinook Salmon density differed among three distinct habitat strata (main channel, off‐channel, and wetland channel), but FL and genetic stock composition did not. Across all habitat strata, environmental covariates (mean percent tree cover, dissolved oxygen level, and mean percent emergent vegetation) were positively associated with juvenile Chinook Salmon density. Although comparisons of environmental metrics and salmon density helped to establish a quantitative relationship between biotic and abiotic conditions, we found that juvenile salmon occupied a range of habitats. Our findings support a strategy that involves restoring a diversity of shallow tidal freshwater habitats to facilitate the recovery of threatened and endangered salmon populations in the Columbia River basin. Received August 28, 2015; accepted February 1, 2016 Published online June 21, 2016