Assessing Juvenile Salmon Rearing Habitat and Associated Predation Risk in a Lower Snake River Reservoir

Abstract

Subyearling fall Chinook salmon (Oncorhynchus tshawytscha) in the Columbia River basin exhibit a transient rearing strategy and depend on connected shoreline habitats during freshwater rearing. Impoundment has greatly reduced the amount of shallow‐water rearing habitat that is exacerbated by the steep topography of reservoirs. Periodic dredging creates opportunities to strategically place spoils to increase the amount of shallow‐water habitat for subyearlings while at the same time reducing the amount of unsuitable area that is often preferred by predators. We assessed the amount and spatial arrangement of subyearling rearing habitat in Lower Granite Reservoir on the Snake River to guide future habitat improvement efforts. A spatially explicit habitat assessment was conducted using physical habitat data, two‐dimensional hydrodynamic modelling and a statistical habitat model in a geographic information system framework. We used field collections of subyearlings and a common predator [smallmouth bass (Micropterus dolomieu)] to draw inferences about predation risk within specific habitat types. Most of the high‐probability rearing habitat was located in the upper half of the reservoir where gently sloping landforms created low lateral bed slopes and shallow‐water habitats. Only 29% of shorelines were predicted to be suitable (probability >0.5) for subyearlings, and the occurrence of these shorelines decreased in a downstream direction. The remaining, less suitable areas were composed of low‐probability habitats in unmodified (25%) and riprapped shorelines (46%). As expected, most subyearlings were found in high‐probability habitat, while most smallmouth bass were found in low‐probability locations. However, some subyearlings were found in low‐probability habitats, such as riprap, where predation risk could be high. Given their transient rearing strategy and dependence on shoreline habitats, subyearlings could benefit from habitat creation efforts in the lower reservoir where high‐probability habitat is generally lacking. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Juvenile Chinook Salmon Mortality in a Snake River Reservoir: Smallmouth Bass Predation Revisited

“…The preference of juvenile Chinook Salmon for this type of habitat (Tiffan et al. ) may have provided Smallmouth Bass with better foraging opportunities that contributed to the often‐higher consumption rates in natural habitat.…”

“…These are important rearing areas for subyearlings (Tiffan et al. ) and for active migrants that delay or slow their downstream movements through these habitats (Tiffan et al. ).…”

Ecology of nonnative Siberian prawn (Palaemon modestus) in the lower Snake River, Washington, USA

“…Mean water column velocity (cm s -1 ) was also included as a covariate in the Lower Granite Reservoir model because velocity data were available for that reservoir. Velocities were estimated for each trawl transect from a two-dimensional hydrodynamic model (Tiffan et al 2015) as follows. First, at each transect, velocities were estimated at each of the five discharges (1388,3342,4050,4361, and 5664 m 3 s -1 ) modeled by Tiffan et al (2015).…”

“…Velocities were estimated for each trawl transect from a two-dimensional hydrodynamic model (Tiffan et al 2015) as follows. First, at each transect, velocities were estimated at each of the five discharges (1388,3342,4050,4361, and 5664 m 3 s -1 ) modeled by Tiffan et al (2015). These were then used to construct 96 transect-specific regressions to predict velocity from discharge (mean r 2 = 0.97, range 0.07-0.99).…”

“…Additionally, many juvenile Snake River fall Chinook salmon Oncorhynchus tshawytscha (Walbaum, 1792) rear for extended periods in reservoir habitats (Connor et al 2013;Tiffan et al 2015). The role of prawns in the food web that supports juvenile salmon in Snake River Reservoirs is unknown but should be cause for concern given the limited data on their current impacts.…”