Abstract
The distribution of juvenile steelhead (Oncorhynchus mykiss), native to the lower American River (LAR) in California’s Central Valley, is currently restricted to the 37 km of river immediately below Nimbus Dam, which acts as a barrier to fish passage. Limited suitable rearing habitat for juvenile steelhead constrains productivity and the quality of this habitat is largely moderated by water temperature and flow. Responding to declining fish populations and diminishing habitat suitability resulting from the Folsom/Nimbus Dam complex, restoration efforts in the LAR have resulted in the establishment of temperature goals and projects designed to improve habitat conditions for adult and juvenile salmonids, including steelhead. The amount of suitable habitat remains a limiting factor for steelhead productivity in the LAR and there is a high degree of variation in the quality of these spawning and rearing habitats. In the highly altered LAR, relatively little is known about the relationship between variation in rearing habitat and juvenile steelhead density. There is a lack of understanding regarding how the environmental variables such as temperature, flow, and channel type interact with time of year and location along the river to influence juvenile steelhead density patterns in predictable and significant manners. Environmental variables interact in dynamic ways throughout the juvenile steelhead rearing period and the amount of suitable high-quality rearing habitat in the LAR is limited. Considering these constraints, the purpose of this study was to investigate how patterns in juvenile steelhead density respond dynamically to the spatially and seasonally changing patterns in water temperature, flow, and channel type through regular seining surveys at multiple sites on the LAR. At twelve sites, distributed over 32 km of river, abiotic data collection and seining surveys were performed monthly during the juvenile steelhead rearing period in 2015, a year of relatively low flows and high temperature and 2016, a year of relatively moderate flows and cooler temperatures. These data were used to generate a model describing the relationship between juvenile steelhead density and the habitat conditions at each site and elucidate how the patterns in juvenile steelhead populations are influenced by heterogeneity in physical habitat characteristics of the LAR. In the LAR, patterns of juvenile steelhead presence or absence, and density were influenced by water temperature, discharge rate (flow), distance downstream from Nimbus Dam, time (Julian day), and channel type. The influence and interaction and of these environmental variables changes over time and space, generating patterns of juvenile steelhead habitat use. We found two different mechanisms driving these observations: one underlying the likelihood of catching juvenile steelhead along the longitudinal gradient of the LAR over time, and another driving the changing spatial and temporal patterns of juvenile steelhead density. Binomial catch probability was driven by temperature, flow rate, distance downstream from Nimbus Dam, and time of year. Juvenile steelhead density responded likewise to these four variables in addition to channel type.