Riverscape heterogeneity in estimated Chinook Salmon emergence phenology and implications for size and growth

Abstract

Many salmonid‐bearing rivers exhibit thermal and hydrologic heterogeneity at multiple spatial and temporal scales, but how this translates into spatiotemporal patterns of fry emergence is poorly understood. Understanding this variability is important because emergence timing determines the biophysical conditions fish first experience (e.g., temperature, flow, and food supply), thereby influencing growth opportunities and survival during this critical life stage. We predicted spring Chinook Salmon (Oncorhynchus tshawytscha) emergence phenology across four northeastern Oregon subbasins over 5–9 years using empirical spawning and temperature data. We then related interannual emergence timing estimates to juvenile salmon size and growth rates at consistent sampling locations. There were clear longitudinal patterns of predicted emergence timing in each subbasin: The shape of these patterns was consistent among years, but not among subbasins. In two subbasins, emergence occurred progressively later with distance upstream, whereas in the other two subbasins emergence was earliest at upstream sites. Within each year, median emergence dates among sites within each subbasin ranged between 44 and 58 days. This spatial variation was comparable to interannual variation, with median emergence dates for a given location in each subbasin ranging between 47 and 74 days among years. Contrary to our expectations, juvenile salmon were not larger in years with earlier emergence, owing to slower estimated spring and summer growth rates compared to years with later emergence. Despite large interannual variation in estimated emergence dates, these results suggest that other factors (e.g., stream flow, temperature, and density‐dependence) were more important than growth duration in determining juvenile salmon growth rates and size among years. We demonstrated considerable spatial and interannual variation in emergence phenology within these subbasins. Understanding how this variation translates to spatiotemporal patterns of juvenile salmon habitat use, growth, and survival has important implications for guiding restoration efforts and understanding how climate change may impact these populations.

Stream groundwater inputs generate fine‐scale variation in brook trout phenology and growth across a warming landscape

“…For example, differences in phenology should spread the risk of exposure to episodic events such as floods, droughts or heatwaves, thereby reducing the likelihood of synchronous declines in survival and growth across populations (Kaylor et al., 2021; Kovach et al., 2016). Additionally, effects of temperature on growth differed considerably among streams with similar thermal regimes, adding another layer of variation that may help to stabilise productivity through portfolio effects (Kaylor et al., 2022; Lisi et al., 2013). Improved understanding of fine‐scale population and habitat diversity across the species range could provide insights into the resilience of brook trout and the ecosystem services they provide (e.g., supporting fisheries and acting as apex predators in small streams).…”

“…In addition, while the timing of hatch and emergence shifted earlier in every stream, groundwater-dominated streams exhibited slower phenological shifts, perhaps reflecting their more stable temperatures overall (Figure 4). Similar variation in hatch and emergence times in Chinook salmon can mediate spatial patterns in growth (Kaylor et al, 2021(Kaylor et al, , 2022, but the consequences of the disparate phenologies observed among Cape Race brook trout populations are currently less clear (see below). Moreover, reproductive timing was not accounted for when reconstructing degree-day accumulations, as spawning ground surveys (see Fraser et al, 2019) indicate that brook trout reproduce at least 1-2 weeks later in groundwater-dominated streams.…”

Modeling timing and size of juvenile Chinook salmon out-migrants at three Elwha River rotary screw traps: a window into early life history post dam removal

“…Observed patterns in juvenile Chinook salmon out-migration timing and sizes at these traps provide an opportunity to test hypotheses about, and advance our understanding of, juvenile Chinook salmon early life history (e.g., Zimmerman et al, 2015). However, these patterns are the product of multiple processes including spawn timing and location, egg incubation, movement, survival, and growth, all of which are regulated by temperature (Kaylor et al, 2021;Kaylor et al, 2022). Therefore, interpreting patterns in out-migrant timing and sizes requires combining our understanding of these biological processes along with spatially and temporally explicit estimates of water temperature upstream of the traps.…”