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

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, food supply), thereby influencing growth opportunities and survival during this critical life stage. We predicted spring Chinook Salmon (Oncorhynchus tshawytscha) emergence phenology across four NE Oregon subbasins over 5-9 years using empirical spawning and temperature data. We then related inter-annual 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 inter-annual variation, with median emergence dates for a given location in each subbasin ranging between 47 to 74 days among years. Contrary to our expectations, juvenile salmon were not larger in years with earlier emergence, owing to slower spring and summer growth rates compared to years with later emergence. Despite large inter-annual variation in emergence dates, these results suggest that other factors (e.g., stream flow, temperature, density-dependence) were more important than growth duration in determining juvenile salmon growth rates and size among years. We demonstrated considerable spatial and inter-annual 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.

诸多栖息有鲑科鱼类的河流在多时空尺度上均表现出热力与水文异质性，但此类异质性如何转化为仔鱼孵化出苗的时空分布模式，目前仍鲜有研究阐明。探明这种变异性具有重要意义：鱼类的首次出苗时间决定了其早期经历的生物物理环境（如水温、径流、食物供给），进而影响这一关键生活史阶段的生长潜力与存活率。本研究依托实测产卵与水温数据，对俄勒冈州东北部4个子流域5至9年间的春季奇努克鲑（Oncorhynchus tshawytscha）出苗物候进行了预测。随后，我们将年际出苗时间预测值与固定采样点的幼鲑体长及生长速率进行了关联分析。各子流域均呈现出清晰的出苗时间纵向梯度模式：该模式的形态在不同年份间保持一致，但不同子流域之间存在差异。其中2个子流域的出苗时间随上游距离增加而逐渐延后，而另外2个子流域的上游站点出苗时间最早。各年份内，单个子流域内所有采样点的出苗中位日期介于44至58天之间。这种空间变异性与年际变异性相当：单个子流域内某一固定采样点的年际出苗中位日期介于47至74天之间。与我们的预期相悖的是，出苗较早的年份幼鲑体长并未更大——这是因为相较于出苗较晚的年份，该年份春季与夏季的生长速率更慢。尽管出苗日期存在显著年际波动，本研究结果仍表明：在决定幼鲑年际生长速率与体长的因素中，其他变量（如径流、水温、密度制约效应）比生长时长的影响更为关键。本研究证实了上述子流域内出苗物候存在显著的空间与年际变异性。探明此类变异性如何转化为幼鲑栖息地利用、生长与存活的时空分布模式，对于指导河流修复工作、理解气候变化对鲑鱼种群的影响均具有重要意义。