This paper documents a mass en route mortality event of adult summer chum salmon (Oncorhynchus keta) returning to the Koyukuk River, Alaska in the Yukon River basin. In response to reports from local communities, a small team of researchers (including the author) surveyed ca. 275 km of river on July 26 and 27, 2019 and counted 1,364 dead salmon. Although the total magnitude of mortality is unknown, counts from the survey certainly represent only a small fraction of the true number of fish that died. We sampled 73 carcasses to confirm death occurred prematurely prior to complete maturation and spawning, and to quantify sex and length. Visual inspection revealed a substantial fraction exhibited patterns of fungal growth consistent with secondary infections of skin lesions caused by the ubiquitous natural bacterial pathogen Flavobacterium columnare. Water temperatures during the survey averaged 17.1°C and the water was approximately 85% saturated with oxygen (ca. 8.5 mg/L), which likely contributed to the stress for upstream migrants. Evidence suggests size‐selective en route mortality as female migrants that died were 2% and male migrants 5% shorter than individuals that survived to their spawning grounds on Henshaw Creek. This translates to very strong estimates of natural selection using standardized selection differentials, yet it is unclear whether selection acts on body size directly or indirectly through correlated phenotypic traits such as run timing. The mortality event likely underpins the below average returns of summer chum salmon to the Koyukuk River in 2019, suggesting an impact on spawner abundance. The future consequences of this, or potentially increasingly frequent, en route mortality events for population productivity and the extent to which genetic adaptation or adaptive phenotypic plasticity of migration behavior may facilitate persistence of these populations is unknown.

中文翻译：

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阿拉斯加科尤库克河夏季鲑鱼途中死亡率记录及其与2019年热浪的潜在联系

本文记录了成年夏季鲑鱼的大规模途中死亡事件（Oncorhynchus keta）返回育空河盆地阿拉斯加的科尤库克河。为了回应当地社区的报道，一个小型研究人员小组（包括作者）对加州进行了调查。2019年7月26日至27日，这条河长达275公里，共计1,364条死鲑鱼。尽管总的死亡率未知，但从调查中得出的数字当然仅占死亡的真实鱼类数量的一小部分。我们采样了73个cas体，以确认死亡在完全成熟和产卵之前过早发生，并量化性别和长度。目视检查显示，很大一部分真菌生长方式与由无处不在的天然细菌病原体黄杆菌引起的皮肤病变的继发感染相一致。。调查期间的平均水温为17.1°C，水中约有85％充满了氧气（约8.5 mg / L），这可能对上游移民造成了压力。证据表明在途中有大小选择与在Henshaw Creek产卵场幸存的个体相比，死亡的女性移民的死亡率低2％，而男性移民则短5％。这转化为使用标准化选择差异对自然选择的非常强的估计，但是尚不清楚选择是否直接或通过相关的表型性状（如运行时间）间接作用于体型。死亡事件可能是2019年夏季鲑鱼返回科尤库克河的平均收益低于平均水平的原因，这表明对产卵量的影响。途中或未来可能越来越频繁的未来后果 人口生产率的死亡事件以及遗传适应或迁徙行为的适应性表型可塑性在多大程度上促进了这些种群的持久性尚不清楚。