Anatomy of a range contraction: Flow-phenology mismatches threaten salmonid fishes near their trailing edge

Climate change is redistributing life on Earth, with profound impacts for ecosystems and human well-being. While repeat surveys separated by multi-decadal intervals can determine whether observed shifts are in the expected direction (e.g., poleward or upslope due to climate change), they do not reveal their mechanisms or time scales: whether they were gradual responses to environmental trends or punctuated responses to disturbance events. Here we document population reductions and temporary range contractions at multiple sites resulting from drought for three Pacific salmonids at their ranges’ trailing edge. During California’s 2012-2016 historic multi-year drought, the 2013-14 winter stood apart because rainfall was both reduced and delayed. Extremely low river flows during the breeding season (‘flow-phenology mismatch’) reduced or precluded access to breeding habitat. While Chinook (Oncorhynchus tshawytscha) experienced a down-river range shift, entire cohorts failed in individual tributaries (steelhead trout, O. mykiss) and in entire watersheds (coho salmon, O. kisutch). Salmonids returned to impacted sites in subsequent years, rescued by reserves in the ocean, life history diversity, and, in one case, a conservation broodstock program. Large population losses can, however, leave trailing-edge populations vulnerable to extinction due to demographic stochasticity, making permanent range contraction more likely. When only a few large storms occur during high flow season, the timing of particular storms plays an outsized role in determining which migratory fish species are able to access their riverine breeding grounds and persist.

气候变化正在重塑地球上的生命分布，对生态系统与人类福祉产生深远影响。尽管间隔数十年的重复调查可确定观测到的变化是否符合预期方向（例如，因气候变化导致的向极地或高海拔迁移），但此类调查无法揭示其机制或时间尺度——即这些变化是对环境趋势的渐进响应，还是对扰动事件的阶段性响应。本文记录了分布区边缘的三种太平洋鲑科鱼类（salmonids）因干旱在多个位点出现的种群数量减少与暂时性分布范围收缩现象。在加利福尼亚州2012-2016年历史性多年干旱期间，2013-14年冬季尤为特殊——降雨量既减少又延迟。繁殖季节的极端低河流量（“水文-物候错配”，flow-phenology mismatch）减少或阻断了其进入繁殖栖息地的途径。奇努克鲑（Chinook，Oncorhynchus tshawytscha）出现了向下游迁移的分布范围变化，而虹鳟（steelhead trout，O. mykiss）在个别支流的整个世代灭绝，银鲑（coho salmon，O. kisutch）则在整个流域的世代灭绝。后续年份中，鲑科鱼类借助海洋保护区、生活史多样性，以及在某案例中通过保护育种计划的救助，重返受影响位点。然而，大量种群损失会使边缘分布种群因种群统计随机性（demographic stochasticity）而易受灭绝风险，从而增加永久性分布范围收缩的可能性。当高流量季节仅出现少数强风暴时，特定风暴的发生时机对决定哪些洄游鱼类能进入河流繁殖地并持续生存起着关键作用。