Downstream migration of smolts is a critical aspect of the life history pathway for anadromous salmonids. Timing of downstream migration can vary along latitudinal and climatic gradients. Steelhead Oncorhynchus mykiss occur over a broad range of climate and hydrologic conditions, but relatively little is known about migration timing of smolts in the southern extent of the species’ range. Using a 19‐year data set (1994–2014) of smolt arrivals collected in a downstream migrant trap at the Vern Freeman Diversion facility on the Santa Clara River, one of the largest coastal watersheds in southern California, I report patterns of migration and potential environmental drivers determining migration timing. Large sections of the Santa Clara River and the confluences of its perennial tributaries are intermittent except during winter and spring stormflows, limiting migration opportunities. If tributaries were connected, smolts were regularly encountered in the downstream migrant trap between March and May, with rare observations of downstream migrants in January (0.1%) or February (0.3%). Although these migration data are limited by low smolt abundance and sampling efficiency during high‐flow events, potential environmental drivers were identified as cues for smolt migration timing in this region. Day length was a consistent predictor of smolt migration, while hydrology was both a constraint and a cue, with migrants only arriving after tributaries had reconnected to the main stem and with many arrivals occurring weeks or months after storm events had passed. Smolt migration was not consistently synchronized with periods when intermittent sections of the main stem were wetted and passable to the ocean. Between 0% and 70% of smolts arrived at the Vern Freeman Diversion after natural flows were likely insufficient for passage to the ocean. Smolt migration is a critical piece of the management puzzle for southern California steelhead, and these data will serve to inform effective management strategies and research needs for the successful recovery of the species.

中文翻译：

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南加州的硬头mol迁移的模式和潜在动因

鲑鱼的下游迁移是无性鲑科鱼类生活史途径的关键方面。下游迁移的时间可能会随纬度和气候梯度而变化。斯蒂尔海德Oncorhynchus mykiss发生在广泛的气候和水文条件范围内，但对在该物种范围的南部范围内的小软体动物的迁移时间知之甚少。我使用了19年（1994-2014）的数据集，该数据是在南加州最大的沿海集水区之一圣塔克拉拉河上的弗恩·弗里曼·弗里德曼分流设施的下游移民陷阱中收集的来水的，我报告了迁徙的模式和潜力环境驱动因素决定迁移时间。除了冬季和春季的暴风雨，圣克拉拉河的大部分地区及其多年生支流的汇合都是断断续续的，从而限制了迁徙的机会。如果支流相连，则在3月至5月之间经常在下游移民陷阱中遇到臭鼬，而在一月​​份则很少观察到下游移民（0。1％）或2月（0.3％）。尽管这些迁移数据受到高流量事件中低丰度和采样效率的限制，但潜在的环境驱动因素被认为是该地区迁移时间的线索。一天的时间是软体动物迁徙的一致预测，而水文学既是一个制约因素又是一个线索，移民只是在支流重新连接到主干之后才到达，而许多到达发生在风暴事件过去几周或几个月之后。软体动物的迁徙与主要茎的间歇部分被弄湿并可以进入海洋的时期并不一致。在自然流量可能不足以进入海洋之后，有0％至70％的大猩猩到达了维尔恩·弗里曼转移中心。