Do precipitation extremes drive growth and migration timing of a Pacific salmonid fish in Mediterranean‐climate streams?

Climate change is expected to increase weather extremes and variability, including more frequent weather whiplashes or extreme swings between severe drought and extraordinarily wet years. Shifts in precipitation patterns will alter stream flow regimes, affecting critical life history stages of sensitive aquatic organisms. Understanding how threatened fish species, such as steelhead/rainbow trout (Oncorhynchus mykiss), are affected by stream flows in years with contrasting environmental conditions is important for their conservation. Here, we report how extreme wet and dry years, from 2015 to 2018, affected stream flow patterns in two tributaries to the South Fork Eel River, California, USA, and aspects of O. mykiss ecology, including over‐summer fish growth and body condition as well as spring out‐migration timing. We found that stream flow patterns differed across years in the timing and magnitude of large winter–spring flow events and in summer low‐flow levels. We were surprised to find that differences in stream flows did not impact growth, body condition, or timing of out‐migration of O. mykiss. Fish growth was limited in the late summer in these streams (average of 0.02 ± 0.05 mm/d), but was similar across dry and wet years, and so was end‐of‐summer body condition and pool‐specific biomass loss from the beginning to the end of the summer. Similarly, O. mykiss migrated out of tributaries during the last week of March/first week of April regardless of the timing of spring flow events. We suggest that the muted response to inter‐annual hydrologic variability is due to the high quality of habitat provided by these unimpaired, groundwater‐fed tributaries. Similar streams that are likely to maintain cool temperatures and sufficient base flows, even in the driest years, should be a high priority for conservation and restoration efforts.

气候变化预计将加剧极端天气事件与天气变异性，包括更频繁的天气骤变，以及极端干旱与极端多雨年份之间的剧烈波动。降水格局的改变将重塑河川径流情势，进而影响敏感水生生物的关键生活史阶段。明确受威胁鱼类物种（例如硬头鳟/虹鳟（Oncorhynchus mykiss））在环境条件迥异的年份中如何受河川径流影响，对其物种保护具有重要意义。本研究聚焦2015至2018年的极端多雨与极端干旱年份，探讨其对美国加利福尼亚州南福克伊尔河两条支流的径流格局，以及虹鳟多项生态特征的影响，包括夏期鱼类生长、身体状况与春季洄游迁出时间。研究发现，不同年份的径流格局存在显著差异，具体体现为冬-春季大型径流事件的发生时序与量级，以及夏季枯水水位的变化。令人意外的是，本研究未观测到径流差异对虹鳟的生长、身体状况及洄游迁出时间产生影响。该流域溪流中的虹鳟在夏末生长受限（平均生长速率为0.02±0.05 mm/d），且该生长速率在干旱与多雨年份间无显著差异；夏末身体状况以及夏季始末各栖息池域的生物量损失情况亦无明显区别。同样，无论春季径流事件的发生时序如何，虹鳟均于3月最后一周至4月第一周期间从支流洄游迁出。本研究认为，虹鳟对年际水文变异性的响应较为微弱，其原因在于这些未受干扰、依赖地下水补给的支流为其提供了优质栖息生境。类似的、即便在最干旱年份仍能维持低温与充足基流的溪流，应当被列为保护与修复工作的优先对象。