As rivers warm, cold‐water fish species may alleviate thermal stress by moving into localized thermal refuges such as cold‐water plumes created by cool tributary inflows. We quantified use of two tributary confluence plumes by juvenile steelhead, Oncorhynchus mykiss, throughout the summer, including how trout positioned themselves in relation to temperature within confluence plumes. At two confluences, Cedar and Elder creeks, along the South Fork Eel River, California, USA, we monitored temperatures using in situ logger grids throughout summer 2016. Fish were counted within confluences via snorkel surveys five times a day on 5 days at each site. We found diel and seasonal dependence on confluence use by steelhead, especially at the Cedar Creek confluence, where mainstem temperatures exceeded 28°C. At this site, fish moved into the confluence on the warmest days and warmest times of the day. Fish observed within the Cedar Creek confluence plume were most common in locations between 20–22°C, rather than the coldest locations (14.5°C). At Elder Creek, where mainstem temperatures remained below 24°C, there was little relationship between mainstem temperature and steelhead presence in the confluence plume. At both sites, steelhead distribution within plumes was influenced by spatial variation of temperature and mean temperature in surveyed grid cells. Our results show that cool tributaries flowing into warmer mainstem reaches (over 24°C) likely create important thermal refuges for juvenile steelhead. As mainstem rivers warm with climate change, cool‐water tributary inputs may become more important for sustaining cold‐water salmonids near the southern end of their range.

中文翻译：

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支流汇合处是变暖的河网中少年鲑鱼的动态避难所

随着河流变暖，冷水鱼类物种可能会转移到局部热源避难所中，以缓解热应力，例如由冷支流形成的冷水羽流。我们量化了幼年头鱼Oncorhynchus mykiss对两个支流汇流羽的使用整个夏季，包括鳟鱼如何根据汇流羽中的温度来定位自己。在美国加利福尼亚州南叉鳗河沿锡达河和埃尔德河的两个汇合处，我们使用现场记录仪网格监测了整个2016年夏季的温度。汇合处每天在5个站点中每天进行五次浮潜调查，对汇合处的鱼类进行计数。我们发现钢头对合流使用的依赖程度取决于季节和季节，特别是在雪松溪合流处，主干温度超过28°C。在这个地点，鱼在一天中最暖和的日子移入汇合处。在雪松溪汇流羽流中观察到的鱼最常见于20–22°C之间的位置，而不是最冷的位置（14.5°C）。在Elder Creek，主干温度保持在24°C以下，汇流羽流中的主干温度与钢头的存在之间几乎没有关系。在这两个地点，烟头内的杆头分布均受所调查的网格单元中温度和平均温度的空间变化的影响。我们的结果表明，凉爽的支流流入较暖的主干（超过24°C），可能会为幼小的硬头鱼建立重要的避难所。随着主干河因气候变化而变暖，冷水支流投入对于维持其范围南端附近的冷水鲑鱼可能变得更加重要。我们的结果表明，凉爽的支流流入较暖的主干（超过24°C），可能会为幼小的硬头鱼建立重要的避难所。随着主干河因气候变化而变暖，冷水支流投入对于维持其范围南端附近的冷水鲑鱼可能变得更加重要。我们的结果表明，凉爽的支流流入较暖的主干（超过24°C），可能会为幼小的硬头鱼建立重要的避难所。随着主干河因气候变化而变暖，冷水支流投入对于维持其范围南端附近的冷水鲑鱼可能变得更加重要。