1. Understanding the effects of density‐dependent and density‐independent factors on recruitment is often inhibited by difficulties quantifying their relative contributions in highly variable recruitment data. Use of data‐driven statistical methods with data that include one or more extreme recruitment events could help overcome these difficulties.
2. Juvenile Atlantic salmon and trout abundances in Wales have declined over the last 2 decades, and 2016 was a notably poor recruitment year in rivers around southern Europe, including England and Wales. The 2016 recruitment crash coincided with extreme winter weather conditions, leading to speculation that unusually warm temperatures and high flows adversely affect salmonid recruitment and caused the 2016 crash, although this remains untested.
3. We developed data‐driven statistical models to: (1) describe juvenile salmonid recruitment from density‐dependent and density‐independent factors; and (2) assess whether the density‐independent factors probably contributed to the 2016 salmon recruitment crash. We compiled salmon and trout young‐of‐year juvenile abundances from electrofishing surveys, egg deposition estimates and river flow and air temperature data from 2001–2017 for seven Welsh rivers, broadly representative of rivers around Wales. We used river flow and air temperature data to derive ecologically and behaviourally meaningful density‐independent explanatory variables.
4. Salmonid recruitment in Wales was best described using density‐dependent and density‐independent factors, especially for salmon: after accounting for a concave relationship with egg deposition, salmon juvenile abundance was reduced under (1) warmer spawning temperatures that might inhibit spawning, and (2) higher flood frequencies during pre‐emergence and emergence that might washout eggs or alevins. Results were less clear for trout, perhaps because they are behaviourally more plastic.
5. Our findings provide empirical support for general and predictable effects of temperature and flow during spawning and emergence on salmonid—especially salmon—recruitment in Wales. Furthermore, we suggest that the 2016 salmon recruitment crash was caused—in part—by particularly inclement spawning and emergence conditions, which could be more common under future climate change. Our findings suggest that future salmonid stock assessment models could include the effects of density‐independent variables on recruitment to improve their predictive power.

中文翻译：

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环境条件改变了依赖密度的鲑鱼的招募工作：对2016年威尔士招募事故的见解

1. 通常难以理解量化密度和密度独立因素对招聘的影响，因为很难量化其在高度可变的招聘数据中的相对贡献。将数据驱动的统计方法与包含一个或多个极端招聘事件的数据一起使用可以帮助克服这些困难。
2. 在过去的20年中，威尔士少年大西洋鲑鱼和鳟鱼的丰度有所下降，而2016年是南欧包括英国和威尔士在内的河流的招募年明显较差。2016年的招聘事故恰逢冬季极端气候条件，导致人们猜测异常温暖的温度和高流量不利地影响了鲑鱼的招募，并导致了2016年的事故，尽管这未经测试。
3. 我们开发了以数据为依据的统计模型，以：（1）从密度依赖性和密度依赖性因素描述少年鲑鱼的募集；（2）评估与密度无关的因素是否可能导致2016年鲑鱼招募失败。我们从2001-2017年的7条威尔士河流域（主要代表威尔士河流域）通过电钓鱼调查，卵沉积估算以及河流流量和气温数据汇总了鲑鱼和鳟鱼的年幼鱼数量。我们使用河流流量和气温数据得出了生态上和行为上有意义的密度无关的解释变量。
4. 威尔士鲑鱼的募集最好使用密度依赖和密度独立的因素来描述，尤其是鲑鱼：在考虑到与卵沉积的凹入关系之后，在（1）产卵温度升高可能会抑制产卵的情况下，鲑鱼幼鱼的丰度降低了;以及（ 2）出苗和出苗期间较高的洪水频率可能会冲走卵或鱼精。鳟鱼的结果不太清楚，也许是因为它们的行为更具可塑性。
5. 我们的发现为威尔士鲑鱼（特别是鲑鱼）招募中产卵和出苗期间温度和流量的一般和可预测的影响提供了经验支持。此外，我们建议2016年鲑鱼招募失败的部分原因是特别严重的产卵和出苗情况，在未来的气候变化下这种情况可能更加普遍。我们的发现表明，未来的鲑鱼种群评估模型可能包括密度独立变量对募集的影响，以提高其预测能力。