The recruitment of anadromous brown trout smolt from a small Baltic stream, frequently exposed to summer drought, was quantified for 9 years, and the effect from drought on smolt number and age composition analysed. Drought was quantified by, (a) a newly developed index quantifying the severity of summer drought (Drought Severity Index—DSI—based on amount of precipitation, monthly mean temperature and number of hours with sun), and (b) the amount of precipitation. Both DSI and precipitation were measured 1 or 2 years prior to emigration. We found highly significant (negative) relations between DSI (1 year before emigration) and both the total number of smolt, and the number of age 1 smolt. In addition, precipitation was (positively) related to total number of smolt, but DSI proved to be a stronger predictor compared to precipitation. In addition to drought, our results support a negative influence from older parr on the survival of age 0 trout. Our results indicate that recruitment of brown trout smolt from streams, regularly affected by summer drought, may be predicted approx. 6 months before the actual emigration the following spring. This offers an opportunity for managers to adapt protective measures in accordance with local and temporal changes in recruitment.

中文翻译：

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夏季干旱的严重程度是迁徙褐鳟（Salmo trutta）捕捞鲑鱼的预测因子

从一个经常暴露于夏季干旱的波罗的海小溪中募集了九岁的鳟鳟，对其进行了长达9年的定量研究，并分析了干旱对鲑鱼数量和年龄组成的影响。通过以下方式对干旱进行量化：（a）一种新的指数，用于量化夏季干旱的严重程度（基于降水量，月平均温度和日照小时数的干旱严重程度指数（DSI），以及（b）降水量。移民前1年或2年都测量了DSI和降水。我们发现DSI（移居前1年）与沙门氏菌总数和1沙门氏菌年龄之间高度显着（负）关系。此外，降水量（正）与白鲑总数有关，但事实证明，与降水量相比，DSI是更强的预测指标。除干旱外，我们的研究结果还表明年龄较大的幼鱼对0岁鳟鱼的生存具有负面影响。我们的结果表明，经常会受到夏季干旱影响的溪流中鳟鳟鱼的招募预计可达到约。次年春季实际移民之前的6个月。这为管理人员提供了一个机会，可以根据招聘的当地和时间变化来调整保护措施。