1. Conservation management of freshwater ecosystems often focuses on mitigating or reversing the negative effects of altered patterns of river discharge. Assessments of management interventions frequently focus on direct, short-term responses to discharge without consideration of underlying population trends that span multiple years.
2. We sought to determine the effects on fish populations of annual variation in river discharge and water temperature after accounting for underlying population trends. We used data on five native and one non-native fish species, collected over 7–20 years in seven rivers in the Murray-Darling Basin, south-eastern Australia.
3. Population trends explained 3.4%–24.6% of the total variation in abundance and biomass of our six study species, while discharge and water temperature explained a further 1.2%–11.4% of this variation. However, population trends confounded the effects of discharge and water temperature, which suggests that the effects of annual discharge conditions may be contingent on past conditions and factors intrinsic to populations (e.g. age structure).
4. Failing to account for population trends led to a combination of plausible and implausible associations with discharge and water temperature. Plausible associations included positive associations with the magnitude of spring discharge and negative associations with the number of days where discharge was below the long-term 10th percentile. Determining whether estimated associations are real or artefactual requires a greater focus on the processes that underpin multiyear population trends.
5. Our results highlight the potential for underlying trends in populations to confound the short-term effects of discharge. Potential confounding of short- and long-term changes in populations underscores the need to assess responses to river discharge in the context of overarching environmental conditions, including factors other than discharge.

中文翻译：

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潜在趋势混淆了鱼类种群对河流排放反应的估计

1. 淡水生态系统的保护管理通常侧重于减轻或扭转河流排放模式改变的负面影响。管理干预措施的评估通常侧重于对出院的直接、短期反应，而不考虑跨越多年的潜在人口趋势。
2. 在考虑了潜在的种群趋势后，我们试图确定河流流量和水温的年度变化对鱼类种群的影响。我们使用了 7 到 20 年间在澳大利亚东南部墨累-达令盆地的七条河流中收集的五种本地鱼类和一种非本地鱼类的数据。
3. 人口趋势解释了我们六个研究物种的丰度和生物量总变化的 3.4%–24.6%，而排放和水温解释了这种变化的进一步 1.2%–11.4%。然而，人口趋势混淆了排放和水温的影响，这表明年排放条件的影响可能取决于过去的条件和人口固有的因素（例如年龄结构）。
4. 未能考虑到人口趋势导致了与排放和水温的似是而非的关联。合理的关联包括与春季排放量的正相关和与排放低于长期第 10 个百分位数的天数负相关。确定估计的关联是真实的还是人为的需要更多地关注支撑多年人口趋势的过程。
5. 我们的结果强调了人口潜在趋势可能会混淆出院的短期影响。人口短期和长期变化的潜在混淆强调需要评估在总体环境条件下对河流排放的反应，包括排放以外的因素。