1. Fish larval drift is an essential step in the life cycle of riverine fish species as it determines dispersal and colonisation. Anthropogenic flow alterations and interruption of longitudinal and lateral connectivity by river damming and straightening can severely affect larval drift patterns. In this study, we characterised spatio-temporal drift patterns of fish larvae in the heavily regulated large alpine River Inn and within a constructed nature-like fish bypass.
2. Drift was investigated in the main reproduction period of the fish fauna in this river, ranging from mid-April to end of June. Diel patterns were assessed by samples taken during day, dusk, night, and dawn each day of sampling. To obtain robust species identification, we used DNA barcoding for genetic verification of phenotypically pre-sorted groups.
3. From a total of 1,069 fish larvae and 283 fish eggs caught, DNA barcoding revealed 16 species from five families, including several target species of conservation. We found strong evidence that several endangered species, such as Chondrostoma nasus, Thymallus thymallus, Cottus gobio, and Aspius aspius successfully reproduced in the bypass system. Genetic species verification showed a high level of homogeneity in most phenotypically pre-sorted groups.
4. Distinct seasonal patterns were observed, with the majority of fish species in the drift observed in mid-June. Thymallus thymallus and Cottus gobio dominated larval drift at the beginning of the observation followed by mainly unimodal abundance peaks of several Cyprinid species. Nocturnal drift prevailed in all species.
5. Our results on the spatial occurrence and abundances of fish larvae highlight the importance of bypass systems in heavily modified waterbodies to provide valuable spawning habitats and drift corridors around dams. Moreover, the distinct species-specific time patterns of larval drift represent a first basis to direct future discharge and river management plans in large alpine rivers towards a protection of the sensitive larval stages of specific target species of conservation. This includes bypass and turbine operation modes in the period of highest drift activity, as well as the construction of nature-like bypass systems and creation of spawning grounds therein.

中文翻译：

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随波逐流：高山河流中幼鱼的时空漂移模式

1. 鱼类幼体漂流是河流鱼类生命周期中必不可少的一步，因为它决定了扩散和定殖。人为的水流改变以及河流筑坝和拉直造成的纵向和横向连通性中断会严重影响幼虫的漂流模式。在这项研究中，我们描述了在受到严格监管的大型高山河流旅馆和构建的类似自然的鱼类绕道内鱼类幼虫的时空漂移模式。
2. 调查了该河流鱼类群落的主要繁殖期，即4月中旬至6月底。Diel 模式通过每天采样的白天、黄昏、夜晚和黎明期间采集的样本进行评估。为了获得可靠的物种鉴定，我们使用 DNA 条形码对表型预分类组进行遗传验证。
3. 从总共捕获的 1,069 条鱼幼虫和 283 个鱼卵中，DNA 条形码显示了来自五个科的 16 个物种，包括几个保护目标物种。我们发现了强有力的证据表明几种濒危物种，如Chondrostoma nasus、Thymallus thymallus、Cottus gobio和Aspius aspius在旁路系统中成功繁殖。遗传物种验证表明，大多数表型预分类组具有高度的同质性。
4. 观察到不同的季节性模式，6 月中旬观察到漂移中的大多数鱼类。Thymallus thymallus和Cottus gobio在观察开始时主导幼虫漂移，其次是几种鲤科鱼类的单峰丰度峰。夜间漂移在所有物种中都盛行。
5. 我们关于鱼类幼虫的空间发生和丰度的结果强调了旁路系统在严重改造的水体中的重要性，以提供有价值的产卵栖息地和大坝周围的漂流走廊。此外，幼虫漂流的不同物种特定时间模式代表了指导大型高山河流未来排放和河流管理计划的第一个基础，以保护特定目标保护物种的敏感幼虫阶段。这包括在最高漂移活动时期的旁路和涡轮机运行模式，以及在其中构建类似自然的旁路系统和产卵场。