Dams alter river functioning and hinders the free movement of migratory fishes. We analysed the effects of a subtropical dam on the reproduction of migratory fishes using data from ichthyoplankton monitoring surveys. These were conducted at six sampling stations upstream, downstream and within the reservoir on a weekly basis during 6 years. The dam had a direct impact on fish movement, leading to the fragmentation of fish populations. Two different and independent reproductive areas became evident: one located in the reservoir headwater area and the other downstream of the dam. The long temporal scale of our study facilitated the detection of the impact of Salto Grande on the species of ichthyoplankton. Freshwater secondary fish species (mainly Lycengraulis grossidens) increased in abundance in the stations located inside the reservoir and closer to the dam, whereas most fishes found in the headwater of the reservoir corresponded to river migratory species (potamodromous fish). These differences are most likely due to changes in the physicochemical characteristics of the water induced by the transformation of a river into a reservoir. During rainy summers, Microcystis spp. densities were lower than 1,000 cells ml⁻¹ and fish densities were higher than 4 ind. m⁻³, whereas in dry summers, there were important Microcystis spp. blooms (401,573 ± 112,559 cells ml⁻¹) and fish larvae decreased sharply (<1 ind. m⁻³) in the reservoir headwater area. This could indicate that the discharge of water in the reservoir may also indirectly affect the regulation of fish reproduction. The alternating periods between dry and rainy summers associated with El Niño‐Southern Oscillation poses a challenge to ichthyofauna management in the reservoir.

中文翻译：

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水力发电厂对乌拉圭河游鱼的影响

大坝改变了河流的功能，并阻碍了迁徙鱼类的自由活动。我们使用鱼类浮游生物监测调查数据分析了亚热带水坝对on游鱼类繁殖的影响。在六年中，这些工作每周在水库上游，下游和水库内的六个采样站进行。大坝直接影响了鱼类的活动，导致鱼类种群分散。两个不同且独立的繁殖区变得很明显：一个位于水库上游水域，另一个位于大坝下游。我们研究的长期时间尺度有利于发现萨尔托格兰德对鱼鳞浮游生物物种的影响。淡水次生鱼类（主要是小鳞天蛾））在水库内且更靠近大坝的站点中的丰度增加了，而在水库水源中发现的大多数鱼类对应于河流迁徙物种（河豚）。这些差异很可能是由于河流向水库的转化所引起的水的理化特性变化所致。在多雨的夏天，微囊藻属。密度低于1,000细胞ml ^-1，鱼类密度高于4 ind。m ^-3，而在干燥的夏季，有重要的微囊藻。绽放（401,573±112,559细胞ml ^-1）和鱼幼虫急剧减少（<1 ind。m ^-3）在水库源头区域。这可能表明水库中的水排放也可能间接影响鱼类繁殖的调节。与厄尔尼诺-南方涛动相关的夏季和多雨夏季之间的交替时段对水库中鱼鳍金枪鱼的管理构成了挑战。