In late 2018 to early 2019, three significant fish death events occurred in the Lower Darling River, Australia, with mortality estimates of millions of fish. We examined the proximate and ultimate causes of these events. We determined that not only were the conditions existing at the time a significant contributing factor, but that antecedent conditions, particularly during the period 2010–17, also contributed. The extreme hot and dry climate during 2018, extending into 2019, shaped the conditions that saw a large fish biomass, which had flourished in the Darling River and Menindee Lakes since favourable spawning conditions in 2016, isolated in weir pools, with no means of escaping upstream or downstream. Strong and persistent weir pool stratification created hypoxic conditions in the hypolimnion. A series of sudden cool changes subsequently initiated rapid and sudden mixing of the stratified waters, causing depletion of oxygen throughout the water column and resulting in the fish deaths. The events were also shaped by broader climatic, hydrological and basin management contexts that placed the Lower Darling River at risk of such fish deaths. Our observations have implications for future river management, and we make several suggestions how policy makers and river operators can minimise fish death risks into the future.

2018 年底至 2019 年初，澳大利亚达令河下游发生了三起重大鱼类死亡事件，估计有数百万条鱼死亡。我们检查了这些事件的近因和最终原因。我们确定，不仅当时存在的条件是一个重要的促成因素，而且先前的条件，特别是在 2010-17 年期间，也有贡献。2018 年持续到 2019 年的极端炎热干燥气候形成了大量鱼类生物量的条件上游或下游。强而持久的堰池分层在水下造成缺氧条件。一系列突然的凉爽变化随后引发了分层水域的快速和突然混合，导致整个水体中的氧气耗尽并导致鱼类死亡。这些事件还受到更广泛的气候、水文和流域管理环境的影响，这些环境使下达令河面临此类鱼类死亡的风险。我们的观察对未来的河流管理有影响，我们提出了一些建议，政策制定者和河流经营者如何将未来的鱼类死亡风险降至最低。