Invasive Bighead Carp Hypophthalmichthys nobilis, Silver Carp H. molitrix, and Grass Carp Ctenopharyngodon idella are expanding throughout the upper Mississippi River (UMR) basin. Spawning has occurred in the main-stem UMR but could be limited to high-discharge events when dam operations create a free-flowing river. Alternatively, naturally free-flowing tributaries could offer alternative habitat for successful reproduction. Our objectives were to examine temporal and spatial variation in adult gonad development, ichthyoplankton densities, and back-calculated spawn dates of invasive carp collected in three UMR tributaries. We compared ichthyoplankton densities between the upstream and mouth reaches of tributaries and densities between the UMR and adjacent tributaries. Ichthyoplankton samples were collected every 10 d, and adult invasive carp were sampled monthly at nine sites throughout the Des Moines, Skunk, Iowa, Cedar, and Mississippi rivers during April–October 2014 and 2015. Peaks in adult gonadosomatic index, an increase in postspawn females, and ichthyoplankton collections suggested that peak spawning occurred during late May through June, when water temperatures were 18–30°C and channel velocities were at least 0.7 m/s. However, reproduction occurred as late as August in some tributaries. Ichthyoplankton densities were highest in the Des Moines River among tributaries, but tributary densities were similar to the UMR densities, suggesting that both tributaries and main-stem sites provide suitable spawning locations. Invasive carp reproduction in UMR tributaries provides additional sources of potential recruitment for further population expansion upstream in the UMR and should be considered when devising plans for controlling populations along the leading edge of the invasion.

中文翻译：

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密西西比河上游支流的入侵鲤鱼繁殖物候

入侵鳙鱼Hypophthalmichthys nobilis、鲢鱼H. molitrix和草鱼Ctenopharyngodon idella正在整个密西西比河上游 (UMR) 流域扩张。产卵发生在主干 UMR，但可能仅限于大坝运营创造自由流动河流时的高流量事件。或者，自然自由流动的支流可以为成功繁殖提供替代栖息地。我们的目标是检查成年性腺发育的时间和空间变化、鱼类浮游生物密度以及在 UMR 的三个支流中收集的入侵鲤鱼的回算产卵日期。我们比较了支流上游和河口河段之间的鱼类浮游生物密度，以及 UMR 和邻近支流之间的密度。每 10 天收集一次浮游鱼类样本，每月在得梅因、臭鼬、爱荷华州、雪松、2014 年 4 月至 10 月以及 2015 年 4 月至 10 月和密西西比河。成年性腺体指数的峰值、产卵后雌性的增加和鱼类浮游生物收集表明，产卵高峰期发生在 5 月下旬至 6 月，当时水温为 18-30°C，河道速度为至少 0.7 m/s。然而，在一些支流中，直到 8 月才开始繁殖。在支流中，得梅因河的浮游鱼类密度最高，但支流密度与 UMR 密度相似，这表明支流和主干地点都提供了合适的产卵地点。