The bighead (Hypothalmichthys nobilis) and silver carp (H. molitrix) have invaded much of the Mississippi River. It is unclear how reproduction in northern impounded pools of the Upper Mississippi River System (UMRS) compares to unimpounded (open) southern reaches. During spring through summer 2005 and 2006 and once in spring 2007, we quantified larval and juvenile production in the pooled and open UMRS. We then simulated population dynamics in pools as a function of apparent reproductive success. Larvae occurred during about 2 weeks each spring. Peak density and apparent spawn duration were greater in the open reach. Larval production peaked when discharge was high plus rising and water temperatures reached 18C. Most juveniles ([97%) occurred in the open reach. Low flow during drought years in the pools may limit reproductive success. The simulation demonstrated that, by treating dams as barriers to invasion from the lower open river (i.e., a source), climatic conditions may interact with flow in pools to limit populations by creating an isolated sink.
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