Movements of a model fish, the common carp, through a generic Mississippi River lock and dam demonstrate how fish swimming performance, behavior, and discharge-driven flow-fields determine fish passage rates in ways that can be predicted and modified using fish passage models

The distribution of river fishes, including invasive species, is influenced by their abilities and tendencies to pass regulatory structures, which in the Upper Mississippi River (UMR) are locks and dams (LDs). However, how and why fish approach and then pass through individual LDs and their structural components is presently unknown. In this study, groups of 25 common carp, a well-understood model species, were acoustically tagged, transplanted, and released every other week for two summers below a generic UMR LD which has both tainter and roller spillway gates as well as a lock. Common carp distribution, passage routes, and rates were then monitored and analyzed with respect to discharge and gate settings. Although nearly all carp were detected below this LD, weekly passage rates through its spillway gates were negligible until discharge exceeded 85,000 cfs when the gates were ~2/3 open and weekly passage rates increased to ~50% for its tainter gates and ~8% for its rollers. In contrast, carp passed through its lock at a relatively constant and modest rate (~6%), seemingly uninfluenced by discharge. This increase in spillway gate passage, which occurred at discharges shy of open-river, was predicted by a fish passage model which estimated spillway passage using swimming performance and estimated water velocity (flow-fields). When observed passage routes were incorporated into another discharge-based fish passage model, it accurately predicted total seasonal passage. These models could be improved and applied to determine which fish species will pass particular LDs and guide efforts to increase or decrease these rates.