Effects of river control structures on the juvenile migration of Macrobrachium ohione

Summary Macrobrachium ohione is an amphidromous species (i.e. lives and breeds in fresh water with larval development in estuarine, brackish or coastal waters) that has experienced population declines in the Mississippi River (MR) System, possibly due to extensive human modification (e.g. wing dikes, levees, channelisation and dams). Old River Control (ORC) is a complex of locks and dams in Louisiana that separates and controls the volume of water that flows into the Atchafalaya River (AR) from the MR. In this study, we tested the hypothesis that ORC impedes or prevents movement of juvenile M. ohione from the AR into the MR. Juvenile migrants were sampled using unbaited traps placed in the channels leading into and out of the river control structures. Variables such as location (above or below structure), water temperature and channel discharge were documented to determine their effects on juvenile migration. Migrant densities were significantly affected by the interaction between trap location and channel discharge. The densities of juveniles in the channels leading into the structures (downstream) were greater than those leading out of the structures (upstream). In addition, the densities of juveniles were consistently greater in the channels where channel discharge was the greatest. These results suggest that upstream juvenile migration is impeded by the structures, reducing the recruitment of shrimps from the AR into the MR. As a possible solution to allow shrimps to bypass the river control structures, this study tested the hypothesis that M. ohione juveniles are able climb up structures, a behaviour observed in several amphidromous species. A laboratory experiment tested the climbing ability of juvenile migrants (i.e. number of individuals on the ramp) and climbing performance (i.e. number of individuals that completed the climb) when presenting the shrimps for 1 h with a 1.5 m ramp at various ramp angles (30°, 40°, 60°) and water velocities (0, 45, 65, 140 cm s−1). Most shrimps were observed climbing when water velocity was 65 and 140 cm s−1. Climbing performance was also influenced by the angle of the ramp, with the greater number of shrimps climbing at inclinations of 30° and 40°. Our results suggest that the installation of shrimp ladders and migratory conduits at river control structures could contribute to the restoration of M. ohione populations in the upper MR.