The hypothesis of an amphidromous life history pattern, with a female hatching migration from the river to an estuary, larval development in saltwater, and a return upriver migration by postlarvae (juveniles) was tested in the river shrimp Macrobrachium ohione in the Atchafalaya River, Louisiana, U.S.A. in 2006. A possible female migration from the river to the Atchafalaya Delta estuary (AD) to hatch incubated embryos was tested by comparing reproductive status of females sampled monthly from stations 146 km (Butte La Rose = BLR), 42 km (Berwick = BR), and 0 km from AD. Females only occurred in traps at AD only during the reproductive season (March to August) but were present throughout the year at other stations. The highest percentages of prehatching females occurred at the AD and BR stations while prehatching females were relatively rare at the upriver BLR station. Salt water requirements for larval development were tested by incubating first stage larvae from individual hatches in freshwater and saltwater (15 ppt) treatments (n = 10). The first stage (nonfeeding) larvae did not molt to second stage (feeding) larvae in freshwater, with significant mortality beginning after day 5. In saltwater, survival was high and most hatching larvae molted to stage 2 after 4-5 days of hatching. An upstream migration of juveniles began in mid-July 2006 and continued until October. Juvenile migrators were observed swimming near the surface from approximately one hour after sunset until at least early morning in a band of hundreds to thousands of individuals 1-2 m wide along the shore. Body size of migrators increased from downstream to upstream, suggesting that juveniles are feeding and growing during the migration. Hypotheses about whether formerly abundant far northern populations migrated to and from the sea are discussed. The decline of the species in the northern part of its range might be partially explained by human impacts on the juvenile migration and subsequent upstream recruitment.
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Vol. 28 • No. 4