The shrimp Macrobrachium ohione (Decapoda, Caridea) was once numerous in the Mississippi River System (MRS) as far north as the Missouri and lower Ohio Rivers but is now abundant only within the lower Mississippi and Atchafalaya Rivers. Adult M. ohione live and breed in fresh water, but larval development occurs in brackish and marine waters, a life history pattern termed amphidromy. A downstream female “hatching” migration may ensure that the stage-1 larvae reach the required salinity in time for the critical molt to stage-2 (first feeding larval stage). This study tested the hypothesis that embryo-bearing females deliver larvae to the estuaries of the Mississippi and Atchafalaya Rivers in Louisiana. To test the prediction of a downstream migration, this investigation examined the reproductive condition and the spatial-temporal distribution of reproductive-sized females during 2008 and 2009. Shrimps were collected by trapping at downstream (Pass A Loutre, Atchafalaya Delta) and upstream (St. Francisville, Butte La Rose) locations within the Mississippi and Atchafalaya Rivers, respectively. During M. ohione's reproductive season, a significantly larger proportion of females incubating embryos at any stage of development were observed downstream in the Atchafalaya River (AR) but not in the Mississippi River (MR). However, in the MR, a positive association between the proportion of females incubating near-hatching embryos and the downstream sample site was found in both years. In the AR, a similar association was found in 2009 but not in 2008. Females with near-spawning ovaries were positively associated with the downstream sites in the MR in 2009 and the AR in 2008. During the reproductive season, females in both the AR and MR were observed with near-spawning ovaries while simultaneously incubating near-hatching embryos. Thus, females may produce multiple broods during the reproductive season. In general, relative abundance (Catch Per Unit Effort) of reproductive-sized females was higher at downstream sites during the reproductive season. However, the predicted seasonal increase into downstream sites was statistically significant only in the AR in 2008. Overall, results of this study support the hypothesis that reproductive females migrate downstream to deliver larvae to the sea in both the AR and MR. Nevertheless, the exact mechanics of the migrations may vary with river characteristics such as length and water velocity.
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