Female Downstream-Hatching Migration of the River Shrimp Macrobrachium Ohione in the Lower Mississippi River and the Atchafalaya River
Olivier, Tyler J., Bauer, Raymond T., The American Midland Naturalist
ABSTRACT.-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 significandy 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.
Macrobrachium ohione (Ohio shrimp) is one of six species in this freshwater genus that is found throughout coastal rivers and their estuaries in the Gulf of Mexico and the southeastern Adantic Coast (Hedgpedi, 1949; Home and Beisser, 1977; Bowles et al, 2000). This shrimp is the only member of the genus that permanendy inhabits the Mississippi River System (MRS) (Taylor, 1992; Bowles et al, 2000; Barko and Hrabik, 2004). The distribution of M. ohione in the MRS historically ranged north to the Missouri River and well into the lower Ohio River. Prior to the 1940s, commercial fishermen reported large harvests of M. ohione, as part of a fishery for bait and human consumption, from as far north as Kentucky, Missouri and Illinois (McCormick, 1934; Hedgpeth, 1949; Taylor, 1992; Bowles et al, 2000). Since then, collections of M. ohione have become rare in the upper MRS, and when shrimps were taken from the northern reaches of the river, no gravid females were observed (Taylor, 1992; Conaway and Hrabik, 1997; Poly and Wetzel, 2002; Barko and Hrabik, 2004). Macrobrachium ohione has not been studied extensively within the MRS. A better understanding of this species' life history may provide insights into the decline of M. ohione populations along its northern distribution. Such information will be invaluable in the conservation of present populations and dieir restoration in the upper MRS.
Abbreviated or direct larval development has allowed most atyid and many palaemonid shrimp species to adapt to exclusive freshwater living (Hubschman and Broad, 1974; Jalihal et al, 1993; Bauer, 2004). …