The objective was evaluate the reproductive biology of Macrobrachium jelskii (Miers, 1877) in São Francisco River, with emphasis on sex ratio, reproductive period, sexual maturity and fertility. The samples were collected monthly from March 2005 to February 2006 except April and October 2005, in two sites in São Fransico River, upstream of the Três Marias Dam (Pirapitinga Ecological Station) and downstream of the dam, located in Três Marias, central region of Minas Gerais state, Brazil. The netting mouth had 1 m² and the mesh 1.42 mm that it was passed under the marginal vegetation and the water next to the bank during 30 minutes. In the laboratory, specimens were identified, measured and sexed. A total of 2,945 specimens were captured in the upstream, 1,064 males and 1,857 females (1,653 non-ovigerous and 204 ovigerous) and 1,973 individuals in the downstream, 1,120 males and 841 females (643 non-ovigerous and 198 ovigerous). The sex ratio differed significantly for females of M. jelskii in upstream (0.57, χ2 = 215.28, p < 0.01), and males in the downstream (1.33, χ2 = 39.69, p < 0.01). In both sites the reproductive period was a continuous - seasonal with the peak occurring in January and November for shrimps of upstream and downstream, respectively. The mean fecundity of M. jelskii was 37 ± 14 eggs (5-78 eggs) in the upstream of the dam and 35 ± 14 eggs (5-69 eggs) in the downstream of the dam. The sex ratio showed deviation to males of M. jelskii in the downstream suggesting that females presenting behavior of hiding in the marginal vegetation. And in the upstream the sex was deviate to females, demonstrating the actual proportion of males and females of the population, probably because females do not find places to hide. The eggs size carried by females downstream was influenced by the low water temperatures from the dam.
Freshwater shrimp; sex ratio; reproductive period; sexual maturity and fertility