Aquaculiure and Fisheries Management 1985, 16, 341-348 A simple apparatus for self-separation of post-larval prawns, Macrobrachium spp., in mixed larval rearing tanks C. A. MARTINEZ-PALACIOS, M. A. OLVERA NOVOA & CRISTINA CHAVEZ-MARTINEZ Centro de Investigacion y de Estudios Avanzados del IPN, Unidad Merida, Merida, Yucatan, Mexico Abstract. A device for self-separating newly metamorphosed post-larvae prawns from iLirva! culture tanks was designed based on the rheotactic activity exhibited by freshwater prawns after metamorphosis into post-larvae. The performance and effectiveness of the post-larvae selector were evaluated under (a) 24-h periods, (b) four water flows and (c) at two tank depths. A high migratory response (90% confidence level) during the dark period with a water Row range of approximately 1 Ipm was found. Thus the device has shown to be highly efficient in separating post-larvae in mixed population tanks. The avoidance of excessive handling stress. and the continuous automatic removal of post-larvae immediately following metamorphosis into juveniles are two of the most important advantages of this design as compared to other separation devices. Introduction A major problem in the commercial culture of Macrobrachium spp. prawns is the effective production of juvenile animals. In the larval stages, in particular, there are still high mortalities (Wang & Williamson 1977; Smith & Hopkins 1977; New & Singholka 1982). Mortalities in the larval stages are mainly due to the long larval period of 35 to 45 days (Fujimura 1976), which is caused by hatchery problems such as poor management during tank cleaning and siphoning, and poor quality water. A further problem is cannibalism, as two or more stages of growth may be preseni. This is more pronounced in the latter stages of production. For commercial and research purposes it is important to reduce mortalities. It would also be desirable to harvest, select and age precisely the post-larvae produced within any system in order to identify the time of metamorphosis, post-larval production and other parameters (Smith & Hopkins 1977). New & Singholka (1982) report that once the majority (90-95%) of the larvae have metamorphosed into post-larvae, they should be harvested from the larval tank and transferred to holding tanks, using dip nets, and the water-level should be reduced before finally removing the outlet filter sock and flushing the remaining Correspondence: Professor C. A. Martfnez-Palacios, Centro de Investigacion y de Estudios Avanzados del IPN, Unidad Merida. Apartado Postal 73-Cordemex, Merida 97310, Yucatdn. Mexico. 341