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We estimated population abundance and sex ratio for a freshwater live-bearing fish species (Poeciliopsis baenschi) that inhabits fluvial systems in western Mexico. We conducted mark-recapture experiments in 6 distinct populations. We used both our catch data as well as estimates of capture probability to calculate the number of individuals per unit water volume for adult males, adult females, and juveniles. Overall, abundance varied among sites from 23.8 individuals . m-3 to 363.2 individuals . m-3. In most of our study sites, the estimated sex ratio was markedly biased toward females (up to 11 females per male). We discuss the potential causes and implications of the observed demographic features. [ABSTRACT FROM AUTHOR]

In Mexico, several species of fishes are being affected by anthropogenic changes in rivers and streams. In order to understand the potential impacts of human activities on Poeciliopsis fasciata, we examined characteristics of the Río Salado in Puebla, Mexico that were associated with the presence of P. fasciata. Fish were observed in the river in March 2007 and from November 2007 through February 2008, but not from April 2007 through October 2007. The dynamic nature of the Río Salado likely influences the presence of P. fasciata, as evidenced by our finding fish in the study area only during a limited period during the year. The distribution of P. fasciata along the Río Salade is clearly influenced by factors related to the permanence of water, the chemical nature of the water (salinity, dissolved oxygen), and the presence of vegetation. Our results suggest that the presence of salt factories ("salineras") along the RIo Salado have the potential to influence the presence of P. fasciata, either through changes in water flow or salinity. [ABSTRACT FROM AUTHOR]

A reproductive analysis of 1 225 specimens of Poeciliopsis gracilis obtained through monthly samples from Coatetelco, a tropical shallow lake in Central Mexico, was made. There was an evident sexual dimorphism, including a difference in body size at the onset of reproduction. Sex ratio deviated significantly from unity. Monthly variations in gonadosomatic (GSI), hepatosomatic (HSI) indexes and ovarian development stages showed that the spawning season was from July to October, coinciding with the rainy season and phytoplankton biomass increase. The largest sizes were 50 mm for females and 43 mm for males. [ABSTRACT FROM AUTHOR]