This paper describes the length-weight relationship, size at first maturation (L), and breeding season of Poecilia reticulata found in drainage channels on the campus of the Federal University of Pará (UFPA) in the Brazilian city of Belém. A total of 1.936 specimens (942 males and 994 females) were collected in monthly samples of the study area between June, 2006, and March, 2007. Females were significantly longer and heavier than males. The length-weight relationship for males was established by the equation Pt = 5 × 10-5× Ct2,397 and for females by Pt = 3 × 10-6 × Ct3,419. The estimated value of L was 17.5 mm for males and 20.4 mm for females, suggesting that males initiate reproductive activity at a smaller size than females. The abundance of mature females did not vary significantly among months, and there was little variation in the sex ratio, which was close to parity overall. Despite the adverse conditions encountered in the UFPA drainage system, P. reticulata appeared to have adapted its biological processes adequately.
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