Life history, population dynamics and production of eastern mosquitofish, Gambusia holbrooki (Pisces, Poeciliidae), in rice fields of the lower Mondego River Valley, western Portugal

The introduced population of Gambusia holbrooki from the rice fields of the lower Mondego River Valley, Portugal, was studied for 15 months, relating their life cycle and population dynamics with its production, in order to a ssess the role of the species in the energy how and secondary production in this type of agro-ecosystem Two main annual cohorts (1995 and 1996 cohorts ) were identified. The females outnumbered males and the average female/mal e-ratio was 4. The inspection of ovary developmental stages of this vivipar ous fish, revealed that the most important reproductive period was between April and August. The first recruits were recorded in June and were present thereafter until October. Males from the parental cohort died before Augus t, whereas parental females could survive until October. Mean adjusted fecu ndity (number of embryos divided by female standard length) peaked in July 1996 (0.95) and in June 1997 (1.05). Females reached greater sizes, had a h igher growth rate and lived longer than males. Annual production was estima ted at 3.101 g.m(-2).year(-1) (ash-free dry weight, AFDW), the average biom ass at 2.896 g.m(-2) (AFDW), and the P/B ratio was 1.071. A conjugation of life history, population dynamics, production and ecological traits (e.g. f ast growth, reduced longevity, viviparity, high productivity, an intermedia te position in food chain, and no special habitat requirements for reproduc tion) clearly show that the populations of G. holbrooki, introduced into ri ce fields all over the world, may play an important role in the structure a nd functioning of the biological communities of these important agro-ecosys tems, (C) 1999 Editions scientifiques et medicales Elsevier SAS.