Life history of Gambusia holbrooki (Pisces, Poeciliidae) in the Ebro delta (NE Iberian peninsula)

We studied age, growth and reproduction of an introduced poeciliid, Gambusia holbrooki (Agassiz, 1859), in the Ebro delta marshes. Maximum ages observed were 2(+) in females and 0(+) in males. Growth commenced in March and continued until September. Females reached greater sizes, had a higher rate of growth and lived longer than males. The largest sizes found were 63 mm females versus 32 mm males. Multiple spawning occurred throughout the reproductive cycle (from May to September). The average brood size per female reached its maximum in July in the parental cohort (86 embryos/female) and in August in the offspring cohort (29 embryos/female). After reproduction the parental cohort decreased drastically. The relationship between fecundity (F) and total length (mm) was calculated for each month of the reproductive period. Sex-ratio deviated significantly from unity. Viviparity, several broods and large larvae provide G. holbrooki with several advantages over oviparous species. The capacity to adjust its life history to a particular environment could explain the life history differences between those in the Ebro delta and other populations.