LIFE-HISTORY OF GAMBUSIA-HOLBROOKI (PISCES, POECILIIDAE) IN THE EBRO DELTA (NE IBERIAN PENINSULA)

We studied age, growth and reproduction of an introduced poeciliid, Ga mbusia holbrooki (Agassiz, 1859), in the Ebro delta marshes. Maximum a ges 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 o ccurred throughout the reproductive cycle (from May to September). The average brood size per female reached its maximum in July in the pare ntal 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-rati o deviated significantly from unity. Viviparity, several broods and la rge larvae provide G. holbrooki with several advantages over oviparous species. The capacity to adjust its life history to a particular envi ronment could explain the life history differences between those in th e Ebro delta and other populations.