Age, growth and reproduction of Aphanius iberus (Cuv. & Val., 1846) in the lower reaches of the Guadalquivir river (south-west Spain)

SUMMARY. 1. Age, growth and reproduction in an Atlantic population of the small fish, Aphanius iberus (Cuv. & Val., 1846), from the estuary of the Guadalquivir river were studied. 2. The growth period was from April to September and the annulus on the scales was formed in April. 3. The life-span was short with a winter age-structure of 95.9% 0+ fish, 3.9% 1+ and 0.2% 2+. 4. The fecundity of this stock, before the beginning of reproduction, is represented by the formula: Fec=0.907 TL (cm)4.099. During the reproductive period 1+ females contained more, smaller eggs g−1 of gonad than 0+ fish. 5. Males matured before females, but females were larger than males. The overall sex ratio did not differ from 1:1 but males dominated significantly during most of the spawning period. 6. Aphanius iberus spawned intermittently during the reproductive period. From April to July, 1+ group specimens (7–12 months old) reproduced. Their offspring (0+ group; 3–4 months old) reproduced from July to September. Very few of the 1+ group specimens survived to spawn the following year. 7. Mean gonadosomatic indices were much higher in the 1+ group than in the 0+ group and in females compared to males. 8. The loss of condition and the disappearance of the 1 + group during spawning, may indicate the cost of a prolonged high level of reproductive effort. 9. In this productive environment, the life history of Aphanius iberus is similar to that of Gambusia affinis (Baird & Girard, 1853) and Fundulus heteroclitus (Linnaeus) found in the same area.