Life‐history patterns of the mosquito‐fish, Gambusia affinis, in the estuary of the Guadalquivir river of south‐west Spain

SUMMARY. 1. The age, growth and reproduction of the small, introduced fish Gambusia affinis (Baird & Girard, 1853) were studied in the estuary of the Guadalquivir river. 2. The life‐span was very short, the stock contained only two age groups: with annulus (1+ group; 10–12 months old) and without annulus (0+ group). 3. In both sexes growth restarted in April when the annulus appeared, but whilst 1+ males stopped growth, 1+ females grew steadily to June. Growth of 0+ spawners was only evident in September, the last month of the reproductive period. A differential growth rate between sexes was also evident. 4. 1+ specimens reproduced during May and June and their offspring from July to September. In both age groups somatic condition progressively declined during the spawning period. 5. The loss of condition and the disappearance of 1 + and the larger 04‐specimens after reproduction may indicate the cost of a prolonged high level of reproductive effort. 6. The total fecundity (taken as the number of embryos) of 1 + females was represented by the formula: Fec=5.08 T.L. (mm) ‐170.07 and that of 0+ specimens by: Fec=2.23 T.L. (mm) ‐42.92. The maximum average monthly fecundity was reached in June when the length of the mother was at its greatest. 7. Length at first maturity was smaller in 0+ group than in the 1 + group; the difference between the two groups was greater in males (≅5 mm, T.L.) than in females (≅3 mm, T.L.). Also the average total length of 14‐spawners was greater than 0+ spawners. There were significant differences in the overall sex ratio of 956 males to 2057 females. 8. The differences found in growth and reproduction between the two age groups suggest that life‐history tactics may vary not only between different stocks but also within the same stock among its different components.