Gambusia affinis, Baird & Girard, 1853, originally comes from North America and has been distributed all over the world for the biological control of mosquitoes. This study aimed to determine some biological properties, such as structure, age, growth and reproduction of the populations of G. affinis in Turkey. For this purpose, between November 1998 and October 1999, 2026 mosquitofish were caught in the wetlands of Fethiye-Akgöl, Dalaman and Ortaca in Muğla province in the western Mediterranean region of Turkey and the data found were compared. In the Fethiye-Akgöl population the percentage of the 0+ age group was 97.59%, and of the 1+ age group was 2.41%; in the Dalaman population percentage of the 0+ age group was 99.71%, and of the 1+ age group was 0.29%; in the Ortaca population percentage of the 0+ age group was 94.37%, and of the 1+ age group was 5.63%. The mosquitofish population in Fethiye was 81.42% female and 18.58% male, in Dalaman 71.55% female and 28.45% male, and in Ortaca 70.42% female and 29.58% male. Total length (TL) in the Fethiye-Akgöl population was 1.3-5.5 cm, in Dalaman 1.7-5.5 cm and in Ortaca 1.3-5.8 cm; and total weight (W) in Fethiye-Akgöl was 0.02-2.31 g, in Dalaman 0.06-2.58 g and in Ortaca 0.02-5.83 g. In each population the gonadosomatic index (GSI) increased in March and April, had a slight decrease in May and increased again in mid May. At the beginning of January the GSI of the Fethiye population started to decrease again and the GSI decreased in Dalaman and Ortaca populations beginning in mid July. This shows that the mosquitofish is a bivoltine species. The average number of eggs belonging to each age group was 21.90 (0+) and 52.81 (1+) in Fethiye, 27.11 (0+) and 91.0 (1+) in Dalaman, and 28.24 (0+) and 81.78 (1+) in Ortaca. Average fecundity per gram was between 27.71 (0+) and 31.71 (1+) in Fethiye-Akgöl, between 42.97 (0+) and 39.99 (1+) in Dalaman, and between 35.28 (0+) and 37.39 (1+) in Ortaca.

Gambusia affinis, Baird & Girard, 1853, originally comes from North America and has been distributed all over the world for the biological control of mosquitoes. This study aimed to determine some biological properties, such as structure, age, growth and reproduction of the populations of G. affinis in Turkey. For this purpose, between November 1998 and October 1999, 2026 mosquitofish were caught in the wetlands of Fethiye-Akgöl, Dalaman and Ortaca in Muğla province in the western Mediterranean region of Turkey and the data found were compared. In the Fethiye-Akgöl population the percentage of the 0+ age group was 97.59%, and of the 1+ age group was 2.41%; in the Dalaman population percentage of the 0+ age group was 99.71%, and of the 1+ age group was 0.29%; in the Ortaca population percentage of the 0+ age group was 94.37%, and of the 1+ age group was 5.63%. The mosquitofish population in Fethiye was 81.42% female and 18.58% male, in Dalaman 71.55% female and 28.45% male, and in Ortaca 70.42% female and 29.58% male. Total length (TL) in the Fethiye-Akgöl population was 1.3-5.5 cm, in Dalaman 1.7-5.5 cm and in Ortaca 1.3-5.8 cm; and total weight (W) in Fethiye-Akgöl was 0.02-2.31 g, in Dalaman 0.06-2.58 g and in Ortaca 0.02-5.83 g. In each population the gonadosomatic index (GSI) increased in March and April, had a slight decrease in May and increased again in mid May. At the beginning of January the GSI of the Fethiye population started to decrease again and the GSI decreased in Dalaman and Ortaca populations beginning in mid July. This shows that the mosquitofish is a bivoltine species. The average number of eggs belonging to each age group was 21.90 (0+) and 52.81 (1+) in Fethiye, 27.11 (0+) and 91.0 (1+) in Dalaman, and 28.24 (0+) and 81.78 (1+) in Ortaca. Average fecundity per gram was between 27.71 (0+) and 31.71 (1+) in Fethiye-Akgöl, between 42.97 (0+) and 39.99 (1+) in Dalaman, and between 35.28 (0+) and 37.39 (1+) in Ortaca.