Growth and fecundity of the annual fish Austrolebias nigrofasciatus (Cyprinodontiformes: Rivulidae) under laboratory conditions
The annual fish Austrolebias nigrofasciatus is an endemic species of the Patos-Mirim lagoon system, in southern Brazil, where it is threatened with extinction. The effects of temperature on the growth and reproduction of A. nigrofasciatus were studied in captivity. Experiment 1 - growth: Naturally fertilized eggs, stored in a laboratory in a humid environment, were exposed to water and within 12 hours they hatched. The average standard length of the newly hatched fish was 4.67 ± 0.25mm. Juveniles were kept at 16 and 22°C for eight weeks and growth was faster at the highest temperature during the first few weeks, but as soon as spawning started (four weeks after hatching), the growth rate was reduced and at the end of eight weeks they measured 23.68 ± 3.73 and 22.68 ± 5.36 mm (p> 0.05), respectively at 16 and 22°C.Females kept at 22°C reached 23.00 ± 2.83mm and were significantly larger (p <0.05) than those kept at 16°C (17.91 ± 2.47 mm). The first spawning at reduced temperature was observed only in the eighth week after hatching. There was no significant difference in growth for males. The sex of A. nigrofasciatus seems to be determined by temperature, there was a higher proportion of males (1: 0.6) at 16°C than at 22°C (1: 1.1). Experiment 2 - reproduction: Twelve couples were collected in the environment and distributed in 12 aquariums, maintained at 17, 21 and 25°C (four replicates for each temperature) and followed for five weeks. The average weekly fertility was 30 ± 15; 32 ± 10 and 38 ± 19 for 17, 21 and 25°C, respectively, with no significant difference between treatments. At the end of the experiment,accumulated weekly fertility also did not differ between treatments and averaged 150 ± 49, 159 ± 3 and 190 ± 56 for 17, 21 and 25°C, respectively. At the end of the study, the growth of males was not influenced by temperature (p> 0.05), however females kept at 17 and 21°C were significantly higher than those kept at 25°C. The condition factor was also reduced at the highest temperature, suggesting that A. nigrofasciatus breeders should be kept at a reduced temperature for reproductive success.