INFLUENCE OF INCUBATION-TEMPERATURE ON OREOCHROMIS-NILOTICUS (L) EGGS AND FRY .1. GROSS EMBRYOLOGY, TEMPERATURE TOLERANCE AND RATES OF EMBRYONIC-DEVELOPMENT

The influence of constant rearing temperatures on the thermal tolerance of O. niloticus embryos was evaluated using two acclimation conditions: Group A eggs were fertilized at 28°C and then subjected to test temperatures; Group B eggs were fertilized and reared to the retinal pigmentation stage (48 h) at 28°C before subjection to test temperatures. Eggs from both groups were incubated at 11, 17, 20, 24, 28, 30, 34.5 and 39.5°C. Incubation temperature and acclimation conditions affected embryonic survival and hatching success. At 17°C and below or at 39.5°C cleavage in Group A eggs failed. At each constant rearing temperature between 20 and 34.5°C the proportion of embryos advancing from the cleavage stage to hatching decreased. Optimal development to hatching (>90%) for Group A eggs occurred at 25-30°C compared with 23.5-32°C for Group B eggs. The upper and lower median tolerance limits (TL50-with 95% confidence limits) for Group A eggs were 34.0°C (33.3-34.7) and 21.8°C (21.1-22.5) compared with 35.1°C (34.3-35.8) and 14.8°C (13.9-15.8) for Group B eggs. The time to hatch (>50%) for Group A and B eggs was inversely related to their incubation temperature. For Group A and B eggs hatching times ranged from 2.3 days at 34.5°C to 6 days at 20°C. The rate of development to hatching ( 1 t days), however, was best described by a curvilinear relationship of the second order. © 1990.