Cold tolerance of selective breeding of Oreochromis niloticus and oreochromis aureus

Selected fish for higher growth rate from both O. niloticus and O. aureus were collected and subjected to challenge test for cold. In addition to, fish of both species collected from production ponds of Abbassa were used to compare the cold tolerance of fish. Fry of   similar age were grown under identical conditions. O. niloticus with an average weight of 1.29 ±0.73 g and 0.95 ±0.11 g, for O. aureus were used in this study. The selected O. niloticus revealed lower cold tolerance than the non-selected fish. The cooling degree hours were significantly different between the selected and non-selected O. niloticus (P<0.005). The selected O. aureus exhibited greater cold tolerance than the non-selected and death began at 14.1 oC, while nonselected occurred at 15.2 oC.  The lowest lethal temperature of 50% individuals was 8.7 oC for selected O. aureus   at 198 cooling degree hours (CDH) and 9.3 oC for non-selected fish at 147 CDH. Both selected and non-selected fish gave lowest lethal temperature of 60% individuals at 8.5 oC. But   in different degree hours which was longer in selected   (220 CDH) and shorter in non-selected (182CDH) .The selected fish with stood 330 cooling hours which was higher than that for the non-selected fish (311 CDH). There was strong relationship between the cold and both temperature at death and cumulative degree hours represented by negative correlation coefficient for all tested fish (-0.928± 0.014). There was no correlation between cold tolerance and fish size for all tested fish.