Heritability of cold tolerance in Nile tilapia, Oreochromis niloticus, juveniles

The inability of tilapia to tolerate low temperatures is of major economic concern as it reduces their growing season and leads to over winter mortality. In this study, cold tolerance of juvenile Nile tilapia, Oreochromis niloticus, was investigated and heritability estimates obtained. A total of 80 maternal full-sib families were produced by mating each sire with two dams. Fry were grown in hapas suspended in earthen ponds fertilized with chicken manure, and were 41-91 days post-hatch at the start of the experiment (mean standard length 50.6 mm; mean body weight 5.1 g). Fry were tagged and exposed to low temperature in an indoor facility. Temperature was lowered from 16 degrees C to 11 degrees C in 48 h and from 11 degrees C to 8 degrees C at the rate of 1 degrees C/day. Cold tolerance was expressed as temperature at death (TAD) and cooling degree hours (CDH). Fish mortality started at 13.6 degrees C and total mortality occurred at 8.6 degrees C. Mean TAD and CDH were 10.1 degrees C and 298.07 respectively. Fish body weight (BW) had a highly significant effect on cold tolerance (P < 0.0001). Smaller fish (< 5g) were more susceptible to lower temperature than larger fish. The heritability of cold tolerance was 0.08 +/- 0.17 for CDH and 0.09 +/- 0.19 for TAD, estimated with an animal model. There was a considerable common environmental/full-sib effect for this trait (0.33 +/- 0.10 for CDH and 0.27 +/- 0.09 for TAD). These values indicate that estimation of genetic parameters for cold tolerance in tilapia should include both direct additive and common environmental effects. Based on the results of this study, we conclude that the most appropriate way of enhancing cold tolerance of tilapia juveniles is by husbandry practices that increase pre-winter body weights. (c) 2005 Elsevier B.V. All rights reserved.