COLD TOLERANCE IN MATERNAL MOUTHBROODING TILAPIAS - PHENOTYPIC VARIATION AMONG SPECIES AND HYBRIDS

Cold tolerance in the tropical-subtropical tilapias (genus Oreochromis) is a trait of economic importance in temperate climates where growing seasons are 6-8 months long and overwintering of stocks is a necessity. Standardized short- and long-term cold-tolerance tests have been evaluated for quantifying cold tolerance within and among populations. These tests will allow selection of superior individuals and/or families which can subsequently be evaluated for their breeding value. The short-term test can be conducted in 8-12 h compared with conventional and ambient cooling methods which require several days to several weeks. Rank-order of phenotypic means among species was identical for short- and long-term cold-tolerance methods. Irrespective of testing method. O. aureus was most cold tolerant, followed by O. niloticus (Egypt strain), O. mossambicus, and O. hornorum. Cold-tolerance means for hybrid populations were intermediate to the respective parental means, indicating no significant heterosis. Body weight (range=2-90 g), gender, or color phenotype within populations were not significant factors affecting cold tolerance. Genetic and environmental factors influencing the phenotypic expression of cold tolerance are discussed. © 1990.