The daily requirements of a spawning tilapia female are quantified from the sum of the requirements for maintenance plus production of eggs. The protein and energy requirements for maintenance and the cost of depositing energy and protein towards growth or gonadal products were determined by supplying feed at increasing levels from zero to the maximum intake. Comparative body composition analyses of the females in addition to the amount and the content of eggs enabled us to quantify the total energy and protein channelled into weight gain or alternatively into egg production. The amount of eggs produced increased with increasing feeding levels and ranged between 0.7 and 1.1 g eggs per kg−1 fish day−1. Regardless of feed allowance, the composition of eggs was similar and contained 235 mg protein and 10.5 kJ g−1 wet weight. In contrast, the whole body of tilapia contained 167 mg protein and 6.7 kJ g−1 on average. The energy requirement for maintenance was calculated to be 59.46 kJ × BW (kg)0.80 and 0.98 g × BW (kg)0.70 for digestible protein. The partial efficiency of producing gonads was 0.67 and 0.59 for digestible energy and digestible protein respectively.

Genetic parameters and selection responses were obtained for harvest body weight of blue tilapia (Oreochromis aureus) from data collected over three generations in a selected population. A total of 18 194 records representing 186 sires and 201 dams were used in the analysis. Within generation heritability estimates for harvest body weight ranged from 0.18 to 0.58. When data from more than one generation were included in the analysis, heritability estimates became more stable (0.33-0.40) and it was 0.33 when all data were included in the analysis. The common full-sib effect accounted for 10% of the phenotypic variance in the full data set. Heritability for survival from stocking to harvest was estimated at 0.01 and 0.09 in actual units (fitting an animal model) and in the logit (sire model) scale respectively. The genetic correlation between harvest body weight and survival was 0.22 and not significantly different from zero. The total selection response for harvest body weight over the three generations of selection measured as the difference between least-squares means of selected and control lines was 17.7%. The corresponding figure when response was measured as the difference between mean breeding values of selected and control lines was 19.6%. The average inbreeding coefficient was 0.003 after three generations of selection. These results indicate that there are good prospects for the genetic improvement of harvest body weight in blue tilapia.