Exponential growth model of Nile tilapia (Oreochromis niloticus) strains considering heteroscedastic variance

The objective of this research was to evaluate the initial growth of Nile tilapia of Thai and commercial strains, being the latter derived from GIFT program (Genetically Improved Farmed Tilapia). A set of weight/time data to an exponential model had been fitted. The fish were cultivated in cages of 2.7 m3, arranged on a dam of Parque Ecológico Cidade da Criança in Presidente Prudente — SP, Brazil. Each cage received 250 fingerlings of a single strain. With the fish growth, five random samples of 20 fish from each strain were taken, weighed and the model to the adjustment was weighted by inverse weight variances in each sample. The values of the initial weights were overestimated in the unweighted model in relation to the initial means values observed. Although the Thai strain had presented higher estimate of the initial weight than the Commercial strain, it obtained lower growth rate. In the weighted model, this difference in the growth rate was 32.27%, 10.85% bigger than in the unweighted model. In this experiment, this model described adequately the growth of the Thai and Commercial strains. However, it should be applied only with the intention of comparing the species or strains and not as a generalized growth model. The weighted model by the inverse weight variances had the best adjustment, estimates and confidence parameters. In this cultivation form, the Commercial strain can be the most indicated to farmers and resellers that deal with fish up to 200 g.