The objective ofthis study was to evaluate the morphometric growth and weight gain of strainsof tilapia (Thai, Red, UFLA and Commercial) by nonlinear models. Initially,500 male fingerlings of each strain, at 85 (Red and UFLA) and 86 (Thai and Commercial)days of age, were stocked separately in raceways with 56 m3. Twentyfish of each strain were randomly sampled, weighed and measured monthly. Fivenonlinear models (Brody, von Bertalanffy, Gompertz, logistic and exponential)were tested, choosing one that best fit to the data. The variables studied were:weight, standard length (SL), head length (HL), height 1 (H1), height 2 (H2),height 3 (H3), first distance (D1), second distance (D2), first width (W1),second width (W2) and third width (W3). The exponential model had the best fitto weight and morphometric data, with the exception of W2, in which the bestfitted model was von Bertalanffy. The convergence of the exponential model todata indicates that the cultivation period studied was not enough for the strainsto reach maturity weight. The UFLA strain presented the lowest value for parameter"a" (initial weight estimate), 8.71 g, and the highest for parameter k (specificgrowth rate), 0.0127, when compared with other evaluated strains. However, thehighest k of UFLA was not enough to overcome the final weight observed for theCommercial strain (603.1 g), which was higher than all other strains. Regardingthe morphometric measurements, the UFLA strain also had the highest k for thevariables SL, HL, HH, H1, H2, H3 and D2, and similar k to Commercial and Thaistrains for the variables D1 and W3 respectively. The strains differ as to weightgain and morphometric growth.
