Growth can be expressed as the increase in length, volume or weight against time (Hartnoll, 1982). In aquaculture growth is measured by weight gain. Some authors have developed equations to relate data on length and weight for M. rosenbergii raised in tropical and sub-tropical pond (Sampaio and Valenti, 1996 and Valenti and Sampaio, 1996). As an example, the equation presented by Sampaio and Valenti (1996) based on data obtained in four experimental ponds was as: W = 1.21 x 10-6 L 3.43, where W = weight (g) and L = length (mm).
Growth curve facilitate forecasting and may be used in the determination of production models. Some authors reported using von Bertalonffy equations (Bertalanffy, 1938) to represent the growth of M. rosenbergii under batch conditions (Valenti, Mello and Castagnoll, 1993; Faria and Valenti, 1995; Sampaio and Valenti, 1996 and Valenti and Sampaio, 1996). Figure 2 shows an example of these curves, which indicate that growth rate is high during the first month after stocking and decrease later.