World Aquaculture 59 ally weighed and divided into three groups depending on weight (group 1 = 0.06 to 0.10 g; group = 2, 0.11 to 0.20 g; group 3 = 0.21 to 0.30 g). Three hundred in each weight group were placed in three replicates of 100 juveniles each. Replicates consisted of permanently aerated round plastic tanks with 800 L of filtered freshwater and pieces of plastic net to provide shade and shelter (Figure 5). Shrimp were counted and individually weighed every 14 days. They were maintained for 84 days and fed with pellets (Purina® 35 percent protein) ad libitum every 12 hours at dawn and dusk. Tanks were kept indoors at room temperature (2427°C) with a photoperiod of 12:12 (sunlight only). Dead prawns and uneaten food were removed daily during the water exchange (20 percent). Differences in survival among treatments with time and weight change over time within the same stocking size group were established by one-way ANOVA (Zar 1999). Results Shrimp survival is summarized in Table 1. Stocking weight seemed to have an effect on survival with highest survival at higher stocking weights. Average weight versus time per group is shown in Table 2. The only group with a significant weight increase throughout the complete trial was group 3 (F=185.55) while groups 1 (F=117.14) and 2 (F=101.21), had no continuous significant differences in weight during the trial (one-way ANOVA). Discussion Stocking size may influence prawn production (D’Abramo et al. 1989, Tidwell et al. 2004) as shown in this study. The larger the juveniles at the beginning of the trial, the higher the survival. In addition, juveniles in the largest size group treatment were the only ones with statistically signficiant differences in growth during the trial (Table 2), a fact which suggest easier adaptation ability with increased size. D’Abramo et al. (1989, 1991) affirmed that a higher stocking weight of shrimp ultimately increased production. Tidwell et al. (2003) found that stocking larger juveniles significantly increased the percentage of animals that achieved harvest size. In agreement with previous works with shrimp (D’ Abramo et al. 2000), to minimize the risk of disease, it appears that larger juveniles may have better developed defense systems and, thus, are more resistant to threats (Prayinto and Latchford 1995). This would allow higher survival rates to be more achievable (Samocha et al. 2000). So, stocking larger animals possibly will improve growth and survival. Comparison of the present results with other studies are difficult because there is no known prior work on this topic for M. americanum. Some studies on M. rosenbergii are slightly comparable despite differences in species and management. New (2002) reported that M. rosenbergii juveniles with a stocking weight of 0.01-0.3 g can grow in an indoor nursery 5-30 mg/day over periods between 60-75 days. However, juvenile growth in the present work seems to be slower in comparison to those presented by New (2002), who stated that 0.25 g juveniles reached an averFig. 3. Juvenile prawn of Macrobrachium americanum (Bate 1868) from Sinaloa River. Fig. 4. The Sinaloa River, in which Macrobrachium americanum (Bate 1868) lives. Fig. 5. Plastic tanks utilized as experimental replicates for juvenile prawn holding and maintenance during the trial.
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