CACHARA GROWTH CURVE Pseudoplatystoma reticulatum AND HYBRID P. reticulatum × P. corruscans

Growth curves of animals can be described through non-linear mathematical models, which allow aggregation of weight-age information into a small set of parameters. This study aimed to compare the growth curves of cachara (Pseudoplatystoma reticulatum) and hybrid cachapinta (P. reticulatum x P. corruscans) produced in cages. The experiment was conducted using floating cages placed in a 24-ha hillside pond at a commercial fish farm. A randomized experimental design was established with two treatments (cachara and hybrid) and three replicates, using six cages (13.5-m³ each). Mean weight of fish at the start of the experiment were 75 ± 0,004g and 85 ± 0,001g for cachara and hybrid, respectively. The results of the experiment were adjusted for a density of 135 fish per cages. Since the mean weight of the groups differed, the number of fish were adjusted at the onset of the experiment so that the density in kg per m³ remained the same for each cage, irrespective of the group present. The experimental period lasted for 216 days. Student's t-test was used to compare between groups for the dependent variables, assuming equivalent variances (P>0.05). The exponential model was used for analysis of the growth curves of the morphometric characteristics of each genetic group. To test the similarity of the models between the two groups we used the likelihood ratio test (LR), which verifies the equality of the parameters between them. In the LR test, four models were evaluated: model 1 (M1) the groups presented initial sizes (A) and different growth rates (K); model 2 (M2) where A was the same and different K; model 3 (M3), the two groups had K equal and A different; model 4 (M4), even A and K. The choice of the model used for each evaluated characteristic was the one that presented the smallest number of parameters and did not differ from the complete model (M1). When the M2, M3 and M4 models differ from the complete model (M1), it was concluded that the parameters were different between the two groups. There was no difference (P> 0.05) between the growth curves of the cachara and the hybrid when compared using the characteristics: weight, body height BH, body width BW, head length HL (Table 1). The model used to compare these characteristics was the reduced model (M4). Model 2 was the one that best adjusted for the total length TL in which there was no significant difference in the initial size (A), however, the cachara presented a higher growth rate (B). The complete model (M1) was used for head width HW, where A was smaller and B was larger for this characteristic in the cachara compared to the hybrid. The cachara (Pseudoplatystoma reticulatum) presented a higher growth rate for the characteristics TL and HL in relation to the cachapinta hybrid (P. reticulatum x P. corruscans).