Growth curve of free-ranging Trichechus inunguis

The objective of this work was to describe growth curves for free-ranging Amazonian manatees in Brazil. Data analyzed included age estimates and biometrics from 60 Amazonian manatees (33 males and 27 females) captured between 1993 and 2006 by local residents of the mid-Solimões and Pirativa Rivers, in Amazonia, and collected by the Instituto de Desenvolvimento Sustentável Mamirauá and Centro Nacional de Conservação e Manejo de Mamíferos Aquáticos/ICMBio. The non-linear mathematical model used to analyze the data was the von Bertalanffy's equation; two growth curves were calculated, one for males L(t)male = 299.4[1-e-0.0897507(t+6.55696)], and one for females L(t)female = 256.1[1-e-0.23731(t+ 3.01921)]. Based on these equations, length at birth for the Amazonian manatee is estimated at 133.2 cm (average = 113.0 cm; SD = 34.4 cm) for males, and 131.0 cm (average = 124.7 cm; SD = 22.0 cm) for females. Among the males, the annual growth rate was 0.09 cm (95% CI: ± 0.002), with a maximum length of 299.4 cm (95% CI: ± 2.039), while females presented a growth rate of 0.24 cm per year (95% CI: ± 0.004), reaching up to 256.1 cm (95% CI: ± 0.4832) of total length. Even though von Bertalanffy's equation is not commonly used for sirenians, and considering that when previously applied it did not obtain satisfactory results, in this study it proved adequate for the establishment of the growth curve for free-ranging Amazonian manatees.