Age and growth of the estuarine dolphin on the Parana coast, southern Brazil - Sotalia guianensis - Cetacea, Delphinidae

Fishery Bulletin, April, 2003 by Fernando Cesar Weber Rosas, Andre Silva Barreto, Emygdio Leite de Araujo Monteiro-Filho

After the secondary growth spurt in males, the final asymptotic length did not differ very much from that in females. Previous growth studies carried out by Borobia (1989), Schmiegelow (1990), and Ramos et al. (2000) with the estuarine dolphin did not mention the existence of a second growth spurt in males, possibly because the authors did not analyze the growth of males and females separately. According to Ramos et al. (2000), male and female data were combined because of the absence of sexual dimorphism in the body size of adults of this species.

Borobia (1989) and Schmiegelow (1990), who also used the von Bertalanffy model, obtained different values for the growth equation parameters (Table 1). The sample used by Borobia (1989) did not have many individuals in ages 1 and 2, and none in the 0 age class. The absence of animals that "anchor" the beginning of the curve could result in low estimates of k and [t.sub.0]. Additionally, Borobia (1989) examined individuals from different locations along the distribution of the species and thus did not take into consideration possible geographical variations. The results obtained by Schmiegelow (1990) are similar to those of our study, probably because both of them used animals from the same region.

Ramos et al. (2000) analyzed the growth of S. guianensis using the Gompertz growth model and obtained an asymptotic length (191.7 cm) which was much greater than that obtained in our study and in previous studies (Borobia, 1989; Schmiegelow, 1990) (Table 1). This difference could be due to 1) the small number of individuals older than 12 years (n=3) in their sample; or 2) a difference in asymptotic lengths between southeastern and southern Brazil populations. Similar differences have been observed between asymptotic lengths of Pontoporia blainvillei from Rio de Janeiro (southeastern Brazil) and Silo Paulo and Parana (same area of the present study), where larger individuals were found in Rio de Janeiro (Ramos et al., 2000; Rosas, 2000). Therefore, it is possible that environmental variables could be responsible for larger sizes in the area studied by Ramos et al. (2000), both for S. guianensis and for P. blainvillei.

Although no significant difference was observed in the asymptotic length between adult males and females, the differentiated growth in time between the two sexes is probably responsible for the difference observed in the weight-length relationship.

In most species, the length exponent ([theta]) of the weight-length relationship is usually close to 3 (Santos, 1978). The estimated values of this exponent for the estuarine dolphin (3.2 for males and 2.6 for females) suggest that the longitudinal and transversal body growth in this species follows a similar pattern.

Our results suggest that it is important to study growth by analyzing the sexes separately, because there may be differential growth between the sexes before the adult age.

Table 1
Schnute growth model parameters applied to Sotalia
guianensis on the Parana coast, southern Brazil.
"[[tau].sub.1]" and "[[tau].sub.2]" are predetermined
ages in years; "[y.sub.1]" and "[y.sub.2]" are
estimated sizes at ages "[[tau].sub.1]" and
"[[tau].sub.2]", in cm; "a" and "b" area dimensional
parameters. "SQ" represents the residual sum of squares,
and "Expl. var." represents the variance of the
data explained by the model.

Parameters       Females    Males      All

[[tau].sub.1]       0         0         0
[y.sub.1]          93.11     86.04     89.53
[[tau].sub.1]      28        28        28
[y.sub.2]         181.72    190.45    185.70
a                   0.14      0.07      0.13
b                   7.64      9.45      7.86
SQ               1242.35   1898.75   4375.56
Expl. var. (%)     93.47     92.17     92.47
n                  28        34        71

Table 2
Von Bertalanffy growth model parameters applied to Sotalia guianensis
on the Parana coast, southern Brazil, and parameters from the
literature. "T[L.sub.[infinity]]"= asymptotic length (cm), "k"= growth
constant and "[t.sub.0]"= theoretical age at which the length of the
animal is zero. "SQ" represents the residual sum of squares, and "Expl.
var." represents the variance of the data explained by the model.

                               Our study

                                                              Schmie-
                                                    Borobia    gelow
Parameters            Females    Males      All     (1989)    (1990)

T[L.sub.[infinity]]    177.31    179.10    179.53    187.21    182.6
k                        0.66      1.00      0.79      0.20      0.41
[t.sub.0]               -1.23      0.72      0.95     -4.05     -1.57
SQ                    1944.25   3732.30   6942.93     --        --
Expl. var. (%)          89.78     84.61     88.06     --        --
n                       28        34        71        24        22

Table 3
Von Bertalanffy growth model parameters for male
Sotalia guianensis on the Parana coast, southern Brazil.
"T[L.sub.[infinity]]= asymptotic length (cm), "k"= growth constant and
"[t.sub.0]"= theoretical age at which the length of the animal is
zero. "SQ" represents the residual sum of squares, and
"Expl. var." represents the variance of the data explained
by the model.

Parameters            Up to 5 years   More than 5 years

T[L.sub.[infinity]]       159.64            186.41
k                           2.02              0.53
[t.sub.0]                  -0.38             -1.40
SQ                        510.99           1013.98
Expl.var. (%)              94.20             50.90
n                          15                19