380 



Fishery Bulletin 101(2) 



ence between the sexes (^test, P>0.05). However, it was ob- 

 served that males possibly exhibit a discontinuity in growth 

 around five years. The existence of a secondary growth 

 spurt around this age was considered to be due to the onset 

 of puberty in this species (Rosas and Monteiro-Filho, 2002), 

 and necessitated the calculation of separate growth curves 

 for each sex. It should be noted that sexual maturity of 

 the dolphins here analyzed was determined by Rosas and 

 Monteiro-Filho (2002) to occur at seven years in males. In 

 order to estimate the fit of a two-step model, the sample was 

 divided into two groups: 1) up to five years (prepuberty) and 

 2) older than five years (subadult and adults). 



The growth parameters obtained for males and females 

 are given in Table 2. The results obtained by Borobia ( 1989) 

 and Schmiegelow ( 1990) using the von Bertalanffy growth 

 model are also indicated in Table 2 for comparison. The 

 growth parameters obtained by the analyses of males up 

 to five years of age and those older than five are presented 



in Table 3. By dividing the sample in two, the fit of the von 

 Bertalanffy model improved considerably (Tables 2 and 

 3). The growth curves of S. guianensis males and females 

 obtained by the von Bertalanffy model are presented in 

 Figure 3. 



The t-test applied to parameters a and b of the weight/ 

 length regression equations for males and females revealed 

 a significant difference (t=2,25; df=38; P<0.05). Therefore, 

 this relationship was analyzed for the sexes separately and 

 the equations obtained were 



7W = 3. 156 X 10-6 X XL ^ 2836 

 TW = 8.974 X 10-5 X tL ^ "82 



(males) (r=0.96) 

 (females) (r=0.95). 



Discussion 



Age estimation 



Although there was no difference in the age estimation 

 between teeth orientated in the buccal-lingual and ante- 

 rior-posterior planes, we recommend the buccal-lingual 

 plane to obtain easier on-center or close-to-center sections, 

 which are essential for accurate age estimates. 



The differences found in counting GLGs in teeth from 

 the anterior extremity and the median region of the tooth 

 row of the same animal corroborate the results obtained by 

 Hui ( 1980) for Tkirsiops truncatus. Therefore, we also do not 

 recommend using teeth from the most anterior part of the 

 mandible for age estimation in S. guianensis. 



The mean distance between the neonatal line and the end 

 of the first GLG obtained in the present study (622.4 pm) 

 was approximately double that obtained by Ramos (1997) 

 (297.8 pm) for estuarine dolphins on the coast of Rio de Ja- 

 neiro. The differences, however, must be analyzed carefully: 

 the measurements carried out in our study were always 

 made in the neck of the teeth, whereas those made by Ra- 

 mos ( 1997 ) were from the base of the neonatal line. However, 

 the differences may be related to the interpretation of the 

 position of the first annual layer The accessory layers (no- 

 nannual), observed between the neonatal line and the end 



