Rosas et al.: Age and growth of Sotalia guianensis 



379 



3 4 



o 



E 2 



I Females (n=28) 

 I Males (n=34) 



11 



li 



m M i ll III 



111 I II T" 



2 4 6 8 10 12 14 16 18 20 22 24 26 



Age (years) 



28 30 



Figure 2 



Age distribution of female and male Sotalia guianensis on the Parana coast, southern 

 Brazil, recorded between 1997 and 1999. 



to be estimated. To define the growth model that would best 

 fit the length and age data of S. guianensis, the Schnute 

 model was applied to the length-at-age data. 



Growth equations were calculated separately for the 

 sexes, with 34 males and 28 females. Growth model ad- 

 justment to the data was made by using the nonlinear 

 iterative Quasi-Newton method, minimizing the residual 

 sum of squares. 



Total-weight to total-length relationships were estab- 

 lished by using 42 individuals of S. guianensis (23 males 

 and 19 females) with the equation 



7W = X rL» 



(Santos, 1978), 



where TW = total weight in kg; 

 TL = total length in cm; 

 <p = e° (e=base of the natural logarithm); 

 6 = b (6is the length exponent); and finally 

 a and b = correlation parameters between the weight 

 and length, obtained by the method of the 

 least square by adjusting the logarithm data 

 ofTTVandTL. 



Results 



Age estimation 



The mean distance from the neonatal line to the end of the 

 first GLG was 622.4 ±19.1 pm (.n=48). There is one acces- 

 sory layer, sometimes two, between the neonatal line and 

 the end of the GLG of the first year One of them, located 

 at a mean distance of 248.9 ±32.6 pm (n=25) from the neo- 

 natal line, is not always present. The other, which is some- 

 times very conspicuous at the tip of the tooth, is always 

 present, located at a mean distance of 419.6 ±44.6 pm 



(«=54) from the neonatal line, and is first observed between 

 6.7 and 10.3 months of age. 



No difference was observed in age estimates of teeth 

 sections orientated in the anterior-posterior and buccal- 

 lingual planes. However, the buccal-lingual orientation 

 made it easier to obtain on-center sections. 



The number of GLGs counted in the small teeth from the 

 most anterior part of the tooth row was 50% less than those 

 counted in the teeth from the median part of the mandibles 

 of the same animal. 



The proportion of sexes in the sample studied was not 

 significantly different (;f^2=i 39- df=l; P>0.05). However, 

 among the animals with an age equal to or greater than 

 24 years (7!=7), 85.7% were females. The oldest male was 29 

 years old and the oldest female was 30 years old (Fig. 2). 



Age estimates for S. guianensis individuals varied from 



to 30 years. Although the age mode was in the and 



1 year classes (Fig. 2), 53.5% of all animals whose ages 

 were estimated were seven years or more. This proportion 

 remained relatively constant between the sexes; 55% of 

 the males and 50% of the females were equal to or greater 

 than seven years. 



Growth 



When applied to the present data, Schnute's model indi- 

 cated that the von Bertalanffy growth equation fitted the 

 length and age data of S. guianensis better (Table 1, a>0 

 and b>l; Schnute, 1981). Even though the predictive power 

 of Schnute's model is greater than von Bertalanffy 's (see 

 "Explained variance" in Tables 1 and 2), the latter is justi- 

 fied by its historical use, and therefore has a greater value 

 for populational comparison, and incorporates a better 

 understanding of the biological meaning of its variables. 



Comparing the total lengths (TL) of the estuarine dol- 

 phins of six years or more, we found no significant differ- 



