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

We believe that the accessory layer in the dentine found at approximately 419.6 [micro]m from the neonatal line, could be a record of the end of weaning in the estuarine dolphin. It was observed in all the teeth of individuals older than 6.7 months and could be a hypomineralized layer caused by a reduction of calcium in the body due to the absence of milk in the diet (Klevezal, 1996). The other accessory layer found closer to the neonatal line (mean of 248.9 [micro]m) was not observed in all animals and the interpretation of this layer remains uncertain. It may be related to the beginning of weaning, as has been suggested for the bottlenose dolphin (T. truncatus) (Hohn (1)). This hypothesis still needs to be confirmed. However, all the S. guianensis individuals that were still nursing, but which already had remains of solid food in their stomachs (n=5), had only an accessory layer that is closer to the neonatal line--they did not have the layer that we are assuming marks the end of weaning.

According to Rosas (2000), there was no significant difference in incidental catches between mature and immature individuals of S. guianensis caught on the coast of Parana, suggesting a similar vulnerability of young and adult estuarine dolphins to fisheries. Because the animals analyzed in our study were the same ones used by Rosas (2000), this lack of significant difference between mature and immature individuals can suggest a representative age distribution of the individuals analyzed.

Because the maximum estimated age in our study was 30 years, and because the dolphins here analyzed were incidentally caught in fishing nets, it seems reasonable to assume that the longevity of the estuarine dolphin may be 30-35 years. This hypothesis is also corroborated by the study carried out by Ramos (1997) with S. guianensis on the coast of Rio de Janeiro State (southeastern Brazil). Although the age of the oldest male observed in our study was 29 years, the frequency of males older than 21 years was less than 3%, which is extremely low when compared with the frequency of 21.5% for females older than 21 years. These results suggest a greater life expectancy for females, which is also corroborated by a study carried out by Ramos (1997) in Rio de Janeiro.

Growth

The use of Schnute's model is helpful in deciding which growth model should be used. Even though the researcher can usually decide which model is most appropriate by looking at the data, subtle differences in data distribution could cause one or another model to be more adequate. Use of a generic model allows this choice without intervention of the researcher and avoids any unconscious bias towards or against any model.

The discontinuity of growth in male S. guianensis in our study could have been due to the small sample size or may have been due to a second growth spurt, which has already been observed in the total length of Stenella attenuata (Perrin et al., 1976), Lissodelphis borealis (Ferrero and Walker, 1993), and Phocoenoides dalli (Ferrero and Walker, 1999), and in the weight of male Tursiops truncatus (Cockroft and Ross, 1990). The k value obtained for male S. guianensis up to five years was very high, meaning that asymptotic length in this phase of life was reached quickly. The cessation of growth exhibited by the model for males up to 5 years probably is not true in the biological sense but could be an artifact created by the model and the small sample size. Most probably there is a marked reduction in growth with the start of sexual maturation and a greater investment in the weight or reproductive apparatus (or both). The hypothesis of a greater investment in weight is supported by the observed difference in the weight-length coefficient between males and females. Additionally, sexual investment of male estuarine dolphins is very high--testes of adult males can reach up to 32 cm in length and weigh up to 3.3% of the total body weight (Rosas and Monteiro-Filho, 2002).