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

The senior author read teeth slides at least three times during a minimum period of three weeks. Estimated age was taken as the last reading, assuming that reading accuracy improves with practice (Pinedo and Hohn, 2000). Age was estimated without access to biometric and biological data, thereby avoiding reader bias.

By using only central sections or those close by, in which at least 80% of the pulp cavity was exposed (Fig. 1), we obtained the following measurements with an ocular micrometer in a compound microscope: 1) distance (in [micro]m) from the neonatal line up to the end of the first GLG in the dentine; 2) distance from the neonatal line to the first accessory layer in the dentine; and 3) distance from the neonatal line to the second accessory layer in the dentine, if present. All measurements were made perpendicular to the external margin and at the neck of the tooth (an area located between the crown and root of the tooth).

[FIGURE 1 OMITTED]

Ages of individuals less than one year were estimated in months, by using as a base the percentage proportion of the mean distance between the neonatal line and the end of the GLG of the first year (Ramos, 1997).

Several models have been created over the years to describe growth, including the von Bertalanffy, Gompertz, logistical, and Richards models. Schnute's generic growth model helps to choose the model which is best adapted to the length and age data of the species studied. Schnute's model (1981) is defined as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where Y(t) represents a measurement (length, weight, volume) at age t; variables [[tau].sub.1] and [[tau].sub.2] are ages of young and old specimens, respectively, and [y.sub.1] and [y.sub.2] are sizes at these ages. These sizes, together with a and b, are the parameters 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 adjustment 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 established by using 42 individuals of S. guianensis (23 males and 19 females) with the equation

TW = [phi] x T[L.sup.[theta]] (Santos, 1978),

where TW = total weight in kg; TL = total length in cm; [phi] = [e.sup.a] (e=base of the natural logarithm); [theta] = b ([theta] is 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 of TW and TL.

Results

Age estimation

The mean distance from the neonatal line to the end of the first GLG was 622.4 [ or -] 19.1 [micro]m (n=48). There is one accessory 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 [ or -] 32.6 [micro]m (n=25) from the neonatal line, is not always present. The other, which is sometimes very conspicuous at the tip of the tooth, is always present, located at a mean distance of 419.6 [ or -] 44.6 [micro]m (n=54) from the neonatal line, and is first observed between 6.7 and 10.3 months of age.