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

Abstract--Teeth of 71 estuarine dolphins (Sotalia guianensis) incidentally caught on the coast of Parana State, southern Brazil, were used to estimate age. The oldest male and female dolphins were 29 and 30 years, respectively. The mean distance from the neonatal line to the end of the first growth layer group (GLG) was 622.4 [ or -] 19.1 [micro]m (n=48). One or two accessory layers were observed between the neonatal line and the end of the first GLG. One of the accessory layers, which was not always present, was located at a mean of 248.9 [ or -] 32.6 pm (n=25) from the neonatal line, and its interpretation remains uncertain. The other layer, located at a mean of 419.6 [ or -] 44.6 pm (n=54) from the neonatal line, was always present and was first observed between 6.7 and 10.3 months of age. This accessory layer could be a record of weaning in this dolphin. Although no differences in age estimates were observed between teeth sectioned in the anterior-posterior and buccal-lingual planes, we recommend sectioning the teeth in the buccal-lingual plane in order to obtain on-center sections more easily. We also recommend not using teeth from the most anterior part of the mandibles for age estimation. The number of GLGs counted in those teeth was 50% less than the number of GLGs counted in the teeth from the median part of the mandible of the same animal. Although no significant difference (P>0.05) was found between the total lengths of adult male and female estuarine dolphins, we observed that males exhibited a second growth spurt around five years of age. This growth spurt would require that separate growth curves be calculated for the sexes. The asymptotic length (T[L.sub.[infinity]), k, and [t.sub.0] obtained by the von Bertalanffy growth model were 177.3 cm, 0.66, and -1.23, respectively, for females and 159.6 cm, 2.02, and -0.38, respectively, for males up to five years, and 186.4 cm, 0.53 and -1.40, respectively, for males older than five years. The total weight (TW)/total length (TL) equations obtained for male and female estuarine dolphins were TW = 3.156 x [10.sup.-6] x T[L.sup.3.2836] (r=0.96), and TW = 8.974 x [10.sup.-5] x T[L.sup.2.6182] (r=0.95), respectively.

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Until recently the genus Sotalia was monospecific (S. fluviatilis) and had a marine and a riverine ecotype (da Silva and Best, 1996). Using tridimensional morphometric analyses, Monteiro-Filho et al. (2002) were able to separate it into two distinct species: Sotalia fluviatilis, which lives in freshwater, and Sotalia guianensis, which lives in the marine environment. Because tucuxi is the vernacular name used for the freshwater species, Rosas and Monteiro-Filho (2002) suggested "estuarine dolphin" as the vernacular name for S. guianensis, as previously mentioned by Watson (1988).

Age is important in characterizing population dynamics of mammals. Growth layer groups (GLGs) observed in teeth of mammals have been used to estimate ages, and the greatest progress in this area has occurred with studies carried out on marine mammals (Klevezal, 1980; Hohn et al., 1989). The method consists of counting GLGs found in the dentine and cement of the animals' teeth, which are deposited every year in most species (Klevezal, 1996). Calibrating age estimates and identifying accessory layers (not annual) are essential for reliable age determination (Hohn, 1990). Some population parameters are extremely sensitive to errors and age estimate deviations, and the absence of or an inadequate calibration, could lead to incorrect interpretations (Hohn et al., 1989).

Because there is no sexual dimorphism in the body proportions of adult Sotalia guianensis, all previous growth studies analyzed both sexes together (Borobia, 1989; Schmiegelow, 1990; Ramos et al., 2000). However, there is evidence of differentiated growth between male and female estuarine dolphins around puberty (Rosas and Monteiro-Filho, 2002), thereby making it necessary to analyze growth separately for the sexes.

The objectives of this paper were 1) to estimate the ages of S. guianensis caught incidentally or stranded on the Parana coast, Brazil; 2) to give some guidelines to promote reliable age estimates in this species; 3) to describe the growth in body length (cm) according to the ages (years) of male and female estuarine dolphins, by using classical mathematical growth models; and 4) to describe the body-weight-body-length relationship for both sexes of this dolphin.

Materials and methods

Teeth from 71 individuals of S. guianensis (34 males, 28 females and 9 of undetermined sex), incidentally caught or found stranded on the Parana coast, southern Brazil (25[degrees]18'S; 48[degrees]05'W-25[degrees]5'S8; 48[degrees]35'W), from January 1997 to July 1999, were used to estimate age. The total body weight (kg) and standard measurements of individuals were made in accordance to Norris (1961). Total length (cm) was measured in a straight line from the tip of the beak to the central notch of the tail, in an axial projection. The skulls and teeth were collected, prepared, and deposited in the collection of the Instituto de Pesquisas Cananeia (IPeC).

Preparation of the teeth, from the decalcification to the mounting of the slides, was carried out in the Laboratory of Marine Mammals and Marine Turtles of the Department of Oceanography of the Fundacao Universidade do Rio Grande (FURG). The method of Hohn et al. (1989) was used, with the following adaptations: 1) decalcification time varied from one hour for newborn or young individuals, up to a maximum of 12.5 hours for old adults, 2) Harris's hematoxilin was used for staining, according to Molina and Oporto (1993), and immersion times of the sections varied from three to six minutes.

Because the absence of a pre-established age estimation model for S. guianensis, we tested both anterior-posterior and buccal-lingual planes for cutting teeth. Age estimation was performed by counting GLGs in the dentine. GLGs were defined as being the sequence of a thin nonstained layer, a thick stained layer, and a very thin layer that is strongly stained (very dark). Each complete GLG was assumed to represent one year (Ramos et al., 2000).

Teeth were selected from the middle of the tooth rows. However, to check for differences in age estimation among those positioned along the tooth row, we compared the number of GLGs in teeth from the middle of the tooth row with the number of GLGs in those from the most anterior part of the tooth row of the same animal.