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Fishery Bulletin 101(2) 



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 relation- 

 ship 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°18'S; 48°05'W-25°5'S8; 48°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 projec- 

 tion. 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 Depart- 

 ment of Oceanography of the Funda^ao 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 m2iximum 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 as- 

 sumed 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. 



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 accu- 

 racy 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 80'7r of the pulp cavity was exposed (Fig. 1), we 

 obtained the following measurements with an ocular mi- 

 crometer in a com[)ound microscope: 1) distance (in pm) 

 from the neonatal line up to the end of the first GLG in 

 the dentine; 2) distance from the neonatal line to the first 



Figure 1 



Central section of a Sotalia guianensis tooth cut 

 in the buccal-lingual plane showing the position of 

 the neonatal line (NL), the always-present acces- 

 sory layer (AL), first GLG (1) and second GLG (2). 

 Magnification: .30x. 



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). 



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: 



y'(/) = 



v:'+(v^-vf)- 



where Y(t) represents a measurement (length, weight, 

 volume) at age t; variables r, and T., are ages of young and 

 old specimens, respectively, andyj andy.^ are sizes at these 

 ages. These sizes, together with a and 6, are the parameters 



