588 



Fishery Bulletin 102(4) 



Table 2 



Comparison between average age, weight, and length at 

 sexual maturity between male and female franciscanas 

 from Rio Grande do Sul and Uruguay. The means of the 

 animals from Rio Grande do Sul were estimated by using 

 the DeMaster method (modified! and those from Uruguay- 

 were estimated by using a linear regression to determine 

 the moment when 50 ( 7 ( of the animals are mature. 



' Data from Uruguay were compiled from Kasuya and Brownell 



11979' 

 2 Data from Damlewicz (2003). 



length where 50 r * of the dolphins were mature through 

 a linear regression. Applying this same approach, a LSM 

 of 125.4 cm was estimated for Rio Grande do Sul — a 

 value still very similar to the Uruguay estimate. Male 

 franciscanas attain sexual maturity at less length and 

 weight than do females in Rio Grande do Sul (Danile- 

 wicz, 2003), as observed previously in Uruguay (Kasuya 

 and Brownell, 1979) and Rio de Janeiro (Ramos et al., 

 2000). 



This is the first estimate of mean age at sexual ma- 

 turity presented for male franciscanas. Kasuya and 

 Brownell (1979) could not calculate ASM for Uruguay 

 because of their small sample size («=25). Nevertheless, 

 Kasuya and Brownell suggested that sexual maturity 

 is attained when males are between 2 and 4 years of 

 age. Franciscanas from Rio de Janeiro were considered 

 mature when they were older than 2 years of age and 

 larger than 115.0 cm in length (Ramos et al., 2000). 

 However, histological analysis of the testes was not 

 performed and Ramos et al. employed indirect methods 

 to determine sexual maturity. Nevertheless, despite the 

 uncertainties produced by the use of different criteria 

 to determine sexual maturity, it was evident that there 

 was substantial difference in the size at maturity be- 

 tween males from Rio de Janeiro and those from Rio 

 Grande do Sul and Uruguay. This difference is probably 

 the result of the well-known distinct growth patterns of 

 the franciscanas from these two regions (Pinedo, 1991) 

 and does not necessarily reflect an early attainment 

 of sexual maturity in males from the Rio de Janeiro 

 population. 



The trade-off between growth and reproduction is 

 the best-documented phenotypic trade-off in nature 

 (Stearns, 1992) and has been studied in a wide range 

 of taxa. Because animals from Rio de Janeiro invest 

 less in growth than do animals from Rio Grande do 

 Sul, it is still an open question whether the francisca- 

 nas from the Rio de Janeiro have higher reproductive 



rates or start reproducing earlier than those from Rio 

 Grande do Sul. 



The oldest male and female franciscana ever aged 

 were 16 and 21 years-old, respectively (Kasuya and 

 Brownell, 1979; Pinedo, 1994). These ages contrasts 

 with the age distribution found in the present study, 

 where the oldest specimen analyzed was 6 years old. 

 Similar to what is observed in catches for several other 

 small cetacean species (e.g.. Hector's dolphins — Slooten 

 and Lad, 1991; harbor porpoise — Read and Hohn. 1995), 

 a general feature of incidental catches for these spe- 

 cies is the high entanglement rate of immature ani- 

 mals. In all fishing communities studied in Argentina, 

 Uruguay, and Brazil, a large proportion (>50%) of the 

 specimens caught were less than three years old (e.g., 

 Kasuya and Brownell, 1979; Corcuera et al.. 1994; Ott, 

 1998; Di Beneditto and Ramos, 2001). Although the 

 precise reason for biased catch rates towards imma- 

 ture individuals is not well understood, it could be a 

 combination of factors, including the imbalanced age- 

 structure of local populations (where there are fewer 

 older individuals because of an extensive history as 

 bycatch) and a behavior-related higher vulnerability to 

 bycatch for immature individuals (i.e., juveniles can be 

 more inquisitive and have less ocean experience so that 

 they rove into the area increasing the chances of being 

 entangled). The typically low proportion of old animals 

 in bycatches may explain the characteristics of the data 

 used in this study. 



Index of testicular maturity 



An index of testicular maturity may be very useful 

 in studies where it is necessary to know the sexual 

 maturity of a large sample of animals without the need 

 of histological analysis, which is time consuming and 

 requires expertise. Although Hohn et al. (1985) recom- 

 mended the investigation of the applicability of this 

 indirect index of sexual maturity for male cetaceans, 

 the research on this subject has shown no progress. To 

 date, the index of sexual maturity has been calculated 

 only for the common dolphin, Delphinus delphis (Collet 

 and Saint Girons, 1984), and from the pantropical spot- 

 ted dolphin, Stenella attenuate! (Hohn et al., 1985). For 

 both species, this index distinguished satisfactorily the 

 mature from the immature and pubertal dolphins. Given 

 the results presented, we also recommend the use of the 

 index of testicular maturity as an alternative, nonhisto- 

 logical method, to determine the sexual maturity of male 

 franciscanas. Males with index values lower than 0.05 

 can be safely classified as immature, and males with 

 index values above 0.08 can be classified as mature. It is 

 recommended that for animals with intermediate values 

 their testes be analyzed histologically so that their 

 reproductive status may be determined definitively. 



Besides making intra- and inter-populational com- 

 parisons possible, the index of testicular maturity also 

 permits interspecific comparisons because size differ- 

 ences between species are eliminated. The mean index 

 of testicular maturity of mature franciscanas (0.12) is 



