Reproductive biology of male franciscanas from Rio Grande do Sul, southern Brazil

Fishery Bulletin, Oct, 2004 by Daniel Danilewicz, Juan A. Claver, Alejo L. Perez Carrera, Eduardo R. Secchi, Nelson F. Fontoura

Abstract--The reproductive biology of male franciscanas (Pontoporia blainvillei), based on 121 individuals collected in Rio Grande do Sul State, southern Brazil, was studied. Estimates on age, length, and weight at attainment of sexual maturity are presented. Data on the reproductive seasouality and on the relationship between some testicular characteristics and age, size, and maturity status are provided. Sexual maturity was assessed by histological examination of the testes. Seasonality was determined by changes in relative and total testis weight, and in seminiferous tubule diameters. Testis weight, testicular index of maturity, and seminiferous tubule diameters were reliable indicators of sexual maturity, whereas testis length, age, length, and weight of the dolphin were not. Sexual maturity was estimated to be attained at 3.6 years (CI 95%=2.74.5) with the DeMaster method and 3.0 years with the logistic equation. Length and weight at attainment of sexual maturity were 128.2 cm (CI 95%=125.3-131.1 cm) and 26.4 kg (CI 95%=24.7-28.1 kg), respectively. It could not be verified that there was any seasonal change in the testis weight and in the seminiferous tubule diameters in mature males. It is suggested that at least some mature males may remain reproductively active throughout the year. The extremely low relative testis weight indicates that sperm competition does not occur in the species. On the other hand, the absence of secondary sexual characteristics, the reversed sexual size dimorphism, and the small number of scars from intrassexual combats in males reinforce the hypothesis that male combats for female reproductive access may be rare for franciscana. It is hypothesized that P. blainvillei form temporary pairs (one male copulating with only one female) during the reproductive period.

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The franciscana (Pontoporia blainvillei) is a small dolphin endemic to the coastal waters of the southwestern Atlantic Ocean. The distribution of this species ranges from Golfo Nuevo (42[degrees]35'S; 64[degrees]48'W), Chubut Province, Argentina (Crespo et al., 1998) to Itaunas (18[degrees]25'S; 30[degrees]42'W), Espirito Santo, southeastern Brazil (Moreira and Siciliano, 1991) (Fig. 1).

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The franciscana's coastal habitat makes it vulnerable to being caught as incidental catch in gill nets and trammel nets throughout most of the species range (e.g., Praderi et al., 1989; Corcuera et al., 1994; Secchi et al., 2003). Because of its vulnerability as bycatch, the franciscana has been considered the most impacted small cetacean in the southwestern Atlantic Ocean (Secchi et al., 2002). In the Rio Grande do Sul coast, southern Brazil, this species has been subject to an intense bycatch in gill nets for at least three decades (Moreno et al., 1997; Secchi et al., 1997; Ott, 1998; Ott et al., 2002). The annual mortality of franciscanas in this region was estimated to range from several hundred up to about a thousand individuals (Ottet al., 2002). Simulations studies on the effects of incidental captures on franciscanas in Rio Grande do Sul were carried out by using available data on vital rates, stock size, and bycatch estimates (e.g., Secchi, 1999; Kinas, 2002). All these studies showed that there is a decline in franciscana abundance in this region.

Although the reproductive biology of the female franciscanas have been studied in detail in Uruguay (Kasuya and Brownell, 1979; Harrison et al., 1981), Rio Grande do Sul (Danilewicz et al., 2000; Danilewicz, 2003), and Rio de Janeiro (Ramos, 1997), there are few data about male reproduction. Kasuya and Brownell (1979) presented information on male reproduction for Uruguay, although their small sample size precluded them from estimating age and size at attainment of sexual maturity.

In the Rio Grande do Sul coast, franciscanas are known to reproduce seasonally; births occur from October to early February (about 75% from October to December). Because the gestation period was estimated to last about 11.2 months, mating and conception may take place between November and early March (Danilewicz, 2003). Seasonal changes in testicular size and activity have been used to infer or corroborate mating seasons in some cetacean species (e.g., Neimanis et al., 2000). Nevertheless, it is not known if male franciscanas also undergo seasonal changes in the testicular activity.

In this study, we describe the reproductive biology of male franciscanas from Rio Grande do Sul and present evidences for the species' mating system.

Materials and methods

Sampling procedures

Data and samples collected from 121 specimens incidentally caught (88%) or beached (12%) along the Rio Grande do Sul coast between 1992 and 1998 were used for the analysis on reproduction of male franciscanas. The sampling of the incidentally caught animals was carried out through the monitoring of the commercial fishery fleet from Rio Grande (32[degrees]08'S; 52[degrees]05'W) and Tramandai/Imbe (29[degrees]58'S, 50[degrees]07'W). Stranded dolphins were sampled from systematic beach surveys conducted in an area with an extension of 270 km of sandy beaches, between Torres (29[degrees]19'S, 49[degrees]43'W) and Lagoa do Peixe (31[degrees]15'S, 50[degrees]54'W).