Age and growth of franciscana dolphin, Pontoporia blainvillei, (Cetacea) incidentally caught off southern Brazil and northern Argentina.

BOTTA, SILVINA; MUELBERT, MÓNICA; SECCHI, EDUARDO R.; DANILEWICZ, DANIEL; NEGRI, MARÍA FERNANDA; CAPPOZZO, HUMBERTO LUIS; HOHN, ALETA A.

Cape Town, South Africa

Congreso; 17th Biennial Conference on the Biology of Marine Mammals.; 2007

Society for Marine Mammalogy

The franciscana, Pontoporia blainvillei, is possibly the cetacean most vulnerable to incidental catch in coastal gillnets in the western South Atlantic.  Four Franciscana Management Areas (FMA I-IV) have been proposed based on differences including genetics, morphometrics, and population parameters.  Evidence for splitting FMAIII and FMAIV is weak.  We compared age and growth of animals bycaught from 1998-2005 from FMAIII (Rio Grande do Sul (RS), Brazil) and FMAIV (Buenos Aires coast (BA), Argentina) as possible support for maintaining these two FMAs.  Data on body length (BL) and age from 179 individuals (104 males and 75 females) from Brazil and 26 specimens from Argentina (14 males and 12 females) were used for growth analysis. Ages were estimated by counting growth layer groups (GLGs) in dentine and cement (1GLG=1yr). Ages of individuals less than 1 yr were fractionated. The von Bertalanffy growth model was fit to length-at-age data, separating males and females because they are sexually dimorphic in body size. A large proportion of the specimens (83% of RS and 77% of BA) were less than three years old. A 17-year-old male from Brazil was found, representing the oldest franciscana male ever reported. No between-region differences were observed in asymptotic lengths of males (p=0.22, RS=130.65 cm and BA=135.82) or of females (p=0.83, RS=152.64 cm and BA=150.53 cm).  Females had a greater asymptotic body length than males (p<0.001).  Our findings, while preliminary due to small sample sizes, do not support splitting fransiscana in FMAIII and FMAIV into separate units. The present study reports the first attempt to model BA population’s growth and provides recent data on growth of franciscanas found in FMAIII, a stock considered “Vulnerable” according to IUCN’s Red List of Threatened Species based on levels of decline due to bycatch.