Morphometric relationships of franciscana dolphin, <i>Pontoporia blainvillei</i> (Cetacea), off Rio Grande do Sul coast, southern Brazil

Abstract

Due to the limited availability of cetaceans, body mass as a measurement of body size is often hard to estimate resulting in reduced sample sizes. Therefore, models for estimating body mass from an analysis of morphometric relationships are useful and practical tools to overcome such difficulties. In the present work, we propose specific models to estimate body mass from morphometric variables of franciscanas inhabiting Rio Grande do Sul, southern Brazil. A total of 226 franciscanas (130 males and 96 non-pregnant females) of all size classes, incidentally caught from 1994 to 2005 off Rio Grande do Sul coast (31°13'S - 33°45'S), were examined. Body mass (BM in kg) was regressed against total length (TL in cm), maximum girth (MG in cm) and a body-volume index: BV(cm³)=TL(cm) x (MG(cm))². Although linear regressions of body mass on both TL and MG were reliable predictors, a combination of those linear measurements to obtain a body volume index proved to be the best estimator of body mass. Models are the following: BM = 3.68*10-5 BV for males; BM = 3.64*10–5 BV for females and BM = 3.66*10–5 BV for sexes combined. Models presented were valuable methods for indirect body mass estimation for franciscana. Due to the similiarity in the body volume-body mass ratio in males and females, the mixed sexes combined model was deemed most appropriate for estimating body mass. This technique should be of great help in field situations where obtaining body mass is often difficult and sometimes impossible. Developing similar models for franciscanas from the other regions is recommended.