Feeding ecology of franciscana dolphin, Pontoporia blainvillei (Cetacea: Pontoporiidae), and oceanographic processes on the Southern Brazilian coast
The feeding regime of the franciscana dolphin was investigated from stomach contents of incidentally caught animals in gillnets along the southern Brazilian coast, from 1994 to 2000. The analyses were based mainly on Linear and Generalised Linear Models (LM and GLM). There were some diet differences within the population, as juveniles had eaten more small teleosts and shrimps, and adults consumed larger fish prey and more cephalopods. Adult females had similar diet to juveniles, possibly related to parental guidance. Feeding regimes from northern and southern areas also revealed significant differences, where the most important fish prey from the northern area only occurred once in the diet of the southern animals. More detailed analysis dividing the region into 7 smaller latitudinal zones confirmed many differences, with higher occurrence of warm water species in the diet of northern zones, and cold water species in the southern zones. Additionally, the prey frequency and size do not change gradually along the coast, and different zones revealed a diverse diet composition between northern and southern. This raised the question whether variations in environmental conditions along the coast may affect the distribution of franciscana prey, and consequently its feeding regime. To investigate the spatial and temporal patterns of environmental conditions off the study area, Sea Surface Temperature (SST) images from Advanced Very High Resolution Radiometer , and chlorophyll-a (chl a) images from Sea-viewing Wide Field-of-view were extracted. The analysis (MANOVA, PCA, and time series) revealed a marked temporal influence of warm temperatures from the Brazil Current (BC) in the northern zones, and cold temperatures related to Malvinas/Falkland Current (MC) in the southern zones. The central zones showed a dynamic-edge region in terms of temperature behaviour probably caused by the mixing of BC and MC. Northern zones showed low values of chl a concentration and southern zones high values, whereas central zones revealed intermediate values and less clear seasonal variation. The characteristics of the franciscana habitat had shown potential factors affecting the distribution and abundance of its prey, thus the prey species and size were treated as a function of the variables SST, chl a, latitude, depth, and season, using GLM and LM. Models suggested that habitat parameters influence the franciscana prey species and size. As expected, warm water species are more likely to occur in the diet for longer periods at lower latitudes, and cold water prey at higher latitudes. The SST effect on prey specimens size shows that bigger cephalopods and fish were most frequent in warmer temperatures. The chl a only influenced Engraulidae species. The latitude was the most important spatial predictor for many species, and bigger cephalopods were consumed in large quantities in central latitudes. The depth had great influence in the abundance of some prey species, with smaller fish and cephalopods being in higher numbers in shallower. The influence of the predictors on the franciscana prey is very similar to that found in the habitat by research cruises. Because some structures analysed may remain longer in the stomachs, the findings not only suggest a fairly opportunistic behaviour but it is likely that franciscana may occupy, possibly for long periods, small spatial ranges. This behaviour might be a strategy of minimizing energetic costs, ensuring low maintenance and thermoregulatory costs, and these include limiting excursions to distant areas.