The impact of incidental kills by gill nets on the franciscana dolphin (pontoporia blainvillei) in southern Brazil

Abstract

The franciscana dolphin, Pontoporia blainvillei, is endemic to the western South Atlantic Ocean. Its distribution is restricted to waters up to 30 m depth, making it vulnerable to human influence. In southern Brazil the artisanal gillnet fisheries have increased since the early 1980s, and entanglement mortality of franciscanas has become a source for concern. Strandings have been documented for about 20 yrs, but the impact of incidental captures has remained unknown. In the 1990s numbers of franciscanas incidentally killed by the artisanal gillnet fleet, population size, and the intrinsic growth rate were estimated. I integrated all available information into a population model in order to quantify the impact of incidental kills. The statistical analysis was conducted within a Bayesian framework to maintain coherence while current biological knowledge was weighted with observational data. My analysis indicated a 99% probability that the population is decreasing. For analysis of future impact of the incidental kill the population was defined as being at ‘quasi collapse’ when it reached 10% of its current size, and the probability of quasi collapse within 30 yrs was calculated under alternative scenarios. Results indicate that current levels of entanglement mortality cannot be sustained and that protective measures are needed.