Incidental mortality of franciscana dolphin (Pontoporia blainvillei) in Argentina

Abstract

La Plata River dolphin or franciscana, Pontoporia blainvillei , is an endemic small cetacean of the Southwest Atlantic coast. It is threatened all along its distribution by a sustained high level of incidental mortality in fisheries. Here we assess levels of franciscana bycatch in Argentine waters between 1997 and 2003. We surveyed 18 localities along the coast of the Buenos Aires Province, between General Lavalle (35°06'S, 57°08'W) and Bahía Blanca (Puerto Rosales-Ingeniero White harbour: 38°47'S, 62°16'W). We recorded data on incidental mortality, fishing gears and fishing effort through 209 personal interviews with fishermen. We estimated annual mortality, fishing effort and catch per unit of effort (CPUE) for each locality and period of time. Mortality was caused by gillnets and trawling gears, purse seine nets and shrimper gears. The total mortality estimated for 1997-2000 was 354 dolphins/year (95% CI = 318-392) and 307 dolphins/year (95% CI = 273-343) for 2002-2003. In the entire survey, CPUE of the northern coast of Buenos Aires Province (Bahía Samborombón and Cabo San Antonio) was significantly higher than CPUE for the southern coast (from Mar del Plata to Bahía Blanca estuary). In addition, CPUE of the northern coast decreased significantly throughout the years. This study suggested that even though the gears or fleet behavior changed locally, Buenos Aires Province evidenced an overall mortality relatively constant during the survey. If we consider a minimum of 400 dolphins killed each year in fishing gear and the estimated population values of 15000 individuals for the Argentine coast; mortality represent more than 2% of the Argentine franciscana population, suggesting that it would be subject to decline. Trends in mortality need to be periodically monitored in this area in order to articulate programs of conservation for the species.