Abundance estimation of Franciscana dolphins Pontoporia blainvillei in Buenos Aires Province, Argentina, from aerial surveys,

ALBERTO CRESPO, ENRIQUE; NOEMÍ PEDRAZA, SUSANA; FLORENCIA GRANDI, MARÍA; DANS, SILVANA LAURA; VALERIA GARAFFO, GRISELDA

Sorrento

Congreso; 56° Annual Meeting Of The International Whaling Commission; 2004

The franciscana dolphin, Pontoporia blainvillei, is considered the most threatened small cetacean in the Southwestern Atlantic and population estimates have been highly recommended. This paper presents the second abundance estimate, the first in Argentine waters. On February and May 2003 and February 2004 we carried out 17 aerial surveys using transect line sampling methodology at Buenos Aires and Rio Negro Provinces coast. One hundred one  (101) franciscanas were observed in 71 sightings with an average of 1.43 individuals per group. For the northern area, the autumn series of flights was carried out in better meteorological conditions leading to what is considered a good estimation of franciscanas density: 0.10598 ind/Km2 (95% CI: 0.00222 to 0.07038). The mean density between strata (0.08560 ind/Km2) could be considered a lower bound for the northern area (95% CI: 0.045385 to 0.16145). The southern area was surveyed with two solely flights and no replicates and the estimation of density was lower with respect to the northern areas (0.055375; 95% CI: 0.01808 to 0.16960). A correction factor for submerged dolphins was applied to density estimation leading to obtain a corrected density value of 0.2960 / 0.23910 ind/ Km2 (best / mean) in the north and 0.15466 ind/Km2 in the south. Density was extrapolated to the area between the coast and the 50m isobath as the offshore border (162,192.59 Km2) and the abundance for the Argentine coast was estimated in 40230 / 34131 individuals. Considering previous studies that have reported incidental mortality estimates for this region, the abundance estimate suggests that around 2% of the stock that may be removed each year by the fishery. However, mortality rates seem to vary from year to year. More refinement is needed on abundance and as well as on incidental mortality rates estimates in order to design the best management options. Financiado por CMS, FVSA y CONICET