Seasonal distribution of freshwater dolphins in Tipishca Del Samiria, Peru

Abstract

Tipishca del Samiria is an oxbow lake of the blackwater Samiria River system found within the Pacaya-Samiria National Reserve in northeastern Peru. The entire system experiences seasonal inundation and is influenced by the water level of the Marahon River, which joins the Ucayali River to form the Amazon. Botos (Inia geoffrensis) and tucuxis (Sotalia fluviatilis) were studied from September 1997 through August 1998 to determine the seasonal (water level) effects on their distribution and abundance in Tipishca del Samiria. Mean abundance of each species did not vary by water level, and a mean abundance of 25.7 ([] S.E. 1.82, n=78) bolos and 19.9 ([] S.E. 1.65, n=78) tucuxis was calculated for the lake during the study period. These abundance data were equal to or smaller than those obtained from other studies in this area. Overall, botos were found in groups of one (n=348) and two (n=195) with a mean group size of 1.9 ([] S.E. 0.05, n=684). Tucuxis were found in groups of one (n=131) and two (n=134) with a mean group size of 2.5 ([] S.E. 0.09, n=400). Distribution of bolos varied within the lake during high and medium-falling water, and the distribution of tucuxis varied during medium-rising and medium-falling water. A total of 49 botos and 46 tucuxis were photo-identified. Of those, 13 botos and 14 tucuxis were resighted on at least one occasion. Jolly-saber population estimates of 31.9 ([] S.E. 10.62) botos and 37.9 ([] S.E. 5.06) tucuxis were calculated for Tipishca del Samiria. Water quality measurements varied within the lake. The greatest mean conductivity and transparency occurred during medium-falling water, while the pH remained constant. All values, including particulate phosphorus, were greater than literature values for similar water types in Brazil. Forty-two species of fish were identified from casual observation of fish catches in Tipishca del Samiria. Seven of these species are know from the literature to have been found in stomach contents of botos and tucuxis necropsied further downriver in Brazil. Based on casual observations, it appears that humans and dolphins generally exploit different size classes and species of fish in this portion of the Reserve.