Ecology of the river dolphin, Inia geoffrensis, in the Cinaruco River, Venezuela

Abstract

The Cinaruco River is a tributary of the Orinoco River, and forms the southern boundary of Venezuela's newest national park, Santos Luzardo. Like other rivers of this region, the Cinaruco River undergoes an extreme seasonal flood cycle. River dolphins were studied from November 1993 to June 1994 to gather baseline data regarding their ecology, including distribution, habitat affinity, group size, population composition, and prey availability. Transects from watercraft were used to count river dolphins, and observation effort was recorded. River dolphin habitat was characterized by broad hydrologic features (i.e., river, lagoon, side channel or confluence), structural composition (i.e., beach, rocks, cutbank, submerged vegetation) and complexity. Population composition by age class was determined according to size. Photo-identification was used to recognize individual dolphins. Potential preyfish availability within habitats was determined by using seine nets and gillnets. Dolphin diet was inferred from one stomach sample and the literature. Dolphins were seen most often during falling water, and least often during rising water. Dolphins were most often seen in confluence areas, and were seldom seen in side channels. The presence of rocks or beaches was associated with a greater frequency of dolphin sightings, and sightings increased with habitat complexity. Average group size for the 8-mo study was 2.0 (O︢.05) dolphins, and was greatest during rising water. Greatest densities occurred during failing water, at 3.8 (+O.54) dolphins/km. Calves were first sighted at the end of the dry season and became more common as the waters rose. Six individuals were recognized and resighted by photo-identification; one of these was sighted 8 times over a period of 186 days. The fish diversity of the study area was high, and comprised at least 161 species. The stomach of one Inia contained 15 fish, representing at least 4 species.