Aquatic Mammals 2002, 28.3, 312–323 Distribution and abundance of the river dolphin ( Inia geoff rensis)in the Tijamuchi River, Beni, Bolivia Enzo Aliaga-Rossel Bolivian Collection of Fauna, Institute of Ecology, Universidad Mayor de San Andres, PO Box 3182, La Paz, Bolivia Abstract Key words: river dolphin, Inia geoff rensis,bufeo, boto, abundance, distribution, mortality, photo- Very few studies have been conducted in Bolivia identification, Tijamuchi river, Bolivia. regarding the distribution, behavior, or ecology of the Amazon river dolphin ( Inia geoff rensis). The Introduction only published studies of bufeo in Bolivia are from Pilleri (1969) and Pillieri &Gihr (1977) and were There are few studies that have been conducted in not quantitative investigations of river dolphin Bolivia regarding the distribution, behaviour, or abundance or distribution. The work presented here ecology of the boto or Amazon river dolphin consists of an estimate of abundance and descrip- ‘bufeo’, Inia geoff rensis.The only published work tion of the seasonal distribution of the bufeo in the consists of that of Pilleri (1969) and Pilleri &Gihr Tijamuchi River, Beni, Bolivia. The study was (1977). This work was descriptive in nature and conducted during January 1998–September 1999 lacked quantitative analysis of river dolphin abun- and represents four hydro-climatic seasons (i.e., dance. Despite this, Pilleri &Gihr (1977) concluded low, high, rising, and falling waters). Two hundred that their survey of river dolphin populations in and twelve hwere spent in survey e ff ort. The total various rivers and lagoons of Bolivia revealed a study area was approximately 185 km along the visible reduction in river dolphin population size in river. Strip transects were used to survey for dol- some regions, which they attributed to changes in phins. Dolphin distributions among three habitats the ecosystem or to anthropogenic pressure. Based were compared; these habitats were riverine- on this, they recommended that immediate action blackwaters, riverine-mixed waters (black and be taken to establish nature reserves to actively white), and oxbow lakes of the river system. Group protect the Bolivian bufeo. One additional study of size and age structure were recorded. Any dead river dolphins in Bolivia was conducted by Yan˜ez dolphins were necropsied. On average, 208 bufeos (1999) and consisted of adescription of the behav- were observed in the Tijamuchi River, with an ior and general ecology of the bufeo in the Iten˜ez average encounter rate of 1.12 dolphins/linear km. River and the Paragua River of the Noel Kempff Dolphins were seen most frequently during low and Mercado National Park. falling water (56% of total observations) and least Laws that explicitly protect the bufeo do not often during high waters (22% of total observa- currently exist in Bolivia. Some protection is tions). These seasonal diff erences were statistically a ff orded by the Veda General Indefinida significant. Dolphins were seen most often in oxbow (D.S. 25458), enacted in July of 1999, which is a lakes, and next often in confluences. Diff erences in general decree that prohibits the harassment, dolphin abundance according to water colour were capture, harvest, and training of wild animals and not statistically significant. There was some evi- their derivatives. Debate exits over the e ff ectiveness dence of associations between group size and sea- of this law, but it continues to be in e ff ect in the son, and group size and water colour. Forty-two absence of more stringent or specific laws. per cent of observations were of solitary dolphins, Throughout South America, Inia geoff rensis is 32% were of pairs, and maximum group size was 19. classified as ‘vulnerable’ by the IUCN. Calves were seen most often during falling and low Uncertainty exists with respect to the taxonomy waters. Causes of mortality of dolphins in the study of Inia geoff rensis.The genus Inia is con- area were identified as entanglement in fishing sidered monospecific with three subspecies: Inia g. nets, intra-specific aggression, and collisions with humboldtiana found in the Orinoco Basin, I. g. outboard motors. geoff rensis in the Amazon Basin, and I. g. boliviensis 2002 EAAM Distribution and abundance of the river dolphin 313 found in the rivers of the Bolivian Amazon (Best & The study area is located from where the San-Borja- Da Silva, 1989a ;DaSilva, 1993). The Bolivian Trinidad Road crosses the Tijamuchi River to bufeo may be isolated from dolphin populations in where the Tijamuchi River empties into the the rest of the Amazon Basin due to geographic Mamoré River (Fig. 1), and is approximately barriers, such as the waterfalls that are found in the 185 linear km. Three oxbow lakes adjoining the north of Bolivia. These falls extend more than Tijamuchi were also surveyed. 200 km between Guayanamerin and Porto Velho in The area surrounding the Tijamuchi River Brazil. These geographic barriers may have led to belongs to the life zone of the subtropical humid the separation of the I.g. humboldtiania and I. g. forest. Gallery forests border the river, and are geoff rensis on one side, and I.g. boliviensis on the interspersed with savannas, some of which are other. In addition, this theory is supported by the natural and others are aresult of deforestation from fact that I. g. boliviensis diff ers from the other cattle ranches. The mean temperature in the Llanos subspecies of Inia in some morphological character- de Mojos is 26.5% Cand the annual precipitation istics, such as number of teeth, body size, and skull varies between 1200 mm and 2400 mm per year. size (Anderson, 1997; Emmons &Feer, 1997). The months of November, December, January, and Based on these morphological diff erences and the February are always rainy, with maximum rainfall geologic barriers formed during the late Pliocene, of 100–500 mm per month. The hydrologic regime which supposedly isolated the Bolivian bufeo, some is tightly linked to precipitation, and displays a taxonomists proposed the classification of two unimodal curve, with the highest water levels occur- species, Inia geoff rensis and Inia boliviensis. This ring between December and April, and the lowest distinction of anew species has been disputed based water levels from June to October (Loubens et al., on the fact that the characteristics used to diff eren- 1992; Guyot, 1993; Hanagarth, 1993). tiate the species are highly variable and also the The Tijamuchi River is born in the Llanos de small sample size. Currently, the systematics of Inia Moxos, and is classified as blackwater from its have been revised with molecular studies, which headwaters to its middle reaches. Blackwaters are suggest that the genus of Inia found in Bolivia is a characteristically low in nutrients and suspended distinct species, Inia boliviensis (Banguera et al., sediments, and are stained dark from the tannic 2000). The recognition of anew river dolphin acids of decaying vegetation. As the Tijamuchi species endemic to Bolivia increases the importance River flows, it receives input from small channels and necessity of protecting this species (Klinowska, and rivers that originate in whitewaters (white- 1991). waters are turbid waters of Andean origins that are The current conservation status of the bufeo in high in nutrients and suspended sediments). The the Amazon Basin of Bolivia is poorly known, and Tijamuchi River is eventually transformed into baseline population information is lacking. The mixed black-and-whitewaters, until it empties into work presented here estimates the abundance the whitewater Mamoré River. The width of the and distribution of the bufeo in asection of the Tijamuchi River is between 35 and 200 m. The Tijamuchi River of the Bolivian Amazon, taking annual water temperature varies between 25 and into account three ecosystem types (i.e., black- 27% C, dissolved oxygen is 20–23% and depth varies waters, mixed white and blackwaters, and oxbow from 0to5m(Loubens et al., 1992). lakes of the river system), four habitat types During the rainy season, the lagoons that are low (i.e., river channel, confluences, bends/curves/and for part of the year are converted into enormous lagoons) and four hydro-climatic seasons (i.e., high, bodies of water. Lagoons are places of high fish low, rising and falling waters). This work con- abundance, as nutrients that were incorporated in tributes to the knowledge of the bufeo in Bolivia, the soil are released into the water column, allowing and provides important baseline information for the accelerated development of phyto- and that can be used to aid in their conservation, the zooplankton (Lara, 1996). The diversity of wildlife creation of management plans, and the initiation of (including fish) in the Mamoré River system is high. active protection of the bufeo in this country. More than 320 species of fish have been reported in Standardized methods were used in the present the region (Lara, 1996), of which humans consume study, and these can be replicated and used for at least 40 species, and many have commercial value future investigation in Bolivia and elsewhere. in the ornamental market. The principal human activity in the region is cattle ranching. Materials and Methods Fieldwork Fieldwork took place between January 1998 and Study area September 1999. Four surveys were conducted The study was conducted on the Tijamuchi River, during diff erent hydro-climatic seasons: low Moxos province, Department of Beni, Bolivia. water (between August and September); rising 314 E. Aliaga-Rossel Figure 1. Map of the study area, Tijamuchi River, Department of Beni, Bolivia. water (the end of October and December); high conducted. Approximately 185 km of river were water (January); and falling water (May and surveyed during each season. the beginning of June). In each season an Because the Tijamuchi River has an average upriver/downriver transect of approx.