Giant otter Pteronura brasiliensis density and abundance in Llanos Orientales de Colombia in the Orinoco basin

Abstract The giant otter Pteronura brasiliensis, categorized as Endangered on the IUCN Red List, was once widely distributed throughout South America. By the middle of the 20th century the giant otter had become locally extinct along the main rivers of the Orinoco basin. Although some populations seem to have recovered, the paucity of information available does not permit a full evaluation of the species' conservation status. The objective of this study was to estimate the abundance and density of the giant otter population along the Orinoco river in the municipality of Puerto Carreño, Vichada, Colombia, where there is important commercial and recreational fishing. Thirty-nine linear km were surveyed, repeatedly, with a total of 315 km of surveys. Population size was estimated by direct counts of individuals. All individuals detected were photographed and identified individually from their throat pelage patterns. In total, 30 otters were identified, giving a minimum density of 0.77 individuals per km, one of the highest reported for the species in Colombia. Given the high density in this well-developed area, our results highlight the importance of this population for the conservation of the species.

T he giant otter Pteronura brasiliensis, categorized as Endangered on the IUCN Red List (Groenendijk et al., ), was once widely distributed throughout South America from northern Argentina to Colombia and Venezuela (Carter & Rosas, ). However, because of the commercial value of its fur, overhunting in the s and s led to the collapse of its populations and extinction over a large part of its original range (Carter & Rosas, ). In the s in Llanos Orientales de Colombia (Orinoco Basin) the species was present only in the remote upper reaches of the rivers (Medem, ). More recent studies have shown that the species is now present throughout the area (Carrasquilla, ; Díaz, ). However, the methods used in these studies did not provide information that permits an evaluation of the species' conservation status. Our objective therefore was to estimate the abundance and density of the giant otter population along the Orinoco river, and its tributaries and lagoons, in the municipality of Puerto Carreño, in Vichada, Colombia.
This study was carried out along the Orinoco river in the municipality of Puerto Carreño in the north-east of the Department of Vichada (Fig. ). This department lies at altitudes of - m, has a mean annual temperature of °C, and the mean total annual precipitation is , mm, with dry (December-March) and rainy (April-November) seasons (IGAC, ). The study area includes the Bojonawi private nature reserve, and lies in the El Tuparro Biosphere Reserve. Important commercial and recreational fishing activities occur in this area. FIG Giant otters are territorial, diurnal, live in groups and have irregular pale patterning on their throats that allows individuals to be identified (Duplaix, ), facilitating estimation of population size by direct counts of groups and individuals (Groenendijk et al., ). Given the logistical impossibility of sampling continuously, the area was divided into six sectors, which were sampled independently ( Table ). Depending on accessibility, each sector was sampled using either a -m motorized aluminium boat with a -hp engine or a canoe, or on foot along the banks of streams where access by boat was impossible. Surveys were conducted by three experienced observers during .-.. When we detected an individual or group of otters, we followed them until the pale throat patterns of each individual could be photographed and the size of the group established. We recorded group sizes and locations using a GPS, which we also used to calculate the length of each transect. The surveys were carried out during  January- March  during the dry season, when the otters are easiest to detect (Groenendijk et al., ).
The study area comprised a total of . linear km, including the main river, its tributaries and lagoons. Each sector was sampled repeatedly, giving a total survey effort of . linear km ( Table ). Six groups of giant otters and two solitary individuals were detected. The groups consisted of - individuals, with a mean of . ± SD . individuals per group. In total,  otters were identified, yielding a minimum density of . individuals per km.
Our results show that the giant otter density along the river Orinoco and its tributaries and lagoons in the Puerto Carreño area is among the highest reported for the species in Colombia (Table ) Duplaix (1980) with few people. Otters occur in lower densities in areas where they are killed by fishers, or prey is scarce because of poor river productivity or overfishing (Trujillo et al., ). The Orinoco is a highly productive river with high fish densities (Lasso et al., ) and consequently supports healthy giant otter populations. Nevertheless, this abundance of fish in an accessible area close to a population centre also encourages fishing. During our study, we identified at least eight fishers' camps on the rivers Orinoco and Caño Negro, and the city of Puerto Carreño is host to c. , recreational fishers every fishing season (December-March). Given that killing by fishers is one of the main threats to this otter throughout much of its range (Rosas, ; Recharte et al., ; Trujillo et al., ), we would expect this to occur in a popular fishing area. However, a study carried out amongst fishers in Puerto Carreño in  indicated that % believed that otters were not harmful to fishing activity (Garrote, ).
Nevertheless, there is pressure on fish stocks: both the number of fish caught and their mean size have fallen significantly since  (Trujillo et al., ). This could potentially lead to a decrease in the carrying capacity for the giant otter, and to increased competition between otters and fishers, jeopardizing the positive perception that fishers have of this species (Lavigne, ). Further threats to the species in the Puerto Carreño area include the illegal trade of skins and the capture of cubs as pets (Cruz Antía et al., ; Trujillo et al., ).
Given the high density of the giant otter in this welldeveloped area, our results highlight the importance of this population for the conservation of the species. We will continue to monitor this population, seeking to improve our understanding of the dynamics driving the relative high density and the fishers' favourable perception of the species.