River dolphins and flooded forest: seasonal habitat use and sexual segregation of botos (Inia geoffrensis) in an extreme cetacean environment

Abstract

Habitat use by the boto, or Amazon river dolphin Inia geoffrensis, was investigated in and around the Mamirauá Reserve, Brazil. Largely forested with numerous channels and lakes, Mamirauá comprises a variety of seasonal floodplain habitats known collectively as várzea. The annual cycle of flooding in this region (amplitude 11-15 m) dominates all life. Profound seasonal differences in dolphin density between habitats were consistent with known fish movements, in turn dictated by changes in water level and dissolved oxygen. An exodus of botos from floodplain to river at low water prevents dolphins being trapped in areas that become entirely dry. Densities of botos in floodplain channels were seasonally higher (up to 18 km − 2 ) than reported for any cetacean worldwide. Adults were largely segregated by sex except at low water. Females and calves dominated in chavascal habitatthe areas most remote from rivers, which were preferred by males. Probable causes of this segregation are the energetic requirements of calves and the safety of females and/or calves from male harassment. Some 80% of botos occurring on rivers were within 150 m of the margins. The reliance of adult females and calves on várzea in a region with exceptional dolphin densities demonstrates the importance of floodplain habitats for the boto, and may be the key determinant of this species' distribution.

The Endangered Ganges River dolphin Platanista gangetica gangetica in Nepal: abundance, habitat and conservation threats

“…This result is most likely a consequence of dolphins (1) travelling into tributaries and being temporarily unavailable for sampling, (2) hiding amongst debris, or (3) failing to surface within sight of the observation crew. Similar behavior has been shown to occur in the botos of the Amazon, where sexual segregation occurs during seasonal floods (McGuire & Winemiller 1998, Martin & da Silva 2004. Our methodology provides a standardized protocol, and should be applied to future surveys monitoring the status of the Critically Endangered Nepalese dolphin population.…”

Abundance estimate of the Amazon River dolphin (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) in southern Ucayali, Peru

“…Our model results indicated that more Sotalia were seen at earlier hours of the day and during the dry season. Our results are similar to those obtained from previous work in areas where dolphin densities were observed as highest when river levels were at their lowest (AliagaRossel, 2002;Martin & Da Silva, 2004b;Kelkar et al, 2010). This variation in dolphin density can be attributed to seasonal movements, as individuals that prefer to inhabit tributaries have to move to areas that do not dry out (Martin & Da Silva, 2004b).…”

“…Bolivia and Peru had the highest overall densities, and confluences and lakes were highlighted in terms of habitat preferences (Gómez-Salazar et al, 2012). These areas have previously been reported as abundance hot spots for both species (Martin et al, 2004;Martin & da Silva, 2004b;Mcguire & Aliaga-Rossel, 2014).…”

“…When the lagoon is at a lower level, the lagoon shrinks, resources are concentrated and individuals are easier to detect. Comparatively, when the lagoon is at a higher level Inia and Sotalia have shown preference for flooded forests, and tributaries that reduces their detectability (Martin & da Silva 2004b). In terms of habitat preference there is evidence that shows both species present higher densities within 100 m to the shore (Martin & Da Silva, 2004a;Gómez-Salazar et al, 2012).…”

“…As riverine species, these dolphins are greatly affected by human disturbance (Turvey et al, 2007;Kelkar et al, 2010), yet both are still listed as Data Deficient by the IUCN (Reeves et al, 2011;Secchi, 2012) highlighting one of the key research gaps in cetacean conservation (Parsons et al, 2015). Line and strip transects and mark-recapture techniques have been used to estimate population sizes for both species in certain areas of their distribution but methods have not been standardized, making total estimates unreliable and hard to compare (Gonzalez, 1994;Martin & da Silva, 2004b;Aliaga-Rossel et al, 2006;Gómez-Salazar et al, 2012a, 2012bMcguire & AliagaRossel, 2014) The most extensive survey to date covered the main rivers in six countries resulting in significant insights into habitat preferences for both species (Gómez-Salazar et al, 2012). Bolivia and Peru had the highest overall densities, and confluences and lakes were highlighted in terms of habitat preferences (Gómez-Salazar et al, 2012).…”