• Obtaining information on the historical distribution of a river dolphin is important for understanding declines, and can be accomplished through a variety of options including obtaining information from historical literature and photographs, specimens in museum collections, and from carefully designed surveys of local ecological knowledge.
  • Documenting and monitoring present distribution of a river dolphin species is one of the most important aspects of their status to understand. There are a number of options available to document presence/absence of dolphins from specific river reaches including visual surveys from boats or the river banks, information gathering through citizen science of local ecological knowledge interviews, e-DNA, or acoustic monitoring.  Where surveys cannot be conducted, habitat modelling may provide useful insights.
  • Exploring the causes of a documented decline in range can provide valuable information on the drivers of species decline which is vital for understanding which threats the highest priority are to be addressed by wildlife managers.


Why is understanding distribution and range important for conservation and management of river dolphins?

One of the main characteristics shown by species in decline is a reduction of the area they occupy, also known as their range. Range declines can be used to identify species of concern and monitor their conservation status over time. Declining range (in the form of ‘area of occupancy’ or ‘extent of occurrence’ is one of the criteria used by the IUCN Red List to classify species as threatened1. All species and populations of river dolphins are believed to have suffered declines in their distributional range to varying degrees.

Range declines need not be permanent, however. If habitat improves, it is also possible for populations to recolonise habitat that was previously abandoned and for the range to begin to expand as the population recovers. Exploring the causes of a documented decrease in range can provide valuable information on the drivers of species decline, which is vital for understanding what are the highest priority threats that need to be addressed by wildlife managers.

In order to be able to measure range declines, we need information on both:

  • Historical distribution; and
  • Present distribution.

Below we discuss options for gathering this information as well as successful case studies.


Historical Distribution

Reconstructing the historical distribution of river dolphins can be done using two primary methods:

  • Historical published literature can include references to river dolphins, including information on the location of encounters, behaviour of the animals and exploitation by humans.
  • Local ecological knowledge involves conducting interviews with people resident in the areas where the species of concern is thought to occur, or have occurred, to obtain information on their memory of past events and encounters.

In addition, supporting information can be found in documentation of skeletal material, museum collections and depictions of dolphins on historical artifacts.

Present distribution

Understanding the present distribution of a river dolphin species is of vital importance for conservation planning. It is also necessary in order to be able to compare it with the historical range, thereby enabling a decline in range to be calculated.  This requires confirming dolphin presence / absence from a specific river or river section.  It is important to note that confirming dolphin presence can be relatively easy where animals are abundant, but confirming their absence with confidence can be more challenging, as dolphins may persist in small numbers that are hard to detect and therefore absence may be difficult to confirm. Documenting dolphin presence / absence in a river can be conducted using any, or preferably a combination, of the following methods:

  • comprehensive field surveys (by boat or from the air) along an entire river stretch;
  • bank-based surveys on foot or by car;
  • short-range exploratory boat-based surveys at one or several locations along a river;
  • local ecological knowledge interviews with local people;
  • passive acoustics;
  • citizen science; or
  • e-DNA studies

There are some species with such vast ranges that conducting any kind of sufficiently comprehensive survey is just not possible.  In these instances it is useful to use statistical techniques and GIS tools to develop predictive habitat distribution models2. Having sufficient environmental data to be able to construct appropriate models is important and can be challenging in data deficient areas and it is necessary to ground truth and verify the predictions.

Calculating range decline

Determination of the decline in distributional range of a river dolphin species over a historical baseline is a simple calculation involving subtracting the number of kilometres of river currently regularly occupied by river dolphins, from the number of kilometres that were occupied historically. The number of kilometres can be measured using digital mapping programmes such as Google Earth, QGIS or ArcGIS.  A percentage decline can be determined.

Exploring the drivers or causes of range decline

A more detailed understanding of what conservation or management actions are required and will be most effective to prevent further declines in range can be obtained by conducting more sophisticated studies, which document not just the overall range decline, but also the timing of that decline in different places and can link that to specific causes or drivers.

Best practices / case studies

Range declines have been well documented for a number of river dolphin species and populations. These include:

  • The Yangtze River dolphin, or baiji, was declared extinct in 20073. Interviews with nearly 600 members of fishing communities across the middle–lower Yangtze channel and around Dongting Lake and Poyang Lake were used to determine when the dolphins had last been seen in different parts of their former range, and at what stage younger fishermen ceased to even recognise the species, indicating a ‘shifting baseline’ in the run up to the species’ extinction4, 5.
  • Researchers have used a combination of historical texts and studies6, 7, local ecological knowledge interviews and boat-based visual surveys to document the extreme decline in the range of the Indus River dolphins in Pakistan8, 9, 10. The presence of accurate historical distribution data and recent studies carefully documenting their present extent, followed by regular monitoring of the entire range, have provided a clear understanding of how the range declined historically and how it is continuing to decline – both of which provide important information for managing the population.
  • Boat based visual surveys of Yangtze River cetaceans have been conducted since the mid 1980s11 , so there are now approximately 30 years of survey data on the distribution of the Yangtze finless porpoise. The regular comprehensive surveys covering the entire range of the finless porpoise have allowed for changes and gaps in distribution to be identified. This information on the development of new gaps in distribution suggests the ongoing fragmentation and decline of the finless porpoise population in the Yangtze River12.
  • The ranges of the Amazon river dolphin and tucuxi in the Amazon and Orinoco river basins are too vast to allow detailed boat-based surveys of all the potential habitats used by these species.  However, development projects, such as the building of dams in Brazil, are proceeding rapidly, and a lack of data on dolphin distribution could lead to negative impacts on dolphin habitats that have not yet been accurately mapped. The South American River Dolphins Initiative (SARDI) is constructing species distribution models for both the Amazon river dolphin and the tucuxi to fill this important information gap.

For more information on current distributions and range declines of all six river dolphin species please look at our species pages and interactive map. For more detail on best practices for research and monitoring see the full report on Global Best Practices for River Dolphin Conservation.


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