Habitat use and movements

Why is understanding habitat use and movements critical for river dolphin conservation?

Documenting a species’ distribution and range is the first step in understanding which parts of a river need to be studied and protected to conserve the species. However, to design truly effective conservation measures, it is important to have a finer scale understanding of the exact location and characteristics of the habitats that are most used by the dolphins for key life cycle functions, such as feeding, resting, mating, giving birth, or nursing young. With this understanding, conservation measures can be focused on protecting these core habitats and the corridors between them. The next level of refinement is understanding exactly what dolphins are doing in these preferred habitats, which is explored further in the section on species behaviour.

How do we study and document habitat use and movements?

There are two main methods:

1. Vessel surveys can be conducted to document dolphin distribution along a river, either through visual observations or acoustic detections. These surveys can be designed so that they can also be used to map distribution and calculate abundance. For all of these purposes, it is important to carefully document observation efforts, so that researchers can confidently say where they did NOT see/hear dolphins, as well as where they DID see/hear dolphins. The presence/absence data collected on these surveys can then be used to create habitat models. These models can be used to demonstrate which locations are most frequently used by the dolphins, and can be refined by looking for relationships between these preferred locations and various environmental characteristics, such as depth, river flow, river/channel width, and other geomorphological features. The more data that is available to ‘feed’ the model, the more reliable and robust the model will be. Ideally data will also be available for different seasons, as most river dolphin habitats experience ‘high’ and ‘low’ water seasons, which have been linked with significant changes in habitat use.
2. It is also possible to learn something about habitat use through monitoring the movements of individual dolphins over time. This can be done to a certain extent through photo-identification, especially where animals use a relatively small area and researchers can access them frequently for photo-identification studies. However, satellite tagging of individual dolphins provides finer scale data on an individual’s movements. Rather than offering a snapshot of an individual dolphin’s position at one moment in time, it provides data on the position of each tagged dolphin several times a day. The received data can provide information on how long dolphins spend in certain locations (with the assumption that areas where they spend more time are more important for critical life functions) and how far they move between different locations, providing important information on the corridors that need to be protected to allow movements from one part of a dolphin’s habitat to another.

• Presence/absence data and statistical habitat modelling were used effectively to document habitat use of Indus river dolphins during the low water season in Pakistan. The study determined that dolphin distribution was driven by features such as confluences and constrictions, as well as by broader-scale habitat complexes, providing important information on the possible negative impacts of plans to extract more water from the Indus system1. A variation of this method was used in the Sundarbans to study habitat preferences of Irrawaddy dolphins and Ganges river dolphins. In this study, researchers divided surveyed areas of rivers into 5km segments and compared observed rates of encounters with expected encounter rates to determine that the presence of Ganges river dolphins was associated with low salinity, high turbidity, and moderate depth during both low and high freshwater flow, while Irrawaddy dolphin distribution was associated with low salinity during high freshwater flow, as well as higher temperatures. Both Ganges river dolphins and Irrawaddy dolphins preferred channels with more than two small confluences or at least one large confluence, providing important information on the potential impacts of developments that would reduce these confluence areas, and the need to protect them^{2, 3}.
• Satellite tagging was successfully carried out with 35 individual dolphins in the Amazon between 2017 and 2019, revealing that:
  • The tagged dolphins had large home ranges that included different types of habitats, ranging from confluences to lagoons, river channels, tributaries, and even shallow areas near beaches where they mate;
  • Individual dolphin’s ranges included transboundary wetlands in Bolivia, Brazil, Colombia and Peru, reinforcing the need for regional/international collaboration to ensure their protection;
  • Dolphins’ seasonal movements followed fish migrations, highlighting the importance of maintaining the health and connectivity of rivers and their tributaries for the dolphins’ fish prey, as well as the dolphins themselves;
  • Dam construction, as well as illegal mining activities, occurring in the areas used by the tagged dolphins highlight the threats that these activities pose.

For more information on the South American satellite tagging study please refer to this infographic.