Factors affecting the persistence of endangered Ganges River dolphins (Platanista gangetica gangetica)

Abstract

The Ganges–Brahmaputra–Meghna and Karnaphuli (GBMK) River Basin in Nepal, India, and Bangladesh is among the world's most biodiverse river basins. However, human‐induced habitat modification processes threaten the ecological structure of this river basin. Among the GBMK’s diverse flora and fauna of this freshwater ecosystem, the endemic Ganges River dolphin (Platanista gangetica gangetica; GRD) is one of the most charismatic species in this freshwater ecosystem. Though a >50% population size reduction has occurred since 1957, researchers and decision‐makers often overlook the persistence (or evolutionary potential) of this species in the highly fragmented GBMK. We define the evolutionary potential as the ability of species/populations to adapt in a changing environment by maintaining their genetic diversity. Here, we review how evolutionary trap mechanisms affect the dynamics and viability of the GRD (hereafter Ganges dolphin) populations after rapid declines in their population size and distribution. We detected six potential trap mechanisms that might affect the Ganges dolphin populations discretely or in combination: (a) habitat modification; (b) occurrence of finite and geographically restricted local populations; (c) ratio of effective to estimate population size; (d) increasing risk of inbreeding depression in genetically isolated groups; (e) at‐risk behavioral attributes; and (f) direct fisheries–dolphin interactions. Because evolutionary traps appear most significant during low water season, they adversely affect demographic parameters, which reduce evolutionary potential. These traps have already caused local extirpation events; therefore, we recommend translocation among populations, including restoring and preserving essential habitats as immediate conservation strategies. Integrative evolutionary potential information based on demographic, genetic, and environmental data is still lacking. Thus, we identify gaps in the knowledge and suggest integrative approaches to understand the future of Ganges dolphins in South Asian waterways.

Assessing the impact of large barrages on habitat of the Ganga River dolphin

“…The barrage acts as a physical barrier in the longitudinal connectivity of the Ganga River, thus impeding the upstream or downstream navigation of dolphins and large migratory fishes (Smith & Reeves, 2000). This has lead to fragmentation and extirpation of the dolphin population (Behra, Singh, & Sagar, 2014; Paudel & Koprowski, 2020; Smith et al, 2000). Migration of dolphins from the Farakka barrage has been limited towards downstream when water is released in high flow season (Sinha, 1997).…”

“…Currently, in the Ganga Basin, they are found in the main stem of the Ganga River from Bijnor to Farakka barrage (Behra, Singh, & Sagar, 2014). Dolphins are also found in the main stem of the Yamuna River at its confluence with the Chambal and the Ganga River (Paudel & Koprowski, 2020). Since 1967, the population of dolphins has diminished in the Yamuna River from upstream of the Chambal confluence to the Hathni‐Kund barrage near Yamuna Nagar (Figure 1).…”

“…Decades of flow regulation and disruption of the natural flow in the Ganga and Brahmaputra Rivers have limited their extent and affected their population. According to a recent study by Paudel and Koprowski (2020), the overall distribution of the Ganga River dolphins has reduced by 18%. The middle (Haridwar to Varanasi) and the lower (Varanasi to Ganga Sagar) reaches of the Ganga River are one of the highly regulated reaches in the world.…”

“…In Sundarban mangroves of Bangladesh, the Ganga River dolphins show conditional dependability on low salinity, high turbidity, and moderate depth (Smith, Braulik, Strindberg, Mansur, & Ahmed, 2009). Habitat modification by a change in the river morphology and flow conditions can affect the habitat suitability, whereas flow regulation can limit the extent of dolphin distribution, and even extirpate their population (Paudel & Koprowski, 2020). However, there are limited studies that quantify the magnitude and the rate at which such changes occur in different rivers in the Ganga or Brahmaputra basins.…”

In-stream habitat availability for river dolphins in response to flow: use of ecological integrity to manage river flows

“…From 1970 to 2012, populations of vertebrate freshwater declined by 88% due to habitat degradation induced by flow regulation [8,9]. Several river basins in Southern Asia (Yangtze River, Ganges River, Indus River, Brahmaputra River) and Northern South America (Amazon, Orinoco, and Tocantins-Araguaia) have been impaired by large water extraction projects (i.e., hydropower and agriculture irrigation projects) with negative impacts on aquatic species conservation [1,10,11]. Such mega-projects threaten to freshwater species with extinction, isolation, and reduction in abundance and range, especially in the case of apex predators and highly migratory riverine cetaceans [12,13,10,11].…”