Habitat selection of freshwater‐dependent cetaceans and the potential effects of declining freshwater flows and sea‐level rise in waterways of the Sundarbans mangrove forest, Bangladesh

Abstract

ABSTRACT1. Generalized additive models of sighting data for cetaceans collected during two surveys of waterways in the Sundarbans mangrove forest of Bangladesh indicated that Ganges River dolphin Platanista gangetica gangetica distribution was conditionally dependent (P50.05) on low salinity, high turbidity, and moderate depth during both low and high freshwater flow; and Irrawaddy dolphin Orcaella brevirostris distribution was conditionally dependent (P50.05) on low salinity during high freshwater flow, high and moderate depths during low and high freshwater flow, respectively; low and high-low extremes of turbidity during low and high freshwater flow, respectively; and high temperature and increasing numbers of large-small channel confluences during low freshwater flow.2. According to sighting data collected over a 3-year period by the captains of three nature tourism vessels, there were significant differences between the actual and expected frequencies of Ganges River dolphin sightings and individuals according to various channel types (chi-square=64.22, P50.0001 and chi-square=134.14, P50.0001, respectively, df=6) and of Irrawaddy dolphin sightings and individuals (chi-square=15.28, P=0.0182, and chi-square=29.42, P50.0001, respectively, df=6), with shared preferences for wide sinuous channels with at least two small confluences or one large confluence.3. The dependency exhibited by both species for environmental characteristics associated with abundant freshwater flow, including low salinity and the availability of confluences, make them particularly vulnerable to habitat loss due to upstream water abstraction and sea-level rise.4. Although the results of this study may not affect plans for construction in India of large-scale, inter-basin water transfer projects that will result in further declines in freshwater flows, or decisions within the international community about CO 2 emissions affecting global sea levels, they can be used to prioritize locations where protective measures could be employed to benefit the long-term conservation of both species.

Widespread infilling of tidal channels and navigable waterways in the human-modified tidal deltaplain of southwest Bangladesh

“…), as relatively few roads and bridges span the embanked polder islands. Thus, the siltation and closure of >600 km of major conduit channels reported here may have significant impacts on regional commercial and human transportation (Alam, 1996: Rahman et al, 2013, and even stability of local cetacean populations (e.g., Smith et al, 2009). One example of the compound effects of the channel closures comes from the Mongla-Ghasiakhali tidal channel (MGC; Figure 7), where infilling of this east-to-west navigation route since the 1980s forced ship traffic to be rerouted through protected channels of the Sundarbans National Forest, a UNESCO World Heritage Site Sharafat, 2015, 2016).…”

“…Under natural conditions, the tidal deltaplain of southwest Bangladesh consists of a dense network of interconnected channels and mangrove islands inhabited by ecologically important and endangered fauna, including the Royal Bengal tiger and Ganges and Irrawaddy river dolphins (Smith et al, 2009;Ortolano et al, 2016). In this landscape, major tidal channels (1-2 km wide) convey semi-diurnal tides >120 km inland of the coast, delivering sedimentladen water to secondary channels and the primary creeks that normally flood and drain the mangrove-vegetated intertidal platforms (Figure 1; Allison and Kepple, 2001;Rogers et al, 2013).…”

Clicking in Shallow Rivers: Short-Range Echolocation of Irrawaddy and Ganges River Dolphins in a Shallow, Acoustically Complex Habitat

“…Ganges river dolphins ( Platanista gangetica gangetica ) are obligate freshwater dolphins found in the Ganges, Brahmaputra and Karnaphuli river systems where they exhibit a peculiar, side-swimming form of locomotion [32]. The extent of their downstream range in the Sundarbans is also determined by seasonally dynamic freshwater flows [30], with the Ganges river dolphin favouring low salinity, high turbidity and moderate depth [33]. Both Irrawaddy dolphins (Fig.…”

“…1C) have relatively small bodies comparable to small marine delphinids and porpoises [9]. In the Sundarbans, they inhabit geomorphically complex areas with extremely variable depth, salinity and turbidity in contrast to the more stable characteristics of marine environments [33]. Given the complex acoustic environment and high amount of clutter and reverberation, it may be hypothesized that Irrawaddy dolphins and Ganges river dolphins employ echolocation signals characterized by low-amplitude, high frequency sonar signals emitted at high repetition rates like small bat species hunting in cluttered habitats [34].…”

Impacts of climate change on coastal ecosystems of Bangladesh

“…Especially during the dry season saltwater intrusion has increased in the Sundarbans over the last decades (Mirza 1998;Smith et al 2009;Huq et al 1996). This is mainly due to a combination of sea level rise and reduced freshwater flow from upstream rivers.…”