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

Abstract

Population decline and extinction risk of river dolphins are primarily associated with flow alteration. Previous studies predominantly highlighted maintenance of adequate flow for low water seasons when habitats contract and the risk of local extinction escalates. Although river dolphins are sensitive to reduction in river flow, no studies quantify the relationships between flow and ecology of river dolphins to mitigate the potential adverse impacts of flow alteration. We quantify the relationships between flow and the ecology of river cetaceans concerning Ganges River dolphins (GRD; Platanista gangetica gangetica) usable area availability (AWS) for the low water season at wider flows (50–575 m3/s) at finer spatial and temporal scales. This study reveals that distribution of area usable to GRD is highly regulated by the adequate flow and river attributes (velocity and depth) interactions that likely offer energetically efficient modes of locomotion to GRD, suggesting the hydro-physical environment as a major determinant of river dolphin distribution and abundance. Flow and AWS relationships indicate that the flow during the dry season negatively contributed to AWS, whereas that of pre-monsoon maximized the AWS, suggesting that modifying flow regimes does alter in-stream habitats at varying spatial scales and may influence life-history strategies. Substantial fragmentation in suitable pool availability and loss of longitudinal connectivity exhibited by dry season flow suggested a higher risk of adverse biological effects during the dry season, which may reduce population viability by reducing survivorship and reproduction failure. Owing to river dolphins’ dependence on the attribute of freshwater flow, they can be expected to be more affected by flow regulations as interactive effects. Considering the seasonal effects and changes in the availability of usable areas by flow alteration, adopting effective habitat retention plans by water-based development projects appears critical to avoid further ecological risks in aquatic species conservation. Identifying priority riverscapes for river cetaceans and prioritizing investment opportunities is an essential first step towards effective riverine cetacean conservation.

Identifying the seasonal characteristics of likely habitats for the Yangtze finless porpoise in Poyang Lake

“…The water level significantly varies between the two seasons (Hu et al, 2020), so the mean water depth in core habitats also varies significantly (4.38 m in the dry season and 12.60 m in the flood season). Similarly, the distribution of other freshwater cetaceans is highly sensitive to hydrological conditions (Paudel et al, 2021). The Irrawaddy dolphin in Bangladesh occurs in areas of deep water during the low-water season and in areas with the high turbidity during the high-water season (Smith et al, 2009).…”

“…For Poyang Lake, the adjustment of the protected area should aim at increasing habitat coverage in different seasons and ensure spatial habitat connectivity (Bao et al, 2019). This approach to hydrological and habitat modelling can also be applied to the present and historical habitat simulations for multiple aquatic species, so as to establish a more comprehensive protection network for ecosystem biodiversity and functionality (Li et al, 2021;Paudel et al, 2021). In addition, sound protection of freshwater cetaceans should focus not only on the distribution of these megafaunal habitats, but also the wider integrity of wetland ecosystems (Wang et al, 2021).…”

“…Species distribution model selection and uncertaintiesSpecies distribution models (SDMs) play an important role in biodiversity conservation. SDMs built with different types of species data including presence-background and presence-absence are widely used in cetacean species habitat modelling(Huang et al, 2019;Paudel et al, 2021). This study used MaxEnt, a presence-background SDM to model the likely habitat distribution of the Yangtze finless porpoise in Poyang Lake(Liu et al, 2022).…”

The effect of physiographic and hydrologic complexities and their alterations on the distribution of obligate freshwater dolphins

“…Meanders tend to have higher nutrient content and also a prevalence of eddy countercurrents, which can disorient fishes, thus facilitating predation by dolphins (Guizada & Aliaga‐Rossel, 2016 ; Mazumder et al, 2014 ). Deep pools are known to facilitate longer dives and hence yield the highest usable area for dolphins (Paudel et al, 2021 ). A combination of deep and shallow river segments also supports activities such as resting, feeding, and reproduction (Mosquera‐Guerra et al, 2020 ; Sinha & Kannan, 2014 ).…”