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

Shambhu Paudel,Usha Thakuri,John L Koprowski,Ajay Karki,Susana Caballero

doi:10.1371/journal.pone.0241099

Shambhu Paudel, Usha Thakuri + Show 3 more

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https://doi.org/10.1371/journal.pone.0241099

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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.

Highlights

As the human population size increases, freshwater species and ecosystems are increasingly threatened by many development activities, including habitat alteration, river diversions, fragmentation and flow regulation, expansion of agricultural and urban landscapes, and climate change [1]
Relationships between environmental flows and ecology of freshwater cetaceans changes to the dynamic riverine ecosystem
As river cetaceans exhibit a strong dependence on hydro-physical attributes, including geomorphic and seasonal flow variation effects, they are essential to informing the decision-making process of flow regulation management, including the identification and protection of river cetaceans’ priority habitats

Summary

Introduction

As the human population size increases, freshwater species and ecosystems are increasingly threatened by many development activities, including habitat alteration, river diversions, fragmentation and flow regulation, expansion of agricultural and urban landscapes, and climate change [1]. 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 freshwater species with isolation, reduction in abundance and range, and extinction, especially in the case of apex predators and highly migratory river dolphins [10,11,12,13]. Extinction of Yangtze River dolphins and a sharp decline in the populations of several river dolphins [Ganges River dolphin (Platanista gangetica gangetica), Indus River dolphin (Platanista gangetica minor), Irrawaddy dolphin (Orcaella brevirostris), Amazonian river dolphin (Inia geoffrensis), Bolivian river dolphin (Inia boliviensis), Tucuxi (Sotalia fluviatilis), Araguaian boto (Inia araguaiaensis)] are the example of consequences of inadequate flow in the Anthropocene [10, 11, 13, 16,17,18]

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