Globally, approximately 37,189 fish species have been documented, of which 18,948 inhabit freshwater ecosystems (Fricke et al., 2025). In India, an estimated 2,743 fish species are found, of which around 1,072 live in freshwater habitats (Froese and Pauly, 2025). Degradation of riverine ecology and changes in aquatic ecosystems made many fish species vulnerable and threatened (Alam et al., 2013). The Western Ghats of India, recognised as one of the world’s biodiversity hotspots (Myers et al., 2000), are characterised by exceptionally high endemism, encompassing 320 fish species belonging to 11 orders, 35 families and 112 genera (Dahanukar and Raghavan, 2013). Biodiversity in freshwater ecosystems is declining faster than in terrestrial or marine systems, raising serious concern (Postel and Richter, 2012). Recent assessments indicate that nearly 24% of freshwater species are at a high risk of extinction, primarily due to pollution, dam construction, water abstraction, agricultural intensification and the presence of alien species (Sayer et al., 2025). Anthropogenic pressures further aggravate biodiversity loss, decline in fish populations and other declines in aquatic biota within riverine ecosystems (Angeler et al., 2014). The Western Ghats ecosystem is considerably fragmented and faces numerous threats, viz., mining, industrial agriculture, persistent deforestation, poaching, human-wildlife conflict, artificial obstacles (such as dams and check dams), rapid urbanisation, overpopulation and tourism (Durairaja et al., 2022).
       
A large number of endemic species necessitates in-depth research into both biotic and abiotic factors, which may influence species assemblages, community structure and diversity in lotic environments (Minns, 1989). Variability in abiotic factors such as flow, depth, substrate and water quality, as well as various physicochemical parameters, can considerably alter both the fish assemblage structure and resource availability in freshwater ecosystems (Johnson et al., 2012). Ulhas River, a west-flowing river originating from the northern valleys of the Rajmachi hills in the Raigad district of Maharashtra, drains the northern slopes of the Western Ghats, covers an area of approximately 4,390 km² river basin, extending across the districts of Thane, Raigad and Pune, with the river flowing for nearly 122 km before discharging into the Arabian Sea near Thane and Vasai Creek (Doke, 2019).
       
Several studies have documented ichthyofaunal diversity in river systems of the Western Ghats. Jadhav et al. (2011) recorded 58 fish species from the Koyna River at Koynanagar and Patan. Mohite and Samant (2013) studied the fauna of the Warna River and recorded 42 species.  Similarly, Wagh and Ghate (2002) studied the Mula and Mutha River, which recorded 62 fish species. Katwate et al. (2012) conducted an extensive study along the Raigad district, recording 66 fish species across six rivers. Kumbar et al., (2021) studied the Krishna River in Sangli District, Maharashtra and recorded 73 fish species. Furthermore, limited studies have been carried out, focusing on the estuarine section of the Ulhas River (Rathod and Patil, 2017; Lal et al., 2020). The ichthyofaunal diversity of the Ulhas River has not been comprehensively studied to date; in this context, we provide a comprehensive checklist of the fish fauna, encompassing its freshwater stretch in the Western Ghats, Maharashtra.

Seasonal fish surveys and diversity assessment were conducted in the freshwater stretch of Ulhas River (excluding tidal influence zone) from June 2024 to November 2025, covering the monsoon, post-monsoon and pre-monsoon seasons (Fig 1). Fish samples were collected fortnightly from five sampling sites along the river continuum: Kondhewadi (S1), Tamnath (S2), Karjat (S3), Neral (S4) and Apati weir (Ambernath) (S5). Site selection was based on local fisher perception and consideration of habitat variability.


       
Experimental fishing was conducted using various gears, including cast nets and gill nets of different mesh sizes (10, 20, 30 and 50 mm), to capture fish of different species and size classes for a soaking period of 6-8 hours. Loaches and small-sized Puntius species were captured using traditional methods, viz., gamcha and mosquito nets. The scooping method, specifically targeting these species, is carried out by identifying fish-congregated areas for approximately 2 hours. In addition to experimental sampling, fish markets (Karjat, Neral, Vangani, Ambernath and Badlapur) associated with specific river stretches were surveyed to supplement species occurrence records and account for locally exploited taxa.
       
Freshly collected fish specimens were preliminarily identified, photographed using a digital camera and preserved in 10% formalin for detailed laboratory-based taxonomic analysis at ICAR- Central Institute of Fisheries Education, Mumbai. Length and weight of the specimens were measured using a digital vernier calliper (SKADIOO) and an electronic weighing balance (Aczet CY 513c), respectively. Species-level identification was achieved using standard taxonomic keys and online published literature, including Talwar and Jhingran (1991), Vishwanath et al., (2009) and Jayaram (2010). Latest nomenclature validity was confirmed using the Catalogue of Fishes (Fricke et al., 2025) and vernacular names from local fishermen.
       
Conservation status of each species was assessed following the IUCN Red List of Threatened Species (2025). Trophic guild classification for each species was assigned based on information available in FishBase, facilitating functional interpretation of the fish assemblage. Relative abundance of each fish species was calculated following Kumbar et al., (2021) and categorised into five classes: Very Rare (VR), Rare (R), Few (F), Common (C) and Very Common (VC). Habitat characteristics for each site were recorded based on the habitat’s parameters, including substrate composition (%), such as bedrock, boulders, cobbles, gravel, coarse sand and fine sand, following standard river habitat assessment protocols (Arunachalam, 2000). These parameters were used to understand the relationships between habitat and fish assemblage within the river system.

Fish diversity and conservation status
 
A total of 59 fish species, belonging to 11 orders, 21 families and 42 genera, were recorded from the freshwater stretch of the Ulhas River (Table 1; Fig 2a-c). Dominant fish order recorded was Cypriniformes (47%), with 28 species, followed by Siluriformes (17%), with 10 species, Anabantiformes (9%), with five species and Cichliformes (7%), with four species, respectively (Fig 3). The family Cyprinidae (19) was recorded as dominant, followed by Danionidae (7), Bagridae (4) and Chichlidae (4). Katwate et al., (2012) recorded 66 fish species across six rivers in Raigad district, with the highest richness in the Kundalika River (66 species), followed by the Savitri (64 species), Amba (61 species) and the Patalganga (45 species). Wanjari et al., (2025) documented 47 fish species representing 32 genera, 17 families and 10 orders in the Wainganga River, noting that Cypriniformes was the most prevalent order. Similarly, Mudoi et al. (2022) recorded 49 fish species across 36 genera, 20 families and 10 orders from the Umtrew River in Meghalaya and Assam, India. Their findings reported Cypriniformes (42%) as the dominant order, followed by Siluriformes (24%). Likewise, studies by Kumbar et al., (2021) and Eldho and Sajeevan (2022) reported Cypriniformes, Siluriformes and Anabantiformes as the dominant taxonomic orders in the Krishna River (Sangli district) and the Chimmony Wildlife Sanctuary in the Western Ghats, respectively. These river systems in the Western Ghats have shown comparable levels of richness, strengthening the region’s global reputation as a biodiversity hotspot. However, differences in species composition among rivers indicate that local habitat structure, flow regimes and anthropogenic pressures all have a significant impact.

       
Based on the IUCN Red List of Threatened Species, 81% (48 species) were categorised as Least Concern. Vulnerable species constituted 7% (4 species) Pethia lutea, Wallago attu, Cyprinus carpio and Oreochromis mossambicus, while 5% (3 species) Tor tor, Puntius amphibius and Megalops cyprinoides were listed as Data Deficient. Another 5% (3 species), Mystus malabaricus, Ompok bimaculatus and Pseudambassis lala were classified as Near Threatened and 2% (1 species), Pangasianodon hypophthalmus was categorised as Endangered. Local fish species face a multi-faceted threat from destructive fishing methods, growth and recruitment overfishing, establishment of invasive species, compromising the ecological integrity of the river system (Sarkar et al., 2012). A total of seven exotic fish species were recorded from the Ulhas River, namely Amphilophus trimaculatus, Oreochromis mossambicus, Oreochromis niloticus, Cyprinus carpio, Clarias gariepinus, Pterygoplichthys pardalis and Pangasianodon hypophthalmus. The invasion and establishment of exotic species pose a significant risk to indigenous fishes by disturbing natural habitats and threatening native populations through predation, competition and competitive displacement (Raghavan et al., 2008; Knight, 2010; Sarkar et al., 2010; Kumbar et al., 2021).
 
Relative abundance and community structure
 
Overall fish relative abundance analysis revealed Garra mullya (27.39%) as the most abundant fish species, followed by Systomus sarana (9.98%), Puntius amphibius (6.81%), Puntius sophore (6.33%), Mystus cavasius (6.12%), Dawkinsia filamentosa (5.69%) and Rasbora daniconius (5.53%). The dominance of these species suggests favourable habitat conditions, including suitable substrate composition, microhabitat availability and optimal physicochemical conditions for survival and reproduction in the Ulhas River. In the low-order streams of the Pamba River, Thomas and Thomas (2025) observed the dominance of Garra mullya (42.63%), attributing its prevalence to the abundance of riffle habitats characterised by fast-flowing, oxygen-rich conditions that favour rheophilic species.
       
Lowest fish relative abundance was recorded for Pseudambassis lala, Labeo dussumieri, Megalops cyprinoides, Sperata lamarrii and Tor khudree, with values of 0.01% for each species. Notably, the abundance of Indian Major Carps (IMC) was relatively lower compared to catfish species, suggesting that habitat modification and altered flow regimes may favour benthic and opportunistic taxa. Kumari et al., (2025) highlighted variability in fish abundance due to differences in microhabitats and the availability of suitable substrates at sampling sites within the riverine ecosystem.
       
Trophic-level community analysis revealed dominance by omnivorous species (53%), followed by carnivores (38%), herbivores (7%) and insectivores (2%). The low dominance of herbivorous fishes indicates limited autotrophic productivity and a shift toward heterotrophic pathways, such as detritivory and predation, collectively signalling a deterioration in ecosystem health (Noble et al., 2007; Heda et al., 2009; Sajina et al., 2022). The predominance of omnivores demonstrated their ability to flourish by consuming wide range of food resources were available. Consequently, this increased percentage indicated a degraded ecosystem (Sajina et al., 2022).
 
Spatial patterns and habitat associations
 
Habitat characteristics of the Ulhas River were recorded, revealing considerable variability, including microhabitat features such as substrate composition (%). Overall result showed bedrock as the dominant substrate present in all sites, with a range of 25-55%, followed by coarse sand (20-25%), fine sand (10-20%), cobble (5-15%), gravel (5-15%) and boulders (3-5%). Downstream sites (S3, S4 and S5) exhibited greater substrate heterogeneity and more mixed habitat characteristics than upstream sites (S1 and S2). Consequently, higher species richness was recorded at S5 compared to the other sites, owing to greater habitat variability, optimal flow, greater water depth and suitable substrate. The freshwater stretch of the Ulhas River exhibits significant anthropogenic alterations, with sampling site S1 showing habitat degradation due to sand mining, site S3 impacted by domestic wastewater discharge from rural drains and site S4 serving as a solid waste disposal area near the left bank, driven by rapid rural development.

The present work is the first comprehensive study of fish diversity, abundance and habitat characteristics from the freshwater zone of the Ulhas River. Checklists of fish species recorded are categorised as near threatened, endangered and vulnerable, calling for the use of conservation aquaculture. The absence of existing data creates a significant and necessary scope for stock assessment, biology and trophic guild studies. Conserving fish diversity in the Ulhas River is vital for maintaining ecological balance and sustaining local livelihoods.

The authors are grateful to the Director of ICAR-CIFE for providing the required facilities to conduct the current investigation. This work was carried out as part of the PhD Thesis research work of the first author at ICAR-Central Institute of Fisheries Education, Mumbai. 

There is no conflict of interest between authors.