Fish diversity, anthropogenic stressors, Deepor Beel, Assam.

Wetlands are among the most productive ecosystems, providing a wide range of environmental services, including provisioning, regulating, cultural, and supporting functions, which offer numerous vital benefits to both humans and wildlife (Gokce, 2019; Sofia and Nurlianti 2019). Assam wetlands are some of the most biologically diverse and productive ecosystems in India, supporting various fish species, including endemic and economically valuable ones (Bassi et al., 2014). Deepor Beel, a Ramsar Wetland, is a perennial freshwater lake and channel of the River Brahmaputra located south of the river in Kamrup District, Assam. The actual extent of Deepor Beel is 4.1 km², with its depth varying seasonally from approximately 6 m to 1.5 m (Pandit, 2016). It supports a diverse population of fish fauna and is the only major stormwater storage basin for Guwahati (Rongpi, 2024). Deepor Beel plays a crucial role in maintaining regional biodiversity. It is home to a wide variety of aquatic species and supports 200 different species of birds, including 70 migratory species (Nikita et al., 2024). Additionally, the wetlands sustain numerous reptiles, amphibians, and plant species, contributing to its reputation as a biodiversity hotspot in Assam (Saikia, 2005).

Bhattacharjya et al. (2017) reported that fish landings in the closed wetlands of the Brahmaputra Valley, Assam, were predominantly comprised of Barbs (Puntius spp.), Rasboras (Danio spp., Rasbora spp., and Devario spp.), small catfishes (Mystus spp.), Murrels (Channa spp.), Notopterus notopterus, Wallago attu, among others. Species requiring flowing waters for spawning, such as major and minor carps (Cirrhinus spp., Labeo spp.), Bagarius bagarius, Pangasius pangasius, Chitala chitala, Ompok spp., and Aspidoparia spp., were notably absent in these wetlands. They further reported that, due to the lack of significant recruitment from rivers, these habitats are primarily inhabited by fish species that spawn in stagnant waters, typically small and economically significant fishes. The natural fishery of closed wetlands is dominated (50-90%) by small economic fish species such as Puntius spp., Rasbora spp., Trichogaster spp., Mystus spp., and Notopterus notopterus. Additionally, macrophyte-choked wetlands exhibited higher abundances of insectivorous and air-breathing fishes, including Channa spp., Anabas testudineus, Clarias magur, Heteropneustes fossilis, and Notopterus notopterus. According to the RIS (Information Sheet on Ramsar Wetlands) report, updated in August 2002, 50 fish species were found in Deepor Beel. Saikia (2005) reported 61 fish species in Deepor Beel, of which 5 species were exotic, 54 were indigenous, and 8 were endangered. They identified species such as Cirrhinus reba, Ompok bimaculatus, Ompok pabda, Botia dario, Nandus nandus, Megarasbora elanga, Channa barca, and Brachydanio rerio during their study of Deepor Beel. While a total of 46 species from 17 families viz., Notopteridae, Clupeidae, Anabantidae, Cyprinidae, Cobitidae, Bagridae, Siluridae, Schilbeidae, Clariidae, Heteropneustidae, Synbranchidae, Chandidae, Gobiidae, Belontiidae, Channidae, Mastacembelidae, and Tetraodontidae, have been recorded in Deepor Beel by another group of researchers (Acharjee et al., 2009). Medhi et al. (2019) found the fish diversity in two other wetlands within the Brahmaputra Basin of Assam, documenting 54 fish species across 38 genera, 8 orders, and 21 families in Chatla Beel, and 52 fish species spanning 37 genera, 8 orders, and 21 families in Urmal Beel. The order Cypriniformes was the most dominant in both wetlands, represented by 23 species in Chatla Beel and 22 species in Urmal Beel.

However, during the past few years, numerous studies have highlighted the threats wetlands face globally due to anthropogenic stressors (Nath and Deka 2012; Sultana et al., 2017; Kumar et al., 2018; Butt et al., 2021). Baruah and Goswami (2022) highlighted that the only Ramsar site of Assam, Deepor Beel is struggling to survive due to human interventions, unchecked pollution, and the violation of critical environmental regulations. The severe pollution of this vital water body has caused adversely impacts biodiversity, ecological balance, and threat to the local livelihoods of the communities. Rongpi (2024) reported that rapid urban development is putting the wetland and its ecosystem at risk of shrinking. Additionally, the establishment of the city's garbage disposal center in the Boragaon area has significantly polluted the waters of Deepor Beel and threatening the biodiversity of the wetland. The deterioration and overexploitation of wetlands reduce their ability to support life and provide essential services. The Ramsar Convention was established to combat wetland loss through global cooperation and promote sustainable development. Despite efforts to raise awareness, wetlands continue to face rapid decline due to overexploitation (Daryadel and Talaei 2014). Duarah and Das (2019) reported that fish diversity of the wetlands are under tremendous threats due to environmental degradation and its associated problems. Unsustainable land-use practices and development activities are placing tremendous pressure on Deepor Beel (Saikia, 2019). Fish production in the wetland is rapidly declining due to siltation, the dumping of solid waste, and the release of hazardous chemicals through inlet channels. These factors have considerably reduced the wetland's carrying capacity, causing many fishermen's families to lose their livelihoods. Bhattacharjya et al. (2021) also documented a decline in fish species due to threats faced by Deepor Beel and emphasized possible conservation strategies, such as regulating pollution, preventing/control of siltation, enacting and enforcing appropriate legislation, and other related conservation measures. 

Over time, anthropogenic activities have caused the area of the wetland to shrink. Deka et al. (2011) on the monitoring of spatial changes in Deepor Beel indicated that the total area of open water bodies diminished by 2.904 sq. km from 1991 to 2010. Mandal et al. (2024) reported an increase in built-up areas around Deepor Beel, from 9.07 to 32.19 sq. km between 2008 and 2018, with increasing built-up areas within a 500-meter buffer of the wetland. A study on the estimation of water pixel frequency from 1988 to 2019 revealed that the aerial extent of high, moderate, and low-frequency water presence areas were approximately 4.81, 3.48, and 2.76 km², respectively, from 1988 to 2000. From 2001 to 2018, the rapid expansion of the railway network further reduced the wetland area by about 58%, 33%, and 52% in high, moderate, and low water presence frequency zones, respectively (Das et al., 2024). The Beel is gradually converting from swamp to grassland. Its ability to absorb floodwaters has decreased, resulting in artificial waterlogging throughout the city. The wetland's self-purification capacity is also compromised due to pollution. Moreover, water quality in most areas of Deepor Beel was found to be in poor condition, a critical factor for fish diversity (Roy and Majumder 2022; Das et al., 2024; Sharma et al., 2024). The water quality has deteriorated due to the release of pollutants from several industries and waste disposal sites. Recent studies have revealed significant quantities of heavy and trace metals, which threaten the aquatic flora and fauna diversity, while lower dissolved oxygen levels in the Beel put aquatic life in a hypoxic state (Fatima et al., 2020; Sengupta and Deb 2022).

Bhattacharjya et al. (2017) emphasized that the scientific management of floodplain wetlands could significantly enhance fish production. These wetlands, often transitioning to marshlands, provide critical habitats for migratory and resident species, while supporting diverse aquatic flora and fauna, including plankton, macrophytes, benthic organisms, and various macrophyte-associated species. They are also vital for conserving threatened air-breathing and small-sized fish species. Fish act as an indicator species, regulating the distribution and abundance of other organisms within their ecosystems. They also serve as key indicators of water quality and overall ecosystem health (Moyle and Leidy 1992; Nath et al., 2015; Malakar and Boruah 2017; Mamun and An 2022). Studies on relationship of anthropogenic activities and the abundance of fish species can clarify the overall wetland health and its associated ecosystems. However, limited studies exist on the current fish diversity of Deepor Beel and the possible anthropogenic stresses that affect fish abundance. Therefore, the present study aims to investigate the current status of fish populations and abundance in Deepor Beel, along with an exploration of possible anthropogenic stresses that have impacted the fish diversity, utilizing a questionnaire survey based on the perceptions of the 'sons of the soil'.

Study area. The study was conducted in the Deepor Beel, Assam, India that located in lower Brahmaputra valley zone of Assam. Deepor Beel, is a large natural freshwater wetland situated southwest of Brahmaputra River near Guwahati City and is recognized as a Ramsar site. It extends between 26⁰03′26″ and 26⁰09′26″ N to 90⁰36′39″ 90⁰41′25″ E. The fish markets selected for fish sampling during the study are located near Deepor Beel. The market survey was carried out directly from local fish markets and from fishermen in the Deepor Beel fish market and Godhuli Bazar at Azara. 

Sampling methods. The fishermen employed various fishing gears, including gill nets (with different mesh sizes), drag nets (fry nets), and hook and line. Sampling at the fish markets in Godhuli Bazar and the Deepor Beel area took place in the morning between 7:00 AM and 9:30 AM from July-November, 2024.  Fish samples were identified based on their morphometric and meristic characteristics. Each species was categorized according to its respective IUCN Red List status (2024). The relative abundance of each fish species (% of the catch) was calculated using the method outlined by Sarkar et al. (2012), applying the following formula:

The frequency of occurrence of each species was calculated based on the number of occasions the species was collected during the samplings. The status was determined with the help of a standard catch frequency chart presented by Tamang et al. (2007) (Catch frequency: 91–100 % = Common, 81–90 % = Abundant, 61–80 % = Frequent, 31–59 % = Occasional, 15–30 % = Sporadic, 05–14 % = Rare, Less than 5% = extremely rare).

Shannon-Weinner index and species evenness was calculated by following May, (1993) and used following diversity indices:

Shannon-Weinner Index H = -{ni/N log2 ni/N}

Where  N = Total species and ni = Numbers of individual species

The species evenness ratio is the ratio of the observed species diversity (H’) to the maximum possible for the same number of species in the sample (log S). 

It is expressed as: J’=H’/log S. If J’=1, the biomasses of the individual species are evenly distributed among all of the species in the sample

A questionnaire survey was conducted to analyze the anthropogenic activities that influence the health of fish diversity in Deepor Beel. The sample questions of the questionnaire are furnished in Table 1. Random sampling was conducted and a total of 15 fishermen from each village were surveyed, making a total of 165 fishermen.

Table 1: Questionnaire format used during the survey at Deepor Beel area, Assam.

 Date:                                Location:                                           GPS coordinates: 

(I) General details:

Name of respondent:                                                              Contact number:

Gender (male/female)

Age (yrs):                        

Fishing experience (yrs):     

(II) Anthropogenic climate change perception/belief:

Do you feel any change in climate over the last 10-20 years: Yes/No/Can’t say

(III) What anthropogenic activities influences fish diversity

a) Developmental activities: most/moderate/low

b) Waste dumping site: most/moderate/low

c) Invasive species: most/moderate/low

(IV) Anthropogenic influences on fish diversity:

Abundance change: increase/moderate/decrease

Taste change: Yes/No/can’t say

(V) Details of fishes found: Indigenous/exotic

(VI) Livelihood of fishermen: fishing/ others/both

Any shift in livelihood (from fishery to others or vice versa)? Yes/No

If yes, reason for shift: 

Fish diversity. In the present study, a total of 107875 fish individuals belonging to 45 species, representing 11 orders (one order is not assigned) and 22 families, were recorded from the Deepor Beel wetland (Table 2, Fig. 1). Photographs of the fish species recorded during the present study are presented in Plate I. Literature surveys revealed that more than 60 fish species were present in the Deepor Beel (Saikia, 2005; Bhattacharjya et al., 2021). Present study included 42 native species and three exotic species (Fig. 2) viz., Piaractus brachypomus from the order Characiformes and family Serrasalmidae, and Cyprinus carpio, belonging to the order Cypriniformes and family Cyprinidae, and Oreochromis mossambicus under the order Cichliformes and family Cichlidae (Table 3). These exotic species are known for their rapid growth and ability to consume a wide variety of foods from various sources, including sometimes the juveniles of other fish species (Erarto and Getahun 2020). Some fish culturists have introduced these species to achieve quick economic returns. During floods, these species can escape from ponds and enter into Beels and rivers, posing a significant threat to native fish species and natural water bodies. The relative abundance (%) of fish species identified during the survey is detailed in Table 3. Among the fish species, Amblypharyngodon mola was found with more relative abundance (35.62%) and 10 fish species were found with lower relative abundances (0.0009%) in the Deepor Beel. Based on Tamang et al., (2007) catch frequency status, the survey categorized 12 fish species as extremely rare, 5 as rare, 5 as sporadic, 15 as occasional, 7 as frequent, and 1 as abundant. The study concluded that, although a moderate number of fish species were found, their catch frequency status raises significant concerns. Several species, such as Mystus cavasius, Rita rita, Sperata seenghala, Pachypterus atherinoides, Pangasius pangasius from the Siluriformes order, Xenentodon cancila from the Beloniformes order, Chitala chitala from the Osteoglossiformes order, Channa gachua from the Anabantiformes order, Lepidocephalichthys guntea from the Cypriniformes order, and Mastacembelus armatus from the Synbranchiformes order were observed at lower levels of relative abundance during the study period. However, Puntius terio species belonging to the family Cypriniformes were found abundant, and Mystus tengara, Heteropneustes fossilis from Siluriformes order, Gudusia chapra from Clupeiformes order, Channa punctata from Anabantiformes order Macrognathus pancalus from Synbranchiformes order and Notopterus notopterus from Osteoglossiformes order were found frequent during the study period in Deepor Beel (Table 3). Fishermen reported that species such as the Zig-zag eel, Corsula, Gangetic leaf fish, Flying barb, Mottled loach, Bengal loach, Pabo catfish, Freshwater garfish, and some of the Puntius species have not been found abundantly and have gradually declined in recent years. Malakar and Boruah (2017) reported that the uncontrolled harvesting of brood and juvenile fish, combined with the overexploitation of commercially important local species during the breeding season, is leading to a notable decline in fish diversity in several other wetlands of Assam, including Sol Beel, Etila Beel, and Duani Beel. In the current survey, the fish species Nandus nandus, Leiodon cutcutia, and Ompok pabda were notably absent. This absence could be attributed to a decline in their population or perhaps seasonal fluctuations, as the survey was conducted across only two seasons. Most of the native fish species obtained during the survey belonged to the Cypriniformes and Siluriformes orders (Fig. 1). The contribution of fish species by order and family is detailed in Table 1 and illustrated in Fig. 1. The present study found the Cyprinidae family to be most abundant among the recorded fish families (Table 2), aligning with the findings of  Nikita et al. (2024) who utilized an eDNA approach to study fish diversity in Deepor Beel. The IUCN Red List status of the fish species recorded in this survey is shown in Fig. 3. Most species were classified as Least Concern (LC), while one species, Wallago attu from Siluriformes order was found to be Vulnerable (VU), Clarias magur from Siluriformes order was found to be endangered and another species Chitala chitala from Osteoglossiformes order was found to be Near Threatened (NT).  

Table 2: Order and family-wise fish species contribution recorded during the present study from Deepor Beel, Assam, India.

                        Source: field survey (July-November, 2024)

The study identified one endangered, one vulnerable and one near-threatened fish species, and emphasizes the urgent need for conservation efforts to protect the wetland and its associated faunal diversity. Although most fish species were classified as being of least concern, their low relative abundance raises urgent concerns about the conservation of indigenous fish, which is essential for maintaining the health of the wetland ecosystem. Nayak and Biswas (2020) underscored that ecological restorations of the wetlands are very crucial to conserve the indigenous fishes of Assam. The present study showed Shannon diversity index and species evenness of fish species in monsoon and post-monsoon seasons (Table 4). The post-monsoon season showed higher Shannon diversity and species evenness in comparison to monsoon season during the survey period, which might be due to the favorable environment for their growth and movement that increases the fish diversity in the Deepor Beel. In the post-monsoon season, there may be favorable water conditions that allow fish populations to flourish, attracting both resident and migratory species from Brahmaputra river, which boosts overall fish diversity. In monsoon season, heavy rains bring an influx of nutrients into freshwater bodies like rivers, and lakes. This nutrient enrichment supports the growth of phytoplankton and other primary producers, creating a more productive environment that promotes food availability for fish species (Madhupratap et al., 2001; Sreekanth et al., 2019). In the post-monsoon season, the lower rate of surface runoff and evaporation support higher species diversity (Saha et al., 2021). Monsoon rains causes fluctuations in water levels, which can lead to changes in water parameters, including pH, TDS levels that may be stressful for some fish species or may temporarily reduce the diversity (Sreekanth et al., 2019; Saha et al., 2021).

Fig. 1. Order-wise family and species distribution of fish recorded during the study in Deepor Beel, Assam, India.

Fig. 2. Graphical representation of categories of fish species found during the survey in Deepor Beel, Assam, India.

Table 3: Details of the fish species recorded during present study in Deepor Beel, Assam, India.

* Exotic species; EN: Endangered; VU: Vulnerable; NT: Near Threatened; LC: Least Concern; DD: Data Deficient

Fig. 3. Graphical representation of the IUCN Red List status of fishes found during the survey in Deepor Beel, Assam, India.

Table 4: Diversity of fish species at Deepor Beel in the study period (July-November).

            Source: field survey (July-November, 2024)

Table 5: Social parameters of the fishermen in the study area at Deepor Beel, Assam, India.

                      Source: field survey (July-November, 2024)

Table 6: Percentage distributions of fishermen’ responses related to fish species diversity in Deepor Beel, Assam, India.

                                            Source: field survey (July-November, 2024)

Fig. 4. Percentage distributions of the respondents on studied anthropogenic activities recorded during the study period (July-November, 2024) in Deepor Beel, Assam, India.

Plate I: Fish species found during the survey in Deepor Beel, Assam. 1. Osteobrama cotio, 2. Labeo bata, 3. Salmostoma bacaila, 4. Anabas cobojius, 5.Trichogaster fasciata, 6. Trichogasterbejeus, 7. Labeo calbasu, 8. Labeo rohita, 9. Puntius sophore, 10. Puntiusterio, 11. Parambassis lala, 12. Chandanama, 13. Botia Dario, 14. Cirrhinusreba, 15. Amblypharyngodon mola, 16. Ophichthys cuchia, 17. Xenentodon cancila, 18. Macrognathus pancalus, 19. Mystus tengara, 20. Mystus cavasius, 21. Esomus danricus, 22. Gudusia chapra, 23. Heteropneustes fossilis, 24. Labeo gonius, 25. Sperata seenghala, 26. Cirrhinus mrigala, 27. Rita rita, 28. Channamarulius, 29. Channa punctata, 30. Labeocatla, 31. Pachypterus atherinoides, 32. Glossogobius giuris, 33. Lepidocephalichthys guntea, 34. Mastacembelus armatus, 35. Banganadero, 36. Pangasius pangasius, 37. Notopterus notopterus, 38. Chitala chitala, 39. Wallago attu, 40. Channa gachua, 41. Channa striata, 42. Clarias magur, 43. Piaractus brachypomus, 44. Cyprinus carpio, 45. Oreochromis mossambicus.

Future research should focus on comprehensive water quality parameter testing, to better understand how water degradation impacts fish health and biodiversity in Deepor Beel. Additionally, longitudinal studies on fish population dynamics and the effects of anthropogenic activities, such as pollution and habitat destruction, are crucial to identifying specific threats to the wetland's ecosystem. Exploring the effectiveness of conservation strategies, including habitat restoration and sustainable fishing practices, will be vital for ensuring the long-term sustainability of the fish populations and the socio-economic and cultural heritage of local communities. These efforts will provide a deeper understanding of the causes of habitat degradation and guide more effective management and restoration practices for Deepor Beel.

Bedabati Kalita, Arijit Chakraborty  and Ajanta Dey  (2025). A Study on the Present Status of Fish Population, Abundance, and Anthropogenic Stressors Affecting the Ichthyofaunal Diversity in Deepor Beel, Assam, India. Biological Forum, 17(2): 01-13.