Abstract

West Bengal is a state in India well known for its topographical and biological diversity. The southern part of West Bengal has many rivers and rivulets, most of which are tributaries or distributaries of the Bhagirathi-Hooghly River. Rivers in Southern Bengal are now facing multiple problems, like water abstraction for irrigation, encroachment of the river bed, construction of dams and barrages, indiscriminate deforestation in the catchment areas, etc. Apart from indiscriminate fishing, increased proliferation of exotics, global climate change, fragmentation of habitat, and alterations in river hydrology are the major threats to fishes in riverine stretches of Southern Bengal. With the decreasing natural stocks and diversity of riverine fishes, the fisherfolk, depending on river productivity, are now facing severe problems in sustaining their livelihood. A rational management program for the rivers of southern Bengal should be put into place to protect fish diversity and increase fish production. The present review aims to provide a comprehensive picture of ichthyofaunal diversity, distribution and potential threats in riverine systems of Southern Bengal based on published literature in the form of research papers, books, monographs, and species inventories. This paper emphasizes the need for a regular exhaustive survey of ichthyofauna present in different rivers of Southern Bengal to create a comparable database that might be used for future research, planning, and management.

1. Introduction

Rivers are dynamic, three-dimensional systems that transport and process multifarious materials exported by terrestrial ecosystems, being dependent on longitudinal, lateral, and vertical transfer of material, energy, and biota ^{1, 2}. Freshwaters, including rivers, are among the most threatened ecosystem in the world. Anthropogenic activities like extensive urbanization, fast-paced industrial growth, weir construction, intensive agricultural practices have been regarded as key players in changing the natural condition of a riverine ecosystem ^{3, 4}.

India is well-known for its intricate riverine network. The country is bestowed with several large and small river basins. The Ganga-Brahmaputra basin is one of the most notable, as it forms the world's largest delta region. West Bengal is a state in India that is known for its topographical and biological diversity. West Bengal, also known as the "Land of Many Rivers," covers 88,752 km² and is home to more than 90 million people ^{5, 6}. West Bengal is divided into two parts by the Ganges: North Bengal and South Bengal. The state is divided into 23 districts, eight of which are located in North Bengal, and the remaining 15 districts are located in the state's southern hub. West Bengal is the only state in India that stretches from the Himalayas in the north to the Bay of Bengal in the south, giving it a unique geographical position among Indian states. Since it has a drainage network of 22 basins, the state has a wide range of topographical diversity ⁷. The southern part of the state has many rivers and rivulets, most of which are tributaries or distributaries of the Bhagirathi-Hooghly, the westernmost distributary channel of the Ganges. The Bhagirathi-Hooghly River receives water from several tributaries such as the Ajay, Mayurakhshi, Rupnarayan, Haldi, Jalangi, Churni, Damodar, and others ⁸. According to Rudra (2016), the state's eastern half is known as "Deltic Bengal," since it comprises several deltas due to a complex network of rivers, whereas the western half is known as "Rarh Bengal" ⁹.

Fishes are the most diverse vertebrates, with about 34000 species ¹⁰. Fishes are valuable for a variety of purposes, such as food, ornamentation, sport, and medicine ¹¹. With a revised estimate of having 2667 species, India is one of the top 25 countries in the world in terms of endemism ¹². It also ranks third in Asia in terms of total freshwater fish diversity ¹³. Fishes are essential part of any freshwater ecosystem, and fish assemblages can be used to monitor riverine ecological health. In Asian countries, including India, there is a serious lack of detailed knowledge about diversity and distribution of freshwater fishes in small and medium-sized stream ecosystems ¹⁴. Most of the current literature on fish diversity in India focuses on major rivers, with little information on diversity and distribution trends in smaller streams ^{15, 16}. Freshwater habitats have been under considerable stress in recent decades compared to terrestrial ecosystems ¹⁷, and fishes are becoming more widely recognized as important indicators of ecosystem health ¹⁸.

The knowledge of fish diversity is woefully inadequate and disorganized, particularly of the tributaries and distributaries of the Bhagirathi-Hooghly. The lack of systematic data on ichthyofauna of Southern Bengal is a significant impediment to explore the potential threats on fish community and for strategizing some conservation measures. The present review attempts to portray a comprehensive picture of ichthyofaunal diversity, distribution, and potential threats in the major fluvial systems of Southern Bengal, India, based on published literature in the form of research papers, books, technical bulletins, monographs, and species inventories.

2. Drainage Systems in Southern Bengal

Just six of its twenty-two drainage basins fall entirely within West Bengal's political boundary due to its elongated orientation and the transverse alignment of most rivers. The state receives primarily outside run-offs ^{7, 19}. Figure 1 shows the drainage network of Southern Bengal, West Bengal, India.

The river Ganga splits into two streams after the Farakka barrage (24º48' N and 87º55' E) – the Bhagirathi (Hooghly) in the west and the Padma in the east. The Bhagirathi (Hooghly) River flows 520 kilometres through Southern Bengal before reaching the Bay of Bengal ²⁰. The river Bhagirathi originates at Mithipur village in Murshidabad district of West Bengal, about 40 kilometres south of the Farakka barrage. After Nabadwip, the river Bhagirathi is mostly tidal for 280 kilometres. This tidal stretch is also known as "River Hooghly" ^{9, 21}.

Eight significant rivers drain the western part of South Bengal. These rivers originate from the Chotanagpur plateau in the west and flow towards the South-East to join the Bhagirathi-Hooghly River (except the Subarnarekha River). These rivers are known as rain-fed rivers; they remain almost dry in the summer and are often flooded during the rainy season. The Damodar River is a major right-hand tributary of the Bhagirathi–Hooghly River ²². River Rupnarayan is one of the important tributaries of the Bhagirathi-Hooghly ²³. The Shilabati and Darakswar Rivers meet near Ghatal, West Bengal, and the combined flow is known as the Rupnarayan River ²⁴. The Kangsabati River comes from the plateau of Chotanagpur and passes through Purulia, West and East Midnapore Districts of West Bengal. River Keleghai then joins the Kangsabati. This combined flow is known as the Haldi river. The River Mayurakshi and Ajay rises from the Chotanagpur plateau and reaches West Bengal in the district of Birbhum, and eventually empties into the Bhagirathi-Hooghly River. The Subarnarekha River originates in Jharkhand near Ranchi and passes through West Bengal's West Midnapore district before entering Odisha. It ultimately meets with the Bay of Bengal.

The Bhairab-Jalangi and the Mathabhanga-Churni are two rivers that flow into the Bhagirathi-Hooghly from the east. During the non-monsoon months, neither of these receives any water from the Bhagirathi-Hooghly. The Ichhamati, the other major river of upper deltaic region, branches off from the Churni and follows the India-Bangladesh border all the way to the sea. During the lean season, the river's upper reaches become dried up, receiving water only during the monsoons. Its lower stretch, on the other hand, is maintained tidally.

The Sundarban is a large estuary with a complex network of creeks and intervening mangrove islands. It is maintained by the flow of enormous tidal prisms into its interior areas twice daily. The rivers in this area are the distributaries of the Hooghly River, which are fed by the tides. Matla, Gosaba, Raimangal, Saptamukhi, Thakuran, Hariabhanga are some of the major rivers in this region.

3. Ichthyofaunal Diversity and Distribution

Table 1 presents the ichthyofaunal diversity of major riverine systems of Southern Bengal, India.

Sarkar and Banerjee (2000 & 2010) identified 79 species from the main channel of Damodar River ^{25, 26}. Sandhya et al. (2019) reported 62 species belonging to 20 families in Panchet Reservoir on the Damodar River. The Ambassidae family was the most prevalent, followed by the Clupeidae. In terms of the number of species, the Cyprinidae family was the most diverse ²⁷. Gudusia chapra was found to be the most abundant species in both the studies of Sandhya et al. (2019) and Sarkar and Banerjee (2010). Saha and Patra (2013) documented 46 species from three sampling stations along the river Damodar in Burdwan District over the period of a year, representing 7 orders, 18 families, and 26 genera. Among the collected species, most of them fall under Order Cypriniformes ²⁸. Dey et al. (2013) reported 36 fish species belonging to 14 families from the Durgapur barrage and adjacent areas in a study on the river Damodar ²⁹.

Kar et al. (2016) found 45 species in the Kangsabati River in Paschim Medinipur District, West Bengal, India, belonging to 8 orders, 17 families, and 29 genera. The Order Cypriniformes was the dominant Order ³⁰. Bera et al. (2014) identified 39 species from Kangsabati Reservior, representing 7 orders and 15 families ³¹. Kangsabati Reservoir was home to 47 freshwater fish species, according to Mukherjee and Praharaj (2009) ³².

Roy et al. (2013) recorded 48 fish species from various stretches of the Silabati and Darakeswar rivers ³³.

Karmakar et al. (2008) reported 66 fish species representing 42 genera, 21 families, and 7 orders, and Ghosh et al. (2011) listed 140 fish species belonging to 55 families and 18 orders from the estuarine portion of the Subarnarekha River (Latitude 21º 34' N- 21º 37' N and Longitude 87º 20' E- 87º 27' E). The Clupieds- Gudsusia chapra and Goniolosa manmina, which are pretty common in the Hooghly estuary, were not encountered by Ghosh et al. (2011) in Subarnarekha River. The dominant Order in their report was Perciformes ^{34, 35}.

Pahari et al. (2017) recorded 55 freshwater fish species from the Keleghai River, representing 9 orders and 21 families. Cyprinidae was the dominant family. Nandus meni was recorded first time in India from this river. Jana et al. (2015) listed 20 species from Keleghai River. Pahari et al. (2017) found Cypriniformes as the most common Order in contrary Jana et al. (2015) found Perciformes to be the most dominant Order. Puntius sophore and Amblypharyongodon mola were eudominant in Keleghai River ^{36, 37}.

Ghorai et al. (2015) revealed the presence of 27 fish species belonging to 20 genera 18 families, and 9 orders in Rupnarayan River ³⁸. Again, Ghorai in 2018, reported 38 ichthyospecies from the same river in Kolaghat, Purba Medinipur district, West Bengal, India, belonging to 29 genera, 24 families, and 10 orders ³⁹. The Order Siluriformes was the dominant Order here in terms of percentage composition. However, the dominant family was Cyprinidae, based on the sequence of the dominance of the encountered families. The most common species was Amblypharyngodon mola ³⁹. A total of 36 fish species belonging to 26 genera, 24 families, and 8 orders were recorded in Rupnarayan River by Bera and Mishra (2021) ⁴⁰.

The river Churni was home to 16 fish species, according to Das and Chakraborty (2007), though there were 44 species in Churni River in 1983 ⁴¹. In the Churni River, Sarkar and Islam (2020) found 44 species ⁴². Bakshi et al. (2016) reported 38 species belonging to 7 orders and 17 families from the river Churni ⁴³. Das and Chakraborty (2007) reported 44 species from the Jalangi River ⁴¹. The diversity and distribution of fishes in the river Jamuna, a tributary of the Ichamati River, was studied by Sarkar (2018), and the results revealed the presence of 46 fish species representing 18 families and 36 genera in this river ⁴⁴.

A total of 64 species belonging to 53 genera, 38 families, and 11 orders were identified from the Matla River of Sundarban estuarine system by Mukherjee et al. (2013) ⁴⁵. Here most widespread family was Sciaenidae. The Matla River shares a significant part of the sundarban's estuarine portion (several kilometers wide in the lower reaches) with an extensive permanent interface with the Bay of Bengal in the mouth region. Because of its more or less permanent opening to the sea, the Matla River has diverse fish species. Open estuaries usually have a higher diversity of species ⁴⁶. Except for some freshwater species like Pangasius pangasius, Gudusia chapra, Anguilla bengalensis euryhaline teleost species dominate the Matla River.

The Indian Sundarban has quite a good number of fish species (165 species) ⁴⁷. In commercial catches, over 120 species have been reported by Seidensticker and Hai (1983) ⁴⁸. The salinity gradients in various parts of the Sundarban are directly related to fish diversity. In areas with high salinity, fish like Harpodon nehereus, Trichiurus savala, Pampus sp., Setipinna sp., Sardinella sp., and Salar sp. live. In contrast, Pangasius pangasius and Lates calcarifer live in freshwater or areas with very low salinity. Tenualosa ilisha, Polynemus sp., Pomadasys hasta, and Coilia sp. are the most common fishes in brackish water zones (with a moderate salinity level). Mangroves are also home to a variety of marine fish ⁴⁹. Dubey et al. (2015) found 62 indigenous fishes from the freshwater areas of Sundarban Biosphere Reserve, representing 45 genera, 28 families, and 9 orders. The order Perciformes has the largest number of species ⁵⁰. Table 2 presents the excerpts of some other studies conducted in Sundarban region.

Hora (1943) conducted an extensive study from 1936 to1942, documenting 110 fish species from the river Bhagirathi-Hooghly ⁵⁵. However, subsequent studies by David in 1954 and Ghosh in 2008 have shown a gradual decline in the total number of fish species in the Bhagirathi-Hooghly River ^{56, 57}. Rosith et al. (2013) reported 155 species belonging to 49 families and 15 orders from the tidal stretch of Hooghly estuary. Cyprinidae was found to be the most speciose family with 34 species in their study ⁵⁸. In comparison to the study conducted before Farakka Barrage construction, such as David ⁵⁶, Gopalkrishnan ⁵⁹, and Menon et al. ⁶⁰; the study of Rosith et al. (2013) revealed that the number of freshwater species had gone up from 55 to 95 whether the number of marine species has dropped to 12 from 27. Sen (1992) also listed 172 fish species belonging to 38 families and 92 genera from the state of West Bengal of which 30 genera comprising 64 species are riverine ⁶¹. Das et al. (2013) recorded 93 species belonging to 36 families from the stretch of Tribeni to Gangasagar of this River ⁶². A report by Nath and Patra (2015) revealed 87 fish species from Hooghly River belonging to 69 genera, 39 families, and12 orders. Order Perciformes was the dominant order. Their study reported Glyptothorax telchitta in this river, which is generally a resident of North riverine habitat; Pterygoplichthys sp (Crocodile fish), an ornamental fish, was also documented. Chagunius chagunio, Secutor ruconius, Rita gogra originally belong to different habitats but found in non-residential places in the Hooghly River system ⁶³. In addition, Nath and Patra found that 60 fish species listed by Hora ⁵⁵ were completely absent from their investigation ⁶³.

4. Potential Threats to Riverine Fish Assemblage in Southern Bengal

Anthropogenic activities such as stream modification, weir construction, riverbank and bed encroachment, and riverbed sand mining promote habitat degradation, which is thought to be the major driver of fish biodiversity loss ^{64, 65}. The current grave situation of some of the Indian rivers is due to dams resulting in hydrological modification, obstacles to fish migration, changes in salinity, and changes in the sediment regime. The rerouting of water through dams has significantly impacted the downstream ecosystem and fisheries. This is also applicable to South Bengal’s rivers, like the river Damodar (Panchet Reservoir, Durgapur barrage), Mayurakhshi (Tilpara Barrage), Kangshabati (Kangsabati Reservoir), and the Ganga (Farrakka barrage). After Farakka barrage construction, there was increased freshwater discharge in the Bhagirathi-Hooghly River, resulting in a significant decrease in salinity ⁶⁶. As a result, the freshwater zone was extended to the mouth of the estuary. A shift in plankton dynamics was also observed. A concomitant decrease in marine fish forms has also been evident since then ⁵⁸. Phytoplankton, zooplankton, and benthic–epibenthic macroinvertebrates are the main prey organisms of fishes ^{63, 67}. As primary producers, phytoplankton influence the aquatic food chain directly or indirectly. It has been well documented that phytoplankton population dynamics significantly impact the migration of commercial fish species in estuarine environments ⁶⁸.

In the upper part of the Hooghly estuary, the capture of purely marine forms and characteristically neritic species, such as Eleutheronema tetradactylum, Tenualosa toli, and Harpodon nehereus, has seen a dramatic drop or entire disappearance after the construction of Farrakka barrage. After the commissioning of Farakka barrage, a few exclusively freshwater fishes like Rita rita, Ailia coilia, Catla catla, Labeo rohita, Labeo bata, and Wallago attu significantly contributed to the fish community structure of Hooghly estuarine system ^{63, 69}. The construction of dams and barrages on Damodar River (like Durgapur barrage in West Bengal) under the Damodar Valley Corporation (DVC) has significantly altered the natural flow regime/environmental flow of the river. This typical alteration in the flow characteristics has threatened the riverine fish diversity and community structure ⁷⁰. A study carried out by the CIFRI before the installation of the dams and barrages indicates that a total of 89 species of fish were found in different stretches of the Damodar River that has decreased to only 56 species during 1993–1995. The IUCN-listed ‘Threatened’ Boal fish (Wallago attu) and other economically significant species like the Catla (Catla catla) are no longer seen in Damodar ⁷¹.

Considering the nature of the source, pollution sources can be classified as point sources or non-point sources. Single or distinguishable sources of pollution are referred to as point sources, whereas diffuse forms of pollution that do not arise from a single distinct source are known as non-point sources ⁷². Because of the easy availability of water, most industries are situated along the river banks. Due to industrialization and urban growth, industrial effluents and sewage water are discharged into rivers, leading to an increased concentration of different pollutants in the rivers ⁷³. Farming activities in river basins and fertile flood plains add harmful pesticides and other chemical contaminants to canals and rivers. Surface water in rivers has become the critical sink for industries for waste disposal ^{74, 75, 76}.

The majority of the rivers in southern Bengal have poor water quality (Table 3), making them unsuitable for aquatic life. The river Bhagirathi-Hooghly receives about 87 MLD wastewater from 22 grossly polluting industries situated in West Bengal like Aditya Birla Nuvo Ltd. [Jayashree Textiles], Rishra, M/S United Brewaries Limited, Kalyani, ITC Limited Paper Boards & Speciality Paper Division, Tribeni etc. It was also observed that 70% of total wastewater comes from the discharge of chemical industries, followed by the pulp and paper industries, contributing to about 20% of the total discharge ^{77, 78}. Only 20% of the waste generated by 42 riverside municipalities was treated at sewage treatment plants (STP), with the remainder dumped directly into the Bhagirathi-Hooghly River ^{13, 79}. The status of sewage generation and treatment capacity of municipalities located along the Bhagirathi-Hooghly River is given in Table 4.

The river Damodar drains almost the whole coal mining area of the Chotanagpur plateau. Heavy industrialization in this area has resulted in a substantial pollution load in the form of oil, TSS, grease, and other pollutants on the river Damodar. Fifteen Coal washeries handle up to 8,000 tons of coal per day ⁸⁰. The Kolaghat thermal power plant is situated on the bank of the river Rupnarayan. The power plant disposed of hot waste water mixed with ash into this river ³⁸. Effluent released from Darshna Sugar Mill contaminates the water of the Churni River ⁴².

Global climate change and stochastic rainfall during the monsoon further aggravate the loss of fish diversity ⁸¹. Fishes are increasingly and intensely threatened by global climate change, resulting in a bleak future for global fisheries as well as for wild fish diversity ⁸². Climate change has profoundly threatened the global environment, chiefly by altering global thermal regimes and the water cycle ^{82, 83}.

Climate change has altered the global hydrological cycle, causing variations in the timing, type, and intensity of precipitation in addition to changes in air and water temperatures ⁸⁴. Fishes perceive climate through seasonal and daily variations in water temperature, winds, currents, and precipitation. Climate change will affect interspecific interactions in fishes such as predation, competition, parasitism and disease both directly through the modification of abiotic conditions and indirectly as the species pool changes ^{85, 86, 87}. The impact of climate change on fish species includes range expansion or habitat reduction through a number of processes. These processes are regulated by various environmental factors, with temperature having a significant influence. However, these factors may not necessarily operate independently, and the possibility of synergy or interactions between factors is certain ⁸⁸.

Decreased river flows have tremendously influenced the flushing properties and resulted in increased sedimentation. Decreased flow aggravates the siltation, which raises the riverbed and further, through negative feedback, affects the flow rate. Increased siltation ultimately causes irreparable changes in fish breeding grounds and hampers the fish breeding process ⁸¹. Ganguly et al. (2017) observed that juvenile hilsa shad are often found upstream at a distance of >60 kilometres from the coastline. However, adult hilsa shad spend almost their entire life span around the mouth of the river, 10–20 kilometres from the coast ⁸⁹. Thus, it may be assumed that the region upstream of the Hooghly River is the spawning ground, and the mouth of the river or the Bay of Bengal is the nursery ground ⁹⁰. The hilsa shad migrates seasonally (winter and monsoon) to spawn in the Hooghly River. It is also believed that there are two distinct populations of hilsa shad, one of which spends its whole life in freshwater and the other inhabits coastal waters and migrates solely to estuaries for spawning ^{90, 91}. The “mesh size of the gill nets vis-à-vis the mean size of fish” has been reduced significantly over the past two decades. Additionally, brood stocks are subject to uncontrolled fishing pressure during migration, adversely affecting breeding success and the subsequent recruitment of desired natural stock. The following are the major human impediments to hilsa migration in the Bhagirathi-Hooghly River system: siltation at estuary mouth, ineffective fish pass, exploitation of brood fishes, juvenile fishing etc. ⁹².

Overfishing affects heritable life-history traits such as growth and sexual maturity levels ⁹³. The vulnerability of fisheries resources has been compounded by overexploitation due to their higher economic value. In West Bengal, Ompok pabda, Chitala chitala, Liza tade, Mugil cephalus etc., in brackish water are dwindling at an unprecedented rate ⁹⁴. According to the findings of Roy et al. (2020), extensive exploitation of recently matured mullets has a detrimental effect on recruitment and future fisheries potentials in the main channel of the Matla River, the Bidya River, the Herobhanga rivulet, and their adjoining mudflats in the Sundarbans ⁹⁵. Trade in native ornamental fish relies mostly on wild collections, which exerts enormous strain on extant natural stock diversity ⁹⁶. The subsistence of the minnow/small indigenous fish population in inland river systems is contingent upon the management of indiscriminate fishing threats ⁹⁷.

Invasive species are believed to be the leading cause of native fish diversity decline ¹⁴. Infiltration of exotic pathogens, shifts in aquatic ecology, ecological threats (alien species found to have outcompeted native species for resources), and chances of hybridization with local species are all risk factors associated with introduced exotic species ^{94, 98}. Three exotic species, viz. Ctenopharyongodon idella, Hypopthamicthys molitrix, and Oreochromis niloticus were recorded at the Panchet Reservoir of the River Damodar. Oreochromis niloticus contributed the maximum percentage to the weight among the exotics ²⁷. Karmakar et al. (2008) have reported the presence of common carp (Cyprinus carpio) and mosquitofish (Gambusia affinis) in the Subarnarekha River ³⁴. However, in the studies of Ghosh et al. (2011), they did not encounter any alien species during their survey ³⁵. A significant increase in the number of exotics like Oreochromis niloticus and Cyprinus carpio was reported by Das et al. (2013) in the middle and lower stretches of the Ganga River ⁶². Bhakta and Bandyopadhyay (2007) recorded eight exotic species under three orders and four families from the river Churni. The most common species were Oreochromis mossambicus and Cyprinus carpio, while the African catfish, Clarias gariepinus, had the lowest population in the Churni River ⁹⁹. The integrity of the aquatic community should be preserved through regular monitoring of exotic species that can compete for food resources and natural habitats for other fish species ¹⁰⁰. Exploratory surveys are, therefore vital for comprehending biological invasions and their harmful consequences ¹⁰¹. Exotic fish species can reduce the indigenous carp population through competition, predation, and habitat degradation. Exotic fish species can cause genetic degeneration of wild populations through hybridization and gene introgression over a short or long period of time ^{102, 103}.

Potential threats to riverine fish assemblage in Southern Bengal is presented in Figure 2.

5. Conservation Status

Four species reported in the Panchet Reservoir of the Damodar River were in the Near Threatened (NT) category, namely Parambassis lala, Ailia coila, Ompok bimaculatus and Wallago attu ²⁷. In the Kangsabati River, two vulnerable (VU) and five near threatened (NT) species were recorded in the study of Kar et al. (2016) ³⁰. Three vulnerable species, Puntius conchonius, Mystus aor, and Bagarius bagarius were recorded in the study of Saha and Patra (2013) in the Damodar River ²⁸. One endangered and six near threatened species were also reported from the Kelegahai River by Pahari et al. (2017) ³⁶. Ghorai (2018) in the Rupnarayan river listed four near threatened species: Ompak bimaculatus, Ompok pabda, Wallago attu and Anguilla bengalensis ³⁹. Dubey et al. (2015) reported that 6% of fish belonged to endangered category and 2% were vulnerable and critically endangered category among the recorded fish species in their study in Sundarban Biosphere Reserve ⁵⁰. The study of Rosith et al. (2013) reported 155 species from the Hooghly estuary, of which 29 species have been listed in the category of threatened species, including 19 vulnerable species, 9 endangered species, and 1 Critically Endangered species. The occurrence of some species which are in the threatened category demands immediate conservation measures ⁵⁸. A list of fish species (river wise) that need immediate conservation measures is given in Table 5.

Proactive measures like declaration of no-fishing zone in riverine stretches and establishment of freshwater protected areas in the form of aquatic biodiversity management are intended to protect such threatened areas and species for generations to come. The conservation policy should foster management practices that preserve the integrity of aquatic ecosystems, avoid endangerment, and facilitate threatened species recovery. Artificial reproduction techniques (ART) can be used to create an Endangered Fish Species Breeding Program for conservation purposes. A live gene bank and a gamete/embryo bank could become the two critical components of this program. The enactment of the West Bengal Inland Fisheries Act, 1984; The West Bengal Inland Fisheries (Amendment) Act, 1993; and the West Bengal Inland Fisheries (Amendment) Act, 2008 were the stepping stones to address these issues ⁹⁴. The use of polyethylene netting with very fine mesh sizes of around 2 mm for drag nets, bag nets, lift nets, and skimming nets, which comprise the principal groups of fishing gears in the riverine system, is detrimental to riverine fish species ¹⁰⁴. To protect the juveniles, net and mesh size regulations must be strictly enforced ¹⁰⁵. Strict vigilance and efficient execution of legislative measures are highly intended to prevent illegal fishing ¹⁰⁶.

6. Conclusion

Rivers in Southern Bengal are now facing multiple problems, like water abstraction for irrigation, encroachment of the river bed, construction of dams and barrages that may cut the connectivity of the river with the associated ecosystems, indiscriminate deforestation in the catchment areas, etc. Many fish species in India for spawning migrate short or long distances. Any obstruction to the spawning routes affects recruitment. Apart from indiscriminate fishing, deforestation, poor land use, increased proliferation of exotics, fragmentation, and alterations in river hydrology are major threats to fishes in riverine stretches of Southern Bengal. Fishes are treated as a commercial commodity in West Bengal. The state has refused to recognize the ecological services provided by the fish. Due to diminishing natural stocks of riverine fish, fishers who rely on river productivity face serious challenges in sustaining their livelihood. Fisher’s livelihood is greatly affected by the decline in riverine productivity. A rational management program for the rivers of Southern Bengal should be put into place to protect fish diversity and increase fish production. In this context, ensuring the sustainable use of indigenous fish requires meticulous planning for conservation and management strategies. The fish assemblage structure and aquatic ecology of the lower order streams of this eco-region need to be explored.

References