Environmental impact assessment of river valley projects in upper Teesta basin of Eastern Himalaya with special reference to fish conservation: a review

Abstract

Teesta river basin in Eastern Himalaya is being subjected to cascade hydropower development. To assess the impacts of this developmental activity on fish fauna of the basin, fish habitats of the four largest tributaries in the basin were selected for the present study. A total of 18 large hydroelectric projects are planned on these rivers. Here, a total of 11 EIAs and 2 carrying capacity studies in Teesta basin were reviewed. Our investigations show that after commissioning all the projects, 52–88% of the free-flowing river stretches will be diverted and 7.6–24% of the riverine ecosystems will be converted into semi lacustrine ecosystems in different rivers. The cascade hydropower development is likely to affect more than 100 fish species. All the EIA reports reviewed have investigated fish species richness, impacts on fish fauna and fishery management plans. EIA studies and management plans, however, do not adequately address important ecological and conservation issues due to various limitations. An integrated fish conservation plan for the upper Teesta basin is suggested in this contribution.

Keywords:

• Teesta basin
• river regulation
• EIA
• fish conservation

1. Introduction

The major Himalayan rivers in India are regulated to harness hydropower with over 300 hydroelectric projects (HEPs) under different stages (planned, commissioned, under construction) in the Himalayan river basins (Pandit and Grumbine 2012). Almost every other major river is being subjected to or earmarked for cascade hydropower development (dams built over the river channel in succession). Dam building activities are known to trigger adverse impacts on terrestrial and aquatic ecosystems and their biological diversity (Pandit and Grumbine 2012), but the most conspicuous impacts are anticipated on the riverine ecosystems and their functions. Modification in water discharge in the downstream sections and interruption in the longitudinal river connectivity due to dam building result in habitat restructuring and destruction (Larinier 2001). When a river is subjected to the cascade development, the cumulative impacts of such modifications impact larger areas, both vertically and horizontally. The large-scale physical changes lead to adverse impacts on the survival of fish fauna, their diversity, and distribution (Lovett 1999; Bhatt et al. 2012; Joshi et al. 2014), migration, recruitment, reproductive, and life history strategies (Humphries et al. 2002; Ziv et al. 2012; Erlewein 2013).

In India, Environment Protection Act (1986) through a regulatory instrument, namely Environmental Impact Assessment (EIA) Notification Section 3, under sub-section (1) and sub-section (2) provides that EIA studies are mandatory not only to address the adverse impacts of developmental projects on the life support system, but also to formulate suitable management plans to mitigate and/or minimize the anticipated impacts. Carrying out EIA studies in India is, therefore, mandatory for all medium and large hydroelectric projects (>25 MW). While the environmental impact assessment policies and regulations of Government of India for the project appraisals are well defined, these are rendered ineffective due to inadequate implementation and poor enforcement (Grumbine and Pandit 2013).

Scoping, one of the earliest stages of EIA process in India, is a tripartite exercise involving the statutory body, the Ministry of Environment, Forests and Climate Change (MoEF&CC), the project proponents and their technical advisors/consultants. This exercise sets the terms of reference (ToR), major goals, concerns, and issues for a particular hydroelectric project that needs to be addressed in the EIA study. One of the major lacunas in scoping relates to low importance accorded to biological diversity in general and still lower to the fish and fisheries (see Wale and Yalew 2010). Biodiversity component in the scoping is largely concerned with rare, endangered and threatened taxa and is invariably biased toward the more charismatic species of mammals such as the wild cats. More controversial aspects in public perception such as human socioeconomic and rehabilitation issues, geological discontinuities and risk from seismicity get more prominence in the project ToRs (see Diduck et al. 2007). Human sociocultural issues and geophysical risks are also debated at various government and civil society forums once ToRs have been set and even after a project has been executed, but there is little debate on the ecological impacts and losses due to the developmental activity (see Wale and Yalew 2010).

The social impacts of Himalayan dams, for example, are a hotly debated topic in the context of human displacement, rehabilitation, resettlement, and their economic upliftment, which are prioritized over ecological and the environmental consequences including impacts of river regulation on the fish and fisheries (Pandit and Grumbine 2012; Bhatt et al. 2016). In case of the Himalaya, recent work by Pandit (2017) has argued for emphasis on the carrying capacity of the region as defined by the trinity of geological vulnerability, ecological fragility, and sociocultural sensitivity of human communities in order to achieve the goal of sustainability.

Environmental impact assessment is practiced in more than 140 countries including India (Chadwick et al. 2012). In South Asian countries, there are legal and institutional frameworks for EIA studies, however, there is ample scope for reforms in EIA as an instrument of achieving sustainability (e.g. Fischer 2014; Diduck and Sinclair 2016; Jha-Thakur 2016; King and Smith 2016). The regulatory framework and existing procedures of EIA in India have been addressed by several workers (Paliwal 2006; Nandimath 2009; Panigrahi and Amirapu 2012; Erlewein 2013; Ghose 2013). The limitations, impacts, and need for strengthening of policy measures, with special reference to hydroelectric projects, have been highlighted by Agrawal et al. (2010), Erlewein (2013), Diduck et al. (2007, 2013); Grumbine and Pandit (2013) and Diduck and Sinclair (2016). The loss of fish diversity is one of the important impacts of hydropower development among various environmental effects, viz. loss of forest land, loss of biodiversity, and changes in social and cultural profile of human communities. A more sharpened focus on evaluating environmental consequences of cascade hydropower development on fish fauna in India is needed seriously. Most of the investigations into this aspect have been carried out internationally (e.g. Dudgeon 1995; Gehrke et al. 1995; Bunn and Arthington 2002; Ahearn et al. 2005; Branco et al. 2011; Dudgeon 2011; Arthington 2012; Ziv et al. 2012; Marshall et al. 2015; but see Vass et al. 2006; Das et al. 2007; Pathak and Tyagi 2010). As such these studies are carried out during post-impoundment phase and do not address a priori issues of environment impact assessment, mitigation measures and policies. There are only a few reviews of EIAs which refer to the predicted impacts and conservation of fish and fisheries and all these works relate to the rivers originating in China (Yangtze and Mekong) (Tullos 2009; Baird 2011; Baran et al. 2011). This study, to the best of our information, is the first review of EIAs with respect to the impacts of river regulation on fish and fisheries and the associated management practices suggested in various impact assessment reports and the data-sets of fish species recorded during pre- and post-regulation phases of the Indian Himalayan rivers.

2. Material and methodology

2.1. Study area

This study is based on EIAs of hydroelectric projects on four major river tributaries of Teesta river system, namely Teesta, Rangit, Rongni Chu, and Rangpo Chu. These rivers are located in Sikkim and West Bengal states of India in the Eastern Himalaya (Figure 1). Teesta river basin is spread over an area of 12,159 km², which straddles India and Bangladesh; Indian basin comprises 10,155 km², while the remaining portion lies in Bangladesh. Of the Indian portion, 8051 km² of Teesta basin lies in the mountainous terrain of Sikkim and West Bengal. In order to harness the hydropower potential of the rivers, Teesta basin is being subjected to the cascade hydropower development in Sikkim and northern West Bengal. About 40 large hydroelectric projects (>25 MW) are being planned in the hill region of the Teesta basin, of which 12 have already been commissioned and 4 are under construction (see Prasai and Surie 2013; Table 1). In addition, numerous small hydroelectric projects are under various stages of development in this basin.

Figure 1. Teesta river basin in Sikkim and West Bengal. Figure depicts the regulated river stretches and dam/barrage sites. Drainage map and location of hydroelectric projects in Teesta basin in West Bengal and Sikkim was prepared with the help of ERDAS imagine ver. 9.1 software. The study area is confined up to TLDP-V.

Table 1. Details of the hydroelectric projects planned in the study area of Teesta basin in Sikkim and West Bengal.

Name of the project	River	Installed capacity (MW)	Length of river diversion (km)	Length of reservoir (km)	Status
Teesta Stage III	Teesta	1200	19.00	1.00	Commissioned
Teesta Stage IV	Teesta	520	5.50	4.40	Proposed
Teesta Stage V	Teesta	510	20.50	4.43	Commissioned
Teesta Stage VI	Teesta	500	20.00	2.00	Under construction
Teesta Intermediate	Teesta	84	5.00	3.17	Proposed
Teesta Low Dam I & II	Rangit	81	2.00	8.57	Proposed
Teesta Low Dam III	Teesta	132	4.47	9.37	Commissioned
Teesta Low Dam IV	Teesta	160	1.10	6.40	Commissioned
Teesta Low Dam V	Teesta	80	-	4.64	Proposed
Ting Ting	Rathong (Rangit)	99	3.00	<0.50	Commissioned
Tashiding	Rathong (Rangit)	97	6.50	<0.50	Under construction
Rangit III	Rangit	60	5.50	<1.00	Commissioned
Rangit IV	Rangit	120	11.00	2.00	Proposed
Jorethng	Rangit	96	11.50	1.75	Commissioned
Rongni	Rongni	96	12.50	2.00	Under construction
Rolep	Rangpo	36	6.00	1.00	Proposed
Chuzachen HEP	Rangpo	110	6.50	<1.00	Commissioned
Suntaley HEP	Rangpo	40	4.50	<1.00	Proposed

Teesta is a glacier-fed river and originates as Chhombo Chu from Khangchung Chho (lake) at an elevation of 5280 m in North Sikkim district. The hydropower development profile of the basin envisaged a total of eight large hydroelectric projects (>25 MW) on the main Teesta river channel (downstream of the confluence of Lachung and Lachen tributaries) within hilly terrain. Of these projects, four are already commissioned (Teesta Stage III, Teesta Stage V, Teesta Low Dam III and Teesta Low Dam IV), one is under construction (Teesta Stage VI), and three are likely to be taken up for execution in near future (Teesta Stage IV and Teesta Intermediate) (Figure 1; Table 1).

Rangit is the largest right bank tributary of Teesta, which originates as Rangit Chu from Talung glacier at an elevation of 4080 m. Rangit joins Teesta river at Teesta Bazar, a small town near West Bengal–Sikkim border. The basin area of Rangit River is 2134 km², of which 1621 km² lies in West Bengal and the remaining area falls in Sikkim state. Rathong Chu is the main tributary of Rangit River and six projects are planned on these river channels from Ting Ting to the confluence of Rangit and Teesta rivers. Of these, three projects are commissioned (Rangit III, Jorethang and Ting Ting), one is under construction (Tashding), and two are proposed (Rangit IV and Teesta Low Dam stage I & II).

Rongni Chu is a left bank tributary of Teesta river and joins the main channel at Singtam (350 m). The Rongni Chu basin is entirely located in Sikkim and the river originates from the western slope of a peak near Kyongnosla Forest Block (3924 m) in East Sikkim. A single large project (96 MW) near Namli Village is under construction on Rongni Chu.

Rangpo Chu is a left bank tributary of Teesta river joining the main river at Rangpo (286 m). It is a glacier-fed stream, rising from the Chhangu lake at an elevation of 3700 m in East Sikkim. A total of three large projects have been planned on Rangpo Chu, of which 1 is commissioned (Chuzachen) while 2 are proposed (Rolep and Suntaley Tar).

2.2. Data collection

In order to assess the impacts of hydroelectric projects on the fish fauna and suggested mitigation measures, a total of 11 EIA reports and 2 carrying capacity study reports were reviewed. Out of 11 EIA studies, our institution, CISMHE, carried out EIA studies for four hydroelectric projects such as Rolep (2002), Jorethang, (2006), Rongni (2007), Teesta Stage IV (2014) and two carrying capacity studies in Teesta basin in Sikkim (2008) and Lower Teesta basin in West Bengal (2015) between 2002 and 2015. In addition, a total of 7 more reports available in the public domain, viz. Tashiding (2009), Ting Ting (2010), Teesta Stages III (2006), V (1998), and VI (2006), Teesta Low Dam III (2002), and IV (2002) were reviewed for the present contribution (see Supplementary Material).

All these EIA reports included studies on various environmental aspects as defined in the respective ‘Terms of Reference.’ The data specific to the fish species composition, impacts on fish fauna, and mitigation measures for fish conservation and fishery development were sourced from these EIA reports. The main aim of the two different carrying capacity studies in Teesta basin reviewed here was to plan optimal number of power projects in the basin.

A drainage map depicting various rivers and location of various hydroelectric projects in Teesta basin was prepared on GIS platform and geo-referenced with the help of ERDAS imagine ver. 9.1 software. The boundaries of each basin/sub-basin, location of each dam/barrage site, river stretches under reservoir and under diversion were digitized with the help of Arc GIS ver. 9.1 software. Teesta basin was divided into Upper Teesta basin, largely in Sikkim and Lower Teesta basin falling inside the state boundary of West Bengal. The geographic coordinates of locations of dam/barrage sites and powerhouse/outlets sites were obtained from the respective EIA reports. Due to non availability of exact exit points of tail race tunnels in some projects, their locations in the map may not reflect the ground reality, but it would not affect the inferences of the study. Using these details, total length of river in the study area, length and maximum width of each reservoir, length of the free-flowing section, and length of the reduced flow/dried up section of each river after its diversion were calculated. Data on fish composition of each river including their taxonomic classification, migratory phenomena of fish, prediction of impacts of hydroelectric projects on the fish fauna, and suggested mitigation measures for the same were sourced from the respective EIA reports of each hydroelectric project.

3. Results

3.1. Regulated zone

The total length of main Teesta river channel within the study area (Chungthang to Sevok bridge) is about 120 km. If all the hydroelectric projects are built, nearly 70.5 km (~59%) river stretch would be diverted through head race tunnels (HRTs) and about 29 km (24%) riverine stretch would be converted from the existing lotic ecosystem into several semi lacustrine ecosystems. The total length of Rangit River (from Ting Ting dam site to Teesta Bazar) is about 61 km, of which about 39.5 km (64%) river stretch is planned for diversion and cumulatively 12 km (19.6%) riverine ecosystem would be converted into semi lacustrine ecosystem. Likewise, the total affected zone of Rangpo Chu river (Rolep dam site to Rangpo) is about 32.5 km and the proposed diversion of the river channel is about 17 km (52%) and the cumulative length of the reservoirs would be nearly 2.5 km (7.6%). Total length of Rongni Chu (from upper most project – Lower Lagyap to Singtam) is about 21 km. The diverted river stretch constitutes 18.5 km (88% of the river length including 13.5 km of Rongni Chu hydroelectric project). In case of Rongni Chu, water would be diverted to other sub basin, Rangpo.

3.2 Fish fauna

The upland region of Teesta basin is inhabited by a total of 103 fish species that belong to 13 families. Fish species are dominated by family Cyprinidae (43 species), followed by Sisoridae (23 species) and Balitoridae (18 species). Schizothorax richardsonii (Snow Trout), Schizothoraicthys progastus (Dinnwah Snow Trout), Neolissochilus hexagonolepis (Copper Mahseer), Garra gotyla gotyla (Gotyla Garra), G. lamta (Lamta Garra), Crossocheilus latius latius (Stone Roller), and Barilius spp. (Baril) are some of the common fish inhabiting the river channels on which the hydroelectric projects listed in Table 1 are located. With regard to conservation status of these fish species, about 60% are categorized as of ‘least concern’ under the IUCN red list. One species (Schistura spilopterus) is designated as ‘critically endangered’, Tor putitora (Golden Mahseer), Schistura kanjupkhulensis are classified as ‘endangered,’ while Schizothorax richardsonii, Cyprinion semiplotum (Assamese Kingfish), Glyptothorax manipurensis, and Danio naganensis are classified as ‘vulnerable.’ Among the threatened species, Tor putitora and Schizothorax richardsonii are of commercial interest in Teesta basin. Major fish captured in the hill region for the local markets comprise Neolissochilus hexagonolepis, Schizothorax richardsonii, and Schizothoraicthys progastus.

A total of 12 species, namely Tor putitora, Tor tor (Deep Bodied Mahseer), Labeo boga (Boga Labeo), L. dyocheilus, L. pungusia, Clupisoma garua (Bachcha Garua), C. Montana (Kocha Garua), Channa gachua (Dwarf Snakehead), C. orientalis (Striped Snakehead), Bagarius bagarius (Devil Catfish), and Anguilla bengalensis (Mottled Eel) are migratory that ascend from the foothills to the upstream tributaries of Teesta. However, Tor putitora and Anguilla bengalensis, diadromous and catadromous fish, respectively, are known to undertake relatively long-distance upstream migrations in these rivers. Tor putitora uses same spawning grounds throughout the lifespan, which are mostly small tributaries at lower elevations in the Himalaya (Nautiyal 1994; Bhatt and Pandit 2016). Anguilla bengalensis is a catadromous species and uses estuaries for spawning (Gubhaju 2002).

3.3. Impact prediction

Perusal of EIA reports of various hydroelectric projects in the Teesta river basin reveals that river regulation is likely to adversely impact the fish fauna of the rivers. The most common impacts predicted include habitat degradation, fragmentation, loss of breeding grounds in the downstream stretches, and obstruction in migration by dams. However, these reports do not appear to have carried out robust empirical analyses of how would damming of rivers lead to the predicted impacts. While predicting impacts, EIA studies of Jorethang, Teesta Stage IV, Teesta Low Dam III, and Teesta stage IV hydroelectric projects identified the constraints on fish migration due to downstream projects like Teesta Low Dam III, Teesta stage V, and Teesta barrage, respectively. These investigations did not analyze and address the anticipated effects of downstream projects on the fish migration because of the already executed projects in the downstream. Such an analysis may have been even irrelevant because the dams in the downstream would be impassable barriers to fish migration.

3.4. Carrying capacity studies

The Carrying Capacity Study of Teesta Basin in Sikkim made an inventory of 55 fish species from different sources including primary surveys of markets during 2003–2004 (CISMHE 2007). The study revealed that these fish species largely inhabited the basin up to 1400-m elevation and only a few species such as the exotic trout (Salmo trutta fario) were found above 1400 m. Schizothorax richardsonii, Schizothoraicthys progastus, Neolissochilus hexagonolepis, Tor putitora, and Garra gotyla gotyla were reported to constitute the major catch in the lower stretches of Teesta, Rangpo, and Rangit rivers. The foothill stretch of Rangit River was designated as constituting spawning grounds of Tor putitora.

The Carrying Capacity Study of Lower Teesta basin in West Bengal listed a total of 67 fish species from the hill terrain of the basin (CISMHE 2015). Schizothorax richardsonii, Barilius spp., and Neolissochilus hexagonolepis comprised the major fish catch in the rivers of the basin. The report highlighted the impacts of existing projects on the fish fauna and revealed that the downstream projects like Teesta Low Dam III have already hampered the upstream migration of Tor putitora and Anguilla bengalensis. As a result, these species failed to reach their breeding grounds in Rangit River and Teesta River beyond the confluence of Rangit and Teesta at Teesta Bazar. Also, due to reduced flow in the downstream areas, rivers have become accessible to both active and casual fishing that resulted in higher fish predation. The studies in Rangit and Teesta rivers also indicated that though major catch (about 50–80%) contained large-sized species like Schizothorax richardsonii, S. progastus, and Neolissochilus hexagonolepis, the individuals were mostly (~90–95%) fingerlings.

3.5. Fish conservation measures

Out of the 11 EIA reports reviewed here, seven (Ting Ting, Teesta Stage III, Teesta Stage V, Teesta Stage VI, Rongni, Rolep and Jorethang hydroelectric projects) suggested establishment of hatcheries as a conservation measure for fish fauna of Teesta basin. These reports recommended that Schizothorax richardsonii, Schizothoraicthys progastus, Neolissochilus hexagonolepis, and Tor putitora be selected for ova production in these hatcheries with the objective of seeding/ stocking of the dam-created reservoirs and of the fragmented river stretches. EIA reports of Teesta Low Dam III and Teesta Low Dam IV suggested construction of fish passes on their respective dam bodies to facilitate fish migration of species like Tor putitora. None of the EIA studies, except Teesta Stage IV hydroelectric project, carried out environmental flow assessment in the downstream of the projects for sustaining the habitat of fish species. However, one of the objectives of the just concluded Carrying Capacity Study of Lower Teesta Basin is to suggest maintaining the environmental flow in the downstream of each project to protect fish fauna and the ecosystem integrity of the river. The draft final report has suggested an environmental flow of 15% of average lean season flow (December to March), 20% of average monsoon (June to September), and non monsoon seasons (April to May and October to November) in the downstream sections of river for a few projects located in the Lower Teesta basin.

3.6. EIA follow-up

As follow-up action, six monthly compliance reports are available online for Rangit, Teesta Stage V, Teesta Low Dam III, Teesta Low Dam IV (nhpcindia.com/newwebsite/six-monthly-progress-report.htm), Teesta Stage III, Teesta stage IV, Tashiding, Jorethang, etc. Most of the projects under the study are either under construction/proposed or recently commissioned; therefore, the follow-up implementations in these projects with respect to fish conservation are not yet prominent. However, a fish hatchery has been constructed as a follow-up action in Teesta Stage V hydroelectric project in Sikkim. This hatchery needs upgradation and presently is not functioning properly. Similarly, fish ladders as stipulated in the environmental management plans have been facilitated in Teesta Low Dam III and Teesta Low Dam IV.

4. Discussion

4.1. Fish species richness and impact assessment

The Himalayan rivers are well known for their cold water fish, which is perhaps the predominant biotic component of the rivers linked to the livelihood of local human communities. Fish diversity in the Himalayan rivers decreases along the elevational gradient (Bhatt et al. 2012); therefore, only a few fish species inhabit elevations above 1400 m in Teesta river basin. The studies reviewed here have covered the elevational diversity and distribution of fish; therefore, this review represents the full spectrum of fish species diversity of the upper Teesta basin. The analyses of data on fish presented in various EIAs show that the fish habitats in the Teesta basin have been and are likely to be further degraded and fragmented by cascade hydropower development. Recent studies have shown that the Himalayan rivers in general and Teesta basin in Sikkim in particular are likely to have the highest dam density per km² basin area in the world (Pandit and Grumbine 2012). The main Teesta river channel is earmarked to have eight hydroelectric projects which correspond to 8 dams in a stretch of about 120 km. The resulting dam density of nearly one dam per 15-km length of the river channel is the highest for the Himalayan rivers (average of one dam/32 km of river length). Cascade hydropower development of rivers resulting in such a high dam density is likely to adversely impact riverine fish in two ways. One, impoundment from dams causing changes in riverine biotic community structure would wipe out fish population of upstream and downstream stretches. Two, the suggested management practices such as stocking enhancement in reservoirs and introduction of non native species (Gehrke et al. 2002).

Two sets of impacts were detected that have arisen as a result of dam building and from the management interventions prescribed in EIAs. First, Carrying Capacity Study of Lower Teesta Basin reported that Teesta Low Dam Stage III had curtailed upstream migration of Tor putitora. The observation was made on the basis of the fact that this fish was no more found in Rangit River (upstream of Teesta Low Dam III) during the survey in 2014. Second, the environmental management plan of Teesta Stage III project had recommended establishing a hatchery of native fish species, Schizothorax richardsonii and S. progastusas a conservation measure. The Sikkim State Department of Fisheries, instead chose to create a hatchery of exotic Oncorhynchus mykiss (Rainbow Trout) for introduction in the river (EIEG 2014). While the impact of the introduced alien fish species in this river is not known so far, the introduction of non native species in other rivers is known to cause adverse impacts on native biodiversity (Singh and Lakra 2011). Clearly, there are twin adverse impacts on fish diversity in Teesta basin from dam-building that manifest in cessation of migration which is linked to their reproduction and population build up, and from conservation (mis)management through introduction of exotic species.

Even as studies investigating the impacts of river regulation on the fish fauna of the Himalayan rivers are limited (but see Bhatt and Pandit 2016; Bhatt et al. 2016), considering the dam density, extent of river water diversion, and creation of reservoirs in Teesta basin, significant impacts are anticipated in river morphology (shape and size of river channel, the distribution of riffle and pool habitats, stability of the substrate, water temperature) and water chemistry (Anderson et al. 2006). The impacts that alter riverine habitat conditions are known to be detrimental to native fish fauna (Branco et al. 2011; Ziv et al. 2012), but the changed conditions are favorable for generalist exotic taxa, as and when introduced (e.g. Vila-Gispert et al. 2005). Adverse impacts from introduction of exotic fish species have been reported in the Sutlej and Beas rivers in Himachal Pradesh Himalaya (see Raina and Petr 1999). These studies showed that prior to the construction of dams on these rivers, fish fauna was dominated by native species like Tor putitora, Labeo dero, L. dyocheilus, Schizothorax, and catfish, but with the introduction of exotic Hypophthalmichthys molitrix (Silver Carp) and Cyprinus carpio (Common Carp.) and non native Labeo rohita and Catla catla (Indian Major Carp) contribution of native species to total fish catch has decreased considerably. A case study on fish fauna of Gobindsagar reservoir indicated that after the introduction of Silver Carp, catch composition altered considerably (Sugunan 1995). The catch of Silver Carp increased from 0% in 1974–1975 to 71.5% of the total catch in 1992–1993, whereas Mahseer (native fish) catch decreased from 16.7% in 1974–1975 to 4.7% of total catch in 1992–1993 (Sugunan 1995). Fish catch data of Government of Himachal Pradesh indicated that Silver Carp still dominates the Gobindsagar reservoir contributing 38.6–85.12% to total catch from 2002–2003 to 2014–2015 (Government of Himachal Pradesh, 2016). Similarly, in case of Pong dam reservoir (Maharana Pratap Sagar) on the Western Himalayan Beas river, the fish catch of Indian Major Carp increased from 8.82% in 1982–83 to 51.39% in 1989–1990. On the other hand, catch of Mahseer in this river decreased gradually from 20.34% in 1982–83 to 11.03% in 1989–1990 (Kumar 1989). In recent years (2006–2007 to 2014–2015), Mystus seenghala has dominated the fish catch in Pong dam reservoir, whereas average catch of Tor putitora is just about 10%. Studying the fisheries of lakes and reservoirs in the Himalaya, researchers have inferred that introduction of exotic and non native fish has built up the regular supply and stock of commercial fish but has adversely affected the native schizothoracines and mahseers (Raina and Petr 1999).

Perusal of the ToRs of the proposed hydroelectric projects revealed that their impacts on fish and fisheries formed an important component of the identified impacts to be studied in detail and were defined as such during scoping process of these projects. In all the EIA reports reviewed here, serious gaps were found between the intent of the ToRs and their execution. The EIA reports did not adequately address the impact of hydroelectric projects on the fish. Most of the studies did not bring out the adverse impacts on fish due to disruption in the longitudinal connectivity of rivers and the resultant fragmented habitats (e.g. Lucas and Baras 2001; Falke and Gido 2006), impacts on the population recruitment in fish species (see Jackson and Marmulla 2001) and the consequences of reduced downstream water flow in the rivers, which encourages higher fish predation including of fries and fingerlings. The only exception is the Carrying Capacity Study of Lower Teesta Basin in West Bengal (CISMHE 2015), which provided adequate space and attention to the impacts of river regulation on the fish fauna as a consequence of habitat degradation and population fragmentation. This study clearly brought out the likely effects of dam operations, which lead to changes in the timing, duration, amplitude, and quality of water flow in the downstream sections and consequently affect the fish species (e.g. Welcomme et al. 2006). The study revealed that Teesta Low Dam Stage III would hamper the migration of Tor putitora from foothills of Teesta river to Rangit River which comprises its breeding ground. Also, the reduced flow in Rangit River due to operation of Rangit dam promoted wanton fishing in Rangit River due to casual fishing in the downstream stretches.

4.2. Mitigation measures

Impediment to fish migration due to dams is one of the long-time concerns of biologists in the context of consequences of river regulation. The mitigation response that emerged out of these concerns resulted in changes in dam design such as provision of fish ladders and fish passes alongside a dam body. Since the large majority of the Himalayan dams are not concrete structures, facilities for such mitigation measures are not considered as practical. Therefore, most of the EIA studies in Teesta basin reviewed here have suggested ex situ conservation measures in fish through hatchery development. This alternative to natural process of fish migration and introduction of exotic fish populations in fragmented stretches is beset with several constraints and pitfalls. Scientific investigations have revealed that hatchery stocks are less viable, lead to loss of genetic integrity every year and genetic material from hatchery may contaminate the natural fish populations adversely while propagated in wild (e.g. Nguyen et al. 2006; Pradhan et al. 2011). The EIA Reports, for instance, did not discuss the likely loss of genetic integrity as stated here, methods of ova propagation, and the related impacts on natural fish populations. That said, fish ladders were suggested in a few cases like Teesta Low Dam III and IV for migratory fish species like mahseer (Tor putitora). We did not come across any details of the types of fish ladder suggested and their suitability for the target species. Also, there was little discussion in the EIA reports on the suitability of the design of fish pass entrance that would appeal to fish to take the artificial upstream route (see Santos et al. 2006). We opine that these aspects of fish pass must form an integral part of the dam design and that an efficacy study of a fish pass needs to be carried out separately.

4.3. Environmental flow

Alteration of flow regimes directly affects the river ecosystem services. Environmental flow investigations have become an integral part of impact assessment studies, management plans, and policies over last a few years (Carolli et al. 2017). Evaluation of environmental flows is fundamental to the conservation of fish and sustaining commercial fisheries as well as a crucial mitigation measure for regulated rivers. For the continuity of ecological processes, survival of biotic communities including sustaining ecosystem integrity, and ecological goods and services, it is essential to allow a certain amount of water flow in a regulated river. Despite scientific evidence that an optimum flow is critical to maintaining biotic diversity and ecosystem functions of riverine ecosystems, only one of the 11 projects reviewed here, namely Teesta IV project included detailed environmental flow study and suggested minimum flow of 5 m³/s in the downstream section. This flow is expected to increase due to precipitation and drainage in the downstream intermediate catchment. The principal reason behind lack of environmental flow studies in the EIAs of the hydroelectric projects in the basin appears to be lack of such a provision in their ToRs.

A decade back, the importance of and need for studying environmental flows for ecosystem integrity in rivers earmarked for regulation in India was not identified as a priority for inclusion in EIA studies. In India’s National Water Policy, ‘ecology’ is ranked 4th in priority (Smakhtin and Anputhas 2006). Even though the first ever National Workshop on the Environmental Flows in India was held in 2005, EIA studies completed till 2010 paid little attention to the importance of environmental flows. The need for investigating environmental flows in the hydroelectric projects and their incorporation in EIA studies in India was only enforced after 2011. The EIA study of Teesta IV project was then an ongoing one; therefore, the parameter of environmental flow was added to the study at a later stage.

4.4. Environmental policies

Because EIA studies in India are project-specific, cumulative impacts of cascade hydropower development and impacts of immediate upstream/downstream projects are not discussed at the time of scoping and are not included in the formulation of ToRs (see Paliwal 2006; Seitz et al. 2011). The silence of policy on cumulative impacts of HEPs and strategic environmental Assessment (SEA) encourages project proponents and their consultants to avoid an ecosystem approach toward investigating the rivers proposed for regulation. In the absence of such an approach, the likely impacts of river regulation on the diversity and distribution of fish fauna, alternative approach, and accountability of project proponent are completely ignored in EIA investigations (see King and Smith 2016). An appraisal of the fisheries management plans of the EIAs under discussion here shows that despite involvement of the State Fisheries Department in their formulation, there is little coordination or link between different plans in the same river. The lack of an ecosystem approach in the EIA studies undermines river continuum concept and ignores the dynamics of river flow, ecological processes, and fish species diversity and distribution. Lopsided scope of EIAs, lackadaisical procedural assessments, and failure of internalizing need for EIA into decision-making have been identified as some of the major constraints in achieving the desired goals of EIA process within the larger framework of sustainability (e.g. WCD 2000). Notably, two independent carrying capacity studies have been carried out in Teesta basin each addressing the areas in Sikkim in upper basin and northern parts of West Bengal in the lower basin, respectively. The two studies carried out a decade apart were limited by the consideration of political boundaries and their geographic extents and not by a basin or river ecosystem continuum approach. Such efforts need to be strengthened and an integrated approach to ecological investigations of rivers from their headwaters to the main channels or oceans need to be carried out for developing a proper understanding of the long-term impacts of regulation.

It is clear from the analysis of the EIAs that the key fish zone of rivers in the Teesta basin would be severely affected by a series of dams leading to habitat fragmentation and interruption in longitudinal connectivity of the river systems. Major parts of river stretches would either dry up or would have significantly reduced flows, and numerous free-flowing riverine stretches would be modified into stagnant water bodies. Thus, the suggested fishery management plans in different EIA reports (reservoir fishery, hatchery for propagation and stocking, etc.) would be of little use, because the reservoirs can sustain only a few native fish like Tor putitora, Schizothorax spp. and other large cyprinids, which are column dwellers and are adapted to stagnant waters. These species are known to survive in other Himalayan reservoirs such as Gobindsagar and Pong, though they do not constitute dominant catch in these water bodies because of introduction of exotic fish species (Kumar 1989; Sugunan 1995; Raina and Petr 1999).

4.5. EIA follow-up

In India, submission of half-yearly compliance report to the concerned regulatory authority by project proponents is an important part of EIA follow-up (Jha-Thakur 2011; Ghose 2013). The compliance report addresses the progress in the conditions and safeguard measures stipulated in the EIA report. The compliance report is submitted in the soft and hard copies and remains in the public domain (Jha-Thakur 2011). Monitoring of EIA follow-up is the responsibility of Ministry of Environment, Forests and Climate Change (MoEF&CC) and its 6 regional offices (Paliwal 2006). Notably, EIA follow-up implementation is largely ignored by stakeholders as compared to EIA report and public hearing (e.g. Jha-Thakur 2011). In Sikkim, the half-yearly compliance reports of various hydroelectric projects are available in public domain. The EIA follow-up implementation with respect to fish conservation seems poor. The probable reason of the poor implementation is that most of the projects are proposed/under construction or recently commissioned; therefore, results of EIA follow-up implementation are yet to come.

5. Conclusions

Notably, nearly 55% fish of Teesta river system are bottom dwellers and are not well adapted to reservoir life. Also, majority of bottom dwellers species are not of commercial importance due to their small size. For these reasons, their rearing in hatcheries and introduction in reservoirs are considered as disagreeable. It is conclude that in order to maintain fish faunal diversity of Teesta river system and to conserve economically important and threatened fish species, reappraisal of the proposed cascade hydropower development is warranted. The survival of fish diversity and their distribution needs to be ensured through adequate environmental flows in the downstream stretches under an ecologically sustainable and integrated water management program (e.g. Richter et al. 2003). The integrated management program with special reference to fish conservation can be followed by applying the good SEA practice which has ample scope for sustainable alternatives (King and Smith 2016). For instance, suggestions have been made to erect partial dams on the Himalayan rivers to maintain their longitudinal connectivity for free movement of sediments and life forms without compromising the hydropower development potential of these rivers (Tyagi and Jhunjhunwala 2014). The new proposals recommend that in order to achieve the requisite head for power generation, the partial dam be moved further upstream to achieve the desired head. Whether such a design would be techno-economically feasible is a matter of investigation, but worth it.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by National Hydroelectric Power Corporation, India Ltd, Ministry of Environment, Forests and Climate Change, Government of India [J.12011/11/99-IA-I] and Water & Power Consultancy Services, Ltd. [WAP/Envt/Teesta Basin/2014/1603].

Supplemental data

The supplementary material for this article is available online at https://doi.org/10.1080/14615517.2017.1354642.

Acknowledgments

Financial supports from NHPC India Ltd, Ministry of Environment, Forests and Climate Change, Government of India and WAPCOS Ltd in different time are gratefully acknowledged. We are thankful to Mr. R. Mehta for his assistance in the graphics.