Abstract

We present results of the first major attempt to record the distribution of aquatic alien species in the Western Ghats Biodiversity Hotspot using Geographical Information System. A total of 32 alien species, including four macrophytes (Salvinia molesta, Pistia stratiotes, Eichhornia crassipes and Cabomba furcata) and 28 fish were recorded from the southern part of the Western Ghats. Seven of these have established natural populations and are invasive, which includes Oreochromis mossambicus, O. niloticus (Cichlidae), Poecilia reticulata, Gambusia affinis (Poeciliidae), Pterygoplichthys pardalis (Loricariidae), Cyprinus carpio (Cyprinidae) and Clarias gariepinus (Clariidae). Oreochromis mossambicus is the most extensively distributed invasive fish in the region. Major reservoirs serve as a spawning hub for species such as C. gariepinus and C. carpio. Significant pathways for the establishment of most alien species in Western Ghats were aquarium trade and aquaculture. Given the increasing records of alien species and proliferation of invasive species in the southern region of the Western Ghats, urgent research and policy interventions are suggested, including the development of a comprehensive database on distribution, trade, invasion biology, as well as determining both short-term and longterm impacts on endemic fauna.

Introduction

Biological invasion is one of the greatest threats to global biodiversity, with multiple impacts on species and ecosystem services (Pejchar and Mooney, 2009), necessitating the development of robust databases and appropriate early warning systems for managing both current and possible future invasions (Liu et al., 2019). The increase in transportation of humans across the world and expanding globalisation has facilitated the large-scale introduction, spread and establishment of alien species across the globe, many of which have eventually turned into Invasive Alien Species (IAS), often threatening biodiversity and livelihoods (IPBES, 2019; WWF, 2020). As freshwater ecosystems harbour the highest species richness per surface area on the planet, the impacts of biological invasions may be disastrous (Thomaz et al., 2015), resulting in homogenisation of fauna through serving as the stepping stones or transits of alien species to nearby watersheds (Havel et al., 2015). Impacts of invasive flora and fauna may also vary between freshwater ecosystems. For example, invading macrophytes may alter hydrology, sedimentation and water quality (Gallardo et al., 2015), besides reducing the space available for co-occurring species, especially those at higher trophic levels such as invertebrates and fish (Carniatto et al., 2014).

Western Ghats (WG) mountain ranges, part of the Western Ghats-Sri Lanka Hotspot, is a distinct biogeographic unit with remarkable biodiversity and high rates of endemism (Myers et al., 2000). The endemic freshwater-dependent fauna of the region includes more than 130 species, of which close to one-third are at a high risk of extinction (Raghavan et al., 2016). In particular, the streams and rivers draining the southern region of the WG in the state of Kerala (henceforth SWG) shows exceptional diversity of fishes, many of which show ‘point endemism’ and trigger the ‘Alliance for Zero Extinction (AZE)’ criteria (Molur et al. 2011). Despite these striking statistics, the ichthyofauna of the region is threatened by a range of anthropogenic stressors, including habitat destruction and fragmentation, over exploitation, pollution, climate change and Invasive Alien Species (IAS) (Molur et al. 2011). In the recent past, climate change associated events have facilitated the entry and proliferation of many aquatic invasive species (Biju Kumar et al., 2019).

The effective implementation of IAS regulation requires accurate, detailed, and timely information on occurrences and distribution for efficient prevention, early detection, rapid response, and also to allow for evaluation of management measures. While the Convention on Biological Diversity (CBD) Aichi Target 9 provides an excellent framework for identification, control and eradication, as well as managing pathways of invasive alien species, only moderate progress was made in these directions in India. This paper, based on primary and secondary data, presents an overview of the occurrence, establishment and impacts of aquatic invasive species of the WG, besides recording the existing knowledge gap and required management framework for addressing the challenges of invasive species.

Materials and Methods

Study area

Our study was carried out in the SWG, within the state of Kerala (38,860 sq. Km; 8°.17’.30” to 12°. 47’.40” N; 74°.27’.47” to 77°.37’.12” E), located in south-western peninsular India. Catchment of all the 44 rivers draining Kerala originate in the WG; 41 flow westward to the Lakshadweep Sea, and three flow east and join the Cauvery (Fig. 1 and 2). All 44 rivers are perennial and monsoon-fed, and only 11 have a length of more than 100 km - the largest of which is Periyar (244 km). The Palghat Gap, located at an altitudinal range of 100 to 300 m ASL and drained by the Bharathapuzha, forms the largest break within the otherwise continuous chain of mountains of the WG. All river basins of Kerala flow through similar geomorphic and land use patterns and therefore show greater similarity in hydrology and ecology (Chattopadhyay and Harilal, 2017). The state also has 53 reservoirs built on the 44 rivers, primarily for hydropower generation and irrigation and three freshwater lakes, Sasthamkotta, Vellayani and Pookode.

Fig. 1.

Distribution of invasive fishes in the southern Western Ghats, Kerala, India.

Fig. 1.

Distribution of invasive fishes in the southern Western Ghats, Kerala, India.

Fig. 2.

Distribution of alien species (excluding invasive species) in the southern Western Ghats, Kerala, India.

Fig. 2.

Distribution of alien species (excluding invasive species) in the southern Western Ghats, Kerala, India.

Fish survey

Freshwater ecosystems of the SWG in Kerala were surveyed from 2016 to 2019 to document the presence of alien aquatic species. Random surveys were conducted in all 44 rivers and 53 reservoirs to record occurrence of alien species and understand the status of invasion, as defined by the IUCN: “an alien species is a species introduced outside its natural past or present distribution; if this species becomes problematic, it is termed an invasive alien species (IAS)” (https://www.iucn.org/theme/species/our-work/invasive-species). The fish collection was carried out with the help of local fishers hired for the purpose of sampling, or from the local fish markets and landing centers along the rivers and reservoirs. Major inland fishing gears, including hook and line, cast nets, gill nets and traps, were used for collection. Permission from the Department of Forest and Wildlife, Government of Kerala was obtained for sampling inside protected areas. The exact coordinates where alien fish were encountered were recorded using a hand-held GPS (Garmin Ltd). Data from secondary literature were also incorporated, based on information on the exact locality of the occurrence of invasive species distribution being available.

Preparation of Geographical Information System maps

QGIS version 3.14 was used in this study to prepare the comprehensive dataset and maps of alien species (macrophytes and fish) in Kerala. Location of species along with relevant attribute information such as taxonomic details, streams, waterbodies, place names and watershed boundaries form the input data. Level 12 hydrobasins (Lehner and Grill, 2013) derived from Hydrosheds project, the highest resolution river basin vector data available were downloaded and used for mapping the distribution of invasive and alien fishes. Hydrobasin shapefiles were clipped with Kerala boundary shapefiles, and the resulting output was modified in the boundary areas to fit into the political borders of Kerala. After clipping, certain polygons that did not make any meaningful contribution to the total area were deleted, and nearby polygons were extended to its region. A list of invasive alien fishes and their distribution was prepared and mapped in GIS (Geographical Information System) as a point shapefile. Using point shapefiles showing the distribution of invasive and alien fishes and level 12 hydrobasin polygon layer, the number of endemic fish species distributed in each hydrobasin was calculated using QGIS 3.14. The number of invasive alien fish occuring in each hydrobasin was also calculated using the same method.

Results

Alien/Invasive species distribution

A total of 32 alien species, including four species of plants (macrophytes) and 28 species of fish, were recorded from the SWG as part of the survey (Table 1). Of the 28 fish species, seven were identified as invasive, represented by two species within Cichlidae (Oreochromis mossambicus and O. niloticus) and Poeciliidae (Poecilia reticulata and Gambusia affinis), and one species each within Loricariidae (Pterygoplichthys pardalis), Cyprinidae (Cyprinus carpio) and Clariidae (Clarias gariepinus).

Table 1.

List of alien/invasive flora and fauna recorded from the waterbodies along southern Western Ghats, India.

Sl.No.Common nameSpeciesNative RangePathway of introductionAlien/InvasivePresence in number of waterbodies
RiversReservoirsFreshwater lakes
FLORA (Macrophytes) 
Kariba Weed Salvinia molesta South eastern Brazil GP Invasive 44 
Water Lettuce Pistia stratiotes Pantropical GP Invasive 20 
Water Hyacinth Eichhornia crassipes South America GP Invasive 38 
Red Cabomba Cabomba furcata South America AQ Invasive 
FAUNA (Fish) 
Mozambique Tilapia Oreochromis mossambicus Tropical and subtropical Africa AS Invasive 44 18 
Nile Tilapia Oreochromis niloticus Africa AS Invasive 
Sailfin Catfish Pterygoplichthys pardalis South America AQ Invasive 
Common Carp Cyprinus carpio Europe to Asia AS Invasive 17 29 
North African Catfish Clarias gariepinuPan Africa AS Invasive 10 
10 Guppy Poecilia reticulata South America MC Invasive 14 22 
11 Mosquito Fish Gambusia affinis North and Central America MC Invasive 
12 Green Swordtail Xiphophorus hellerii Central America AQ Alien 
13 Southern Platyfish Xiphophorus maculatus North America AQ Alien 
14 Giant Gourami Osphronemus goramy South east Asia MC Alien 
15 Three Spot Gourami Trichopodus trichopterus South east Asia AQ Alien 
16 Moonlight Gourami Trichopodus microlepis South east Asia AQ Alien 
17 Shortfin Molly Poecilia mexicana North and Central America AQ Alien 
18 Pacu Piaractus mesopotamicus South America AS/AQ Alien 
19 Pirapitinga Piaractus brachypomus South America AS/AQ Alien 
20 Striped Catfish Pangasianodon hypophthalmus Asia AS/AQ Alien 
21 Arowana Osteoglossum bicirrhosum South America AQ Alien 
22 Rainbow Trout Oncorhynchus mykiss Asia and North America SF Alien 
23 Silver Carp Hypophthalmichthys molitrix East Asia AS Alien 
24 Kissing Gourami Helostoma temminckii Asia (Thailand to Indonesia) AQ Alien 
25 Grass Carp Ctenopharyngodon idella Asia (Eastern China and Russia) AS Alien 
26 Forest Snakehead Channa lucius Asia (Thailand to Indonesia) AQ Alien 
27 Red Tailed Tinfoil Barbonymus altus Asia AQ Alien 
28 Alligator Gar Atractosteus spatula North America AQ Alien 
29 Oscar Astronotus ocellatus South America AQ Alien 
30 Arapaima Arapaima gigas South America AQ Alien 
31 Goldfish Carassius auratus Central Asia and China AQ Alien 
32 Mexican Mojarra Mayaheros urophthalmus Central America AQ Alien 
Sl.No.Common nameSpeciesNative RangePathway of introductionAlien/InvasivePresence in number of waterbodies
RiversReservoirsFreshwater lakes
FLORA (Macrophytes) 
Kariba Weed Salvinia molesta South eastern Brazil GP Invasive 44 
Water Lettuce Pistia stratiotes Pantropical GP Invasive 20 
Water Hyacinth Eichhornia crassipes South America GP Invasive 38 
Red Cabomba Cabomba furcata South America AQ Invasive 
FAUNA (Fish) 
Mozambique Tilapia Oreochromis mossambicus Tropical and subtropical Africa AS Invasive 44 18 
Nile Tilapia Oreochromis niloticus Africa AS Invasive 
Sailfin Catfish Pterygoplichthys pardalis South America AQ Invasive 
Common Carp Cyprinus carpio Europe to Asia AS Invasive 17 29 
North African Catfish Clarias gariepinuPan Africa AS Invasive 10 
10 Guppy Poecilia reticulata South America MC Invasive 14 22 
11 Mosquito Fish Gambusia affinis North and Central America MC Invasive 
12 Green Swordtail Xiphophorus hellerii Central America AQ Alien 
13 Southern Platyfish Xiphophorus maculatus North America AQ Alien 
14 Giant Gourami Osphronemus goramy South east Asia MC Alien 
15 Three Spot Gourami Trichopodus trichopterus South east Asia AQ Alien 
16 Moonlight Gourami Trichopodus microlepis South east Asia AQ Alien 
17 Shortfin Molly Poecilia mexicana North and Central America AQ Alien 
18 Pacu Piaractus mesopotamicus South America AS/AQ Alien 
19 Pirapitinga Piaractus brachypomus South America AS/AQ Alien 
20 Striped Catfish Pangasianodon hypophthalmus Asia AS/AQ Alien 
21 Arowana Osteoglossum bicirrhosum South America AQ Alien 
22 Rainbow Trout Oncorhynchus mykiss Asia and North America SF Alien 
23 Silver Carp Hypophthalmichthys molitrix East Asia AS Alien 
24 Kissing Gourami Helostoma temminckii Asia (Thailand to Indonesia) AQ Alien 
25 Grass Carp Ctenopharyngodon idella Asia (Eastern China and Russia) AS Alien 
26 Forest Snakehead Channa lucius Asia (Thailand to Indonesia) AQ Alien 
27 Red Tailed Tinfoil Barbonymus altus Asia AQ Alien 
28 Alligator Gar Atractosteus spatula North America AQ Alien 
29 Oscar Astronotus ocellatus South America AQ Alien 
30 Arapaima Arapaima gigas South America AQ Alien 
31 Goldfish Carassius auratus Central Asia and China AQ Alien 
32 Mexican Mojarra Mayaheros urophthalmus Central America AQ Alien 

GP: Garden Pond; AS: Aquaculture Systems; AQ: Aquarium System and Ornamental fish trade fish trade; MC: Mosquito larvae control; SF: Sport Fisheries

Among macrophytes, the most commonly distributed species was the Kariba weed (Salvinia molesta), occurring in all 44 rivers, four reservoirs, and three freshwater lakes in Kerala, followed by the Water Hyacinth (Eichhornia crassipes) recorded from 38 rivers and one freshwater lake. The Red Cabomba (Cabomba furcata), a native of South America, is a recent entrant to the aquatic waterbodies in SWG, recorded from seven rivers and two lakes.

Distribution of invasive (Fig. 1) and alien (Fig. 2) fish species in various waterbodies along SWG reveals their distribution to be confined to the highland and midland zones. Among fishes, the most widely distributed species is the Mozambique Tilapia (O. mossambicus), occurring in all 44 rivers, 18 reservoirs and two lakes and having established abundant populations in most habitats. Common Carp (C. carpio) was the second most common invasive species, recorded from 17 rivers, 29 reservoirs and one lake. On the other hand, guppy (P. reticulata) was distributed in 14 rivers and 22 reservoirs, with highly stable populations in high-altitude streams (800-1200m asl). Our study also showed that invasive species such as C. carpio, P. reticulata, O. mossambicus, and C. gariepinus had established good populations in the reservoirs, including those within protected areas. Many alien species, especially those commonly traded in the aquarium hobby, escaped to natural waters after the catastrophic floods triggered by extreme climatic changes during August 2018 and 2019. These floods also resulted in the release or escape of large-sized predatory species such as Arapaima gigas and Atractosteus spatula into the natural waterbodies.

Invasion pathways

Of the 32 alien species, 15 were introduced into the natural waterbodies of the SWG through the aquarium hobby and trade (Fig. 3). While six species were introduced solely for promoting aquaculture, three species were introduced for mosquito control, and three species for either aquarium keeping or promotion of aquaculture. Rainbow Trout, Oncorhynchus mykiss, was introduced during the colonial period to promote sport fishing, the only species introduced for this purpose. Three invasive plants, S. molesta, Pistia stratiotes and E. crassipes were introduced to the region as garden plants or for promotion of research, and their entry into natural systems is believed to be accidental. Cabomba furcata is a common aquarium plant, and this might have entered natural waterbodies either from home aquaria or from aquarium ponds adjoining the river basins, which are also used for rearing aquarium plants. While the release of most alien species may be accidental, species such as C. gariepinus has been illegally introduced for aquaculture. Of the 32 alien species, 11 were native to South East Asia, ten to South America, seven to Central or North America, and four to Africa.

Fig. 3.

Introduction pathways of alien species in the southern Western Ghats in Kerala, India.

Fig. 3.

Introduction pathways of alien species in the southern Western Ghats in Kerala, India.

Impacts of invasive species

Invasive macrophytes form thick mats and reduce light to submerged plants, often outcompeting rooted and submerged native plants and reducing vascular plant diversity. The formation of mats also lowers dissolved oxygen and pH, while simultaneously increasing CO2 and H2S. As plants in the mat die and sink to the bottom, benthos and benthic fish are impacted by changes in O2 concentrations and water depth, as organic material accumulates. The infestations of macrophytes also contribute to human health problems as they serve as host plants for vectors and reduces the aesthetics of the habitat. Their heavy infestation may also become a social problem, particularly in the coastal zones of Kerala, where they have chocked the entire network of inland canals, severely impacting water transport and small-scale and recreational fishing. These macrophytes also tend to increase the nutrient load, velocity of water flow, promote siltation and degrade fish nesting sites.

Oreochromis mossambicus, the most common invasive species in SWG, is naturalised in most waterbodies of Kerala, where they are found to compete with indigenous cichlid Etroplus suratensis, both for food and breeding habitats. In areas heavily invaded by P, pardalis, they have destabilised bank structure by breeding in pits along the stream banks, competing with native species, and damaging fishing gears. In Thiruvananthapuram, the capital city of Kerala, this species has invaded all natural streams, out-competing native species due to their ability to survive in polluted waters with the help of accessory respiratory organs.

Species richness of invasive fish in each of the hydrobasin vary in Kerala state (Fig. 4), with some hydrobasins harbouring up to six species, and most of them associated with larger reservoirs. The Common Carp, C. carpio introduced into Kerala for aquaculture, and primarily stocked in the reservoirs, has escaped into the natural waterbodies establishing good populations. In many ecosystems, particularly in reservoirs, they compete with indigenous cyprinids; particularly in Kallada river they show stronger competition with the native Tor sp. which is considered sacred. Another dominant species in reservoirs of the SWG is C. gariepinus where they pose a serious threat to native species because of their predatory habits. In the Mattupetty reservoir of Idukki district, they are the most dominant, displacing all native species. Poecilia reticulata, though not regarded as a severe pest in India, have now established strong populations even inside protected areas at higher elevations. We recorded this species in many second-order streams of SWG overlapping in distribution with many endemic and threatened species. Gambusia affinis, a remarkably hardy species introduced in Kerala for mosquito control, was observed to survive in low-oxygen waters, in high salinities and temperatures. Adult fishes were observed as too aggressive and attacking other young fish and particularly competing with indigenous Killifish, Aplocheilus spp.

Fig. 4.

Diversity of invasive fishes in different hydrobasins of the southern Western Ghats, Kerala, India.

Fig. 4.

Diversity of invasive fishes in different hydrobasins of the southern Western Ghats, Kerala, India.

Discussion

Non-native or alien fishes were introduced to India mainly for improving food, recreational and ornamental fisheries and mosquito bio-control (Biju Kumar, 2000). Since colonial times, over 300 alien species have been imported legally or illegally into the country, including 291 species for aquarium keeping, 31 species for aquaculture and two larvicidal fishes (Singh and Lakra, 2011). Particularly in the WG, introductions dating to the colonial times have resulted in the establishment of several freshwater alien species (Jones and Sarojini, 1952; Biju Kumar, 2000; Raghavan et al., 2008, 2013; Krishnakumar et al., 2009; Biju Kumar et al., 2015; Roshni et al., 2020). Further, the recent extreme climatic events like floods have resulted in the massive entry of new alien species into the freshwater ecosystems of the SWG (Biju Kumar et al., 2019). The present study adds to the existing literature on alien species in the SWG (Raghavan et al., 2008, 2013; Krishnakumar et al., 2009; Biju Kumar et al., 2015; Roshni et al., 2020), and most importantly, provides a comprehensive distribution pattern of alien and invasive species in the region.

Though O. mossambicus is extensively distributed in Kerala state, their distribution is concentrated in the coastal zone, where they have naturalised and formed part of the food chain, as prey to carnivorous species. However, C. gariepinus and C. carpio are significant stressors as they dominate fish assemblages in reservoirs and middle to upper reaches of major rivers (Fig 1) as with many major river systems of India (Singh and Lakra, 2006; Singh et al., 2010; Singh, 2014). While C. carpio competes with native species for food and space, C. gariepinus feeds on many smaller cyprinids and other endemic species in the freshwater ecosystems. Though this species is banned by the fisheries department of the Kerala Government (and Government of India), they continue to represent the dominant biomass in most reservoirs of the WG. In this context, an urgent management measure for their selective removal from natural waterbodies is warranted (Roshni et al., 2020). An emerging threat is the Amazonian Sailfin Catfish P. pardalis and probable hybrids of Pterygoplichthys spp. (Biju Kumar et al., 2015), an aquarium-associated species that has quickly established in many natural habitats since their first record in 2009 (Krishnakumar et al., 2009; Raj et al., 2019).

Climate vagaries, especially extreme floods, has also triggered the massive release of alien fish into aquatic ecosystems. The recent catastrophic flood in August 2018 in Kerala resulted in the release of large-sized predators such as A. gigas and A. spatula into the natural waterbodies of SWG, revealing the existence of unpredicted changes in the inland waters. Further, unregulated, unscientific, mostly illegal and thriving aquaculture and aquarium fisheries sector based on alien species along the riverine floodplains are posing uncontrolled threats (Biju Kumar et al., 2019; Raj et al., 2021). Given the increasing evidence of habitat changes (dams and the resultant flow alterations), climate change and expansion of alien fishes, further research is urgently needed to understand the spread of invasive fish, and associated declines in ecosystem services (Radinger et al., 2019).

Major pathways for the establishment of alien species in SWG are aquarium trade and aquaculture, similar to those in other parts of India where major identified pathways for aquatic alien species include aquaculture, sport fishing, and mosquito control (Biju Kumar, 2000; Knight, 2010; Singh and Lakra, 2011; Raghavan et al., 2008, 2013; Sandilyan, 2016). Among these, aquarium fish trade has been identified as the major global vector for freshwater fish invasions (Chan et al., 2019). Illegal transport of species such as C. gariepinus across regional and international borders have also led to their proliferation in aquaculture systems and their subsequent release and escape into natural waterbodies (Singh and Lakra, 2006; Lakra et al., 2008; Singh et al., 2013, 2015). In addition, alien species like P. reticulata and G. affinis have been introduced for controlling mosquito larvae, despite the fact that the rich diversity of indigenous larvicidal fish in India can be effectively used for this purpose (Das et al., 2018).

Studies by Raj et al. (2019) and Roshni et al. (2020) have shown that two invasive species P. pardalis and C. gariepinus, which have established healthy populations in the waterbodies of the SWG, can be managed only by targeting smaller-sized individuals rather than large adult fish; rapid growth rate, low fishing mortality and continuous recruitment contributes to the invasion success of the two species. The impact of P. reticulata, which has established even in many second-order streams of SWG, can be revealed only through detailed studies on invasion biology, as previous research indicate their strong invasion potential and impact on aquatic biodiversity (Sievers et al., 2012; El-Sabaawi Rana et al., 2016).

Preventing introduction of IAS in the Western Ghats can be considered as a cost-effective management option along with early detection and rapid response towards eradication in an integrated manner. Extensive research on invasion biology of potentially invasive species and their horizon scanning for prioritisation of IAS to inform decision-making (Peyton et al., 2019) is also recommended for the region. The current strategies in India towards the management of aquatic invasive species remain centred around general regulations and legislations for prevention, quarantine and ban with regard to the use of individual species. However, there is an urgency of an overarching policy that takes care of species-specific assessments based on their invasion biology, a more robust and transparent database on import and export of aquaculture and aquarium fish, specific programmes for early detection and rapid response, extensive monitoring programmes involving citizen scientists, and ICT-based awareness and education programmes through the decentralised local governance system, civil society and self-help groups. As an institutional mechanism, a Western Ghats Invasive Species Management Council is also mooted under the National Biodiversity Authority of India.

Conclusions

In summary, our results revealed the presence of 32 species of alien fish in the waterbodies of the SWG and the establishment and proliferation of 11 species (four macrophytes and seven fish). Alien species show variations in distributional patterns across the freshwater ecosystems, with the widespread occurrence of macrophytes S. molesta and E. crassipes and dominance of fish O. mossambicus, C. carpio and C. gariepinus. Aquaculture, aquarium fish trade and mosquito control were identified as the major pathways for alien species introduction and spread. With the increasing record of alien species in the region and projected changes in climatic regimes, the suggested interventions include development of comprehensive knowledge base invasion biology and the horizon scanning of possible invasive species for prioritising informed decisions on their management. The development of an overarching policy for invasive species management and management of invasive species through collaborative monitoring, prevention, eradication and awarenss involving all stake holders have also been suggested, besides suggesting an institutional mechanism such as the Western Ghats Invasive Species Management Council for implementing the policy.

Acknowledgements

The authors thank the Directorate of Environment and Climate Change (DoECC), the Government of Kerala for project funding support, and Dr A.K Singh for the invitation to contribute to the special issue. We also thank the Department of Forests and Wildlife, Government of Kerala, for the permission for the collection of fish from the protected areas of the state. SR is funded by Rajiv Gandhi National Fellowship, Government of India.

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