An overview of fishes of the Sundarbans, Bangladesh and their present conservation status

Sundarbans, the largest mangrove forest of the world is located in Bangladesh and India. Studies done on the diversity of fish fauna in the Sundarbans mangrove forest of Bangladesh are sparse and patchy. Here we take the opportunity to provide an updated checklist of the fishes of the Sundarbans, Bangladesh based on primary and secondary data. Field surveys were undertaken in the aquatic habitat of Sundarbans core area along with its adjacent marine habitat from June 2015 to July 2017. Based on published information and primary observations the updated list of fishes covers a total of 322 species belonging to 217 genera, 96 families, and 22 orders. Additionally, four species of fishes, are newly reported in Bangladesh waters, viz., Mustelus mosis Hemprich & Ehrenberg, 1899; Lagocephalus guentheri Miranda Ribeiro, 1915; Carangoides hedlandensis Whitley, 1934; Uranoscopus cognatus Cantor, 1849. The global IUCN Red List status of each species has been enlisted. The updated checklist will constitute the reference inventory of fish biodiversity for the Sundarbans, a natural world heritage site.


INTRODUCTION
Mangroves are intertidal forested wetlands confined to the tropical and subtropical regions (Tomlinson 1986). The total area of the mangroves in the globe is an estimated 18.1 million ha (Spalding et al. 1997). The Sundarbans, the single largest tract of mangrove forest in the world covers about 1 million hectares in the delta of the river Ganges, Brahmaputra, and Meghna. Among the total area 60% lies in Bangladesh and the rest in India. This transboundary ecosystem is extremely important both ecologically and economically as it provides a nursery and breeding area for key fishes including those of the Bay of Bengal. The Sundarbans in Bangladesh covers an area of 6,017km 2 along its southwestern part sharing 4,143km 2 of land and 1,874km 2 of water bodies comprising of hundreds of creeks, canals, small and large rivers, and estuaries. This mangrove forest was declared a Ramsar site by the Convention of Wetlands of International Importance in 1992 and declared as a Natural World Heritage Site by UNESCO in 1997 ( Figure 1). Despite continued degradation, the Sundarbans contributes 3% to the country's gross domestic product out of 5% contribution of the country's forestry sector (Roy & Alam 2012).
The fish diversity of the brackish water ecosystem of the Sundarbans is usually associated with tolerance to a wide range of salinity fluctuation and migration. The freshwater fish species having low salinity tolerance enter into upper estuarine zone mainly in the period of ebb tide, while marine fishes are usually confined to the lower zone. Though some species travel freely in the whole salinity area for a major part of the year, very few can be considered as 'native' (Mishra 2017). Basically, most of the fish species enter into the brackish waters of the Sundarbans and spend for a certain period of their life cycle there either for shelter and feeding or for spawning purposes. The major threat to fishes of the Sundarbans region are environmental changes, reduction of freshwater discharge during lean seasons, increased salinity, use of destructive fishing gear (e.g., set bag net, small mesh size gill net), over exploitation, extraction of resources, and pollution.
Information regarding the diversity of fish in the Sundarbans of Bangladesh is scattered. Fishes of the Sundarbans were first described in the study of Hamilton (1822). He described about 71 fish species in the Gangetic estuaries and 51 of them occurred from the Sundarbans. After the independence of Bangladesh in 1971, several researchers and authors published different scientific and conference papers, project reports, guides and books on fish faunal diversity especially since 1978. Only some of them can be considered as valid references on the species availability in the Sundarbans waters such as Seidensticker & Hai (1983), Rahman (1989), Acharya & Kamal (1994), Chantarasri (1994), Bernacsek (2001), Bernacsek & Haque (2001), Shah & Hossain (2006), and Rahman et al. (2009). The main objective of the present study is to assess the existing fish fauna of Sundarbans and accumulate all fish species from the valid records made so far. Additionally, we have reviewed the present fishing practices in the Sundarbans detrimental to fish biodiversity, and the national policies made for fisheries management and conserving fish diversity.

METHODS
We consulted the primary and previously published articles, records, and books on ichthyological studies in the Sundarbans to build this checklist. These collections are mainly taken for preparing the list of the fishes known to occur in the Sundarbans and their valid identification and confirmation. Unbiased and sincere efforts were made in accumulating such a valuable treasure.
In the present survey, specimens of fishes were sampled between June 2015 and July 2017 from the major rivers of the Sundarbans, viz., Baleswar, Shibsa, Passur, Shela, Kobadak, Kalindi, Kholpetua, and a few of the tidal estuaries, and adjacent marine habitat in the Sundarbans, with the help of local fishermen during fishing ( Figure 1). The fishes were also collected from the fish markets inside or near the Sundarbans of Khulna, Bagherhat, and Satkhira districts. The spellings of scientific names and species validity were checked following Fishbase (Froese & Pauly 2018) and the California Academy of Sciences Catalog of Fishes (Eschmeyer et al. 2018). The arrangement of families and order are made according to Nelson (2006) and Laan et al. (2014). The identification was made by using FAO fish species catalogues which present detailed taxonomic accounts of all known species of individual families. As the checklist is intended to be a master reference for the Sundarbans habitat conservation and management, we consulted the latest global IUCN Red List status of each species (IUCN 2018). For habitat preference, we consulted previous data, our primary observation, reference website (Froese & Pauly 2018) and different reference books (Siddiqui et al. 2007;Rahman et al. 2009). DNA barcoding through mitochondrial COI gene sequencing was done for the newly recorded species during the present survey and the sequence was submitted to GenBank.

RESULTS
Based on the previously published information, specimens housed in the Aquatic Bioresource Research Lab., SAU and observations in the present study, the updated list of fishes of Sundarbans, Bangladesh provides information of 322 species belonging to 217 genera of 96 families and 22 orders (Table 1). In the present checklist, we have not considered any description. The column named as "present study" of Table 1, signifies our primary data collected between July 2015 and June 2017 and "previous literature" signifies the names which were enlisted in previous work on Sundarbans conducted by different scholars. Among the enlisted fish, Chondrichthyes (cartilaginous fish) contains 23 genera, 11 families and six orders whereas bony fish (Osteichthyes) covers 194 genera, 85 families and 16 orders. Maximum numbers of fishes (165 species, 50.24%) were recorded from order Perciformes in Sundarbans, Bangladesh. The number of fish species recorded under 22 orders is given at Figure 2.
In the present article, we report four new distributional records of fishes from the Sundarbans region of Identification: Body color reddish-grey above and dull white ventrally. Small sized shark, with an elongate and slender body; snout markedly pointed and long. Mouth triangular, with well-developed labial folds. Skin fairly smooth.    Identification. Fin formula D 1 IV; D 2 I/8; P 1 14; P 2 I/5; A III/8. Body color grayish above and minute black dots on upper third body; sivery below; opercle golden. Body compresses; anterior moderately and posterior deeply.

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marine and brackish water fishes. In the present study, we have categorized all of the compiled fishes according to the Global IUCN status, which is given in the Table 1. About 54.35% species of enlisted fishes belongs to the category of "Not Evaluated" and only 4.04% of fishes are in "Data Deficient" (Figure 3). Around 33.23% of species are categorized as "Least concern", some of which are exploited for commercial purposes such as Elops machnata Forsskal, 1775; Gudusia chapra Hamilton, 1822; Coilia sp., Thryssa sp., Ilisha sp., Arius arius Hamilton, 1822; Sillago sihama Forsskal, 1775; Alepes sp. Among the fishes of the Sundarbans of Bangladesh enlisted in the present study 4.04% (13 species) of the species are "Near Threatened" and 10 species (3.11%) species are "Vulnerable" viz. Recent taxonomic studies of the family Leiognathidae (Pony fishes) suggest several changes; however, a total of seven species have been recorded in this family from the Sundarbans. Among them we found four species where Leiognathus brevirostris Valenciennes, 1835 was newly recorded in the Sundarbans. Pony fishes are small fishes and commercially not valuable. Those species are usually exploited for dried fish.
Puffer fishes belong to the family Tetraodontidae. A total of 13 species of puffer fish has been listed in this checklist from previous literature and the present study whereas Shamsuzzaman et al. (2015) recorded nine marine puffer fish species from Cox's Bazar located on the eastern coast. Among 13 species of puffer fish reported in the Sundarbans until now, we documented four new records of which three species, viz., Triacanthus biaculeatus Bloch, 1786; Diodon hystrix Linnaeus, 1758, and Ostracion cubicus Linnaeus, 1758 are locally new and the species Lagocephalus guentheri Miranda Ribeiro, 1915 is the first record in the country. This study also added a new described species, Chelonodontops bengalensis Habib et al., 2018 from the same family.
The Sundarbans is where mainland Bangladesh meets the Bay of Bengal, making the area a globally unique ecological niche. In the Indian part, 34 elasmobranchs under 10 families and 271 bony fishes belonging to 61 families are known from the Sundarbans (Pal et al. 2014). In the present checklist, 36% species have been found as the habitants of both marine and brackish water followed by 26% as exclusively marine, 25% as marine, freshwater & brackish, 11% freshwater and brackish, and 2% exclusively as freshwater fish (Figure 4).
To the best of our knowledge, no study has been performed to assess diversity and breeding status of fish exclusively for three protected wildlife sanctuaries in the Sundarbans. In the present study, we have tried to cover the sanctuaries to assess its existing species composition of fishes; however, detailed and year-round study is necessary. One of the major limitations in a year-round J TT  survey is the lack of a vessel suitable to go downstream of rivers and canals during the monsoon season due to the strong current of the heavy downwards water flow.
Kobadak, Kholpetua, Rupsa, Shibsa, Pashur, Baleshwar, Raimangal, Arpangasia, Sakbaria are the main rivers passing through the Sundarbans which constitute about 2,000km 2 of waterways (Khan 2011), in addition to numerous small rivers, canals and creeks. The Sundarbans in Bangladesh has been divided into northeastern freshwater, middle to southern moderately saline and western saline zones (Chaffey et al. 1985). Therefore, it has brackish water as well as fresh water fish available in the labyrinth of water bodies. The government made some regulations and passed acts to protect and maintain sustainable production of fish in the Sundarbans area which are executed and enforced by the Bangladesh Forest Department (BFD). For example, 18 'khals' (canals) in the buffer zone of the Sundarbans have been permanently closed for fishing to ensure natural breeding of fish under Khal Closure Regulation (1989). Further, canals of less than 25 feet width have been banned for fishing throughout the Sundarbans. Entire fish of these small canals can be caught easily using poison and trapping fish setting net from two ends. Fishing is prohibited in three wildlife sanctuaries of Sundarbans by Wildlife Sanctuary Regulations (1999). Close Season Regulation (2000) banned catching of three finfish species, viz., Pangasius pangasius, Plotosus canius, Lates calcarifer, from 1 May to 30 June every year inside the Sundarbans to ensure natural breeding. BFD also implements the banning of Hilsa fishing each year imposed by Bangladesh Government's Department of Fisheries during peak breeding season during a certain time of the month between September and October every year (e.g., 9-30 October in 2019).
Fishers of the Sundarbans use different kinds of harmful nets and gear for catching fish which cause damage to aquatic lives, such as monofilament gill nets (called current Jal) are responsible for the killing of different aquatic animals and small sized fishes. Finemeshed set bag nets (locally called Behundi Jal), pull and push nets (Thela Jal), fine-mesh mosquito nets (Chingri Pona Jal), long shore nets (Khuti Jal) have been identified as the most destructive among all the fishing gears in the Sundarbans. Catch mortality is very high for these nets. Set bag nets used for collecting shrimp fry in the estuary and rivers of the Sundarbans also catch eggs, spawn, and larvae of all species along with adult fish. It is highly detrimental for declining fish diversity. Local fishers also use pull nets to catch post larvae (PL) of shrimps which also hampers fishery growth. In such cases they dispose unwanted larvae onto land rather than being freed into water, resulting in wastage. Thus, these larvae do not get the opportunity to mature into fish. It has been observed that about 99 fin fish and fry of other shrimp species are discarded for collecting a single shrimp post larva (Rashid 2000;Azad et al. 2017). Considering such a detrimental effect, the Government of Bangladesh declared a regulation in 2000 where it was stated that "no person shall catch or cause to be caught fry or post larvae of fish, shrimp and prawns of any kind in any form and in any way in the estuary and coastal waters of Bangladesh" (MoFL 2000). Unfortunately, thousands of people still catch post larvae of fish, shrimps and prawns and market their catch.
Poison fishing is another ecosystem threatening practice of the locals. It is very alarming that some fishermen are illegally using lethal poison to catch fish including crabs and shrimps in the Sundarbans canals. They release poison into the water and collect the dying fish. The poison is so deadly that a few drops of it are sufficient to kill a large amount of fish. It also contaminates the water, planktons, and mangrove tree roots. As toxic water flows into the large rivers from canals, it is not only the fish species that are being destroyed, but the entire J TT aquatic ecosystem is also under threat. Such dangerous practices cause a great risk to the flora and fauna of the Sundarbans and may create long-term negative effects on its ecology. This illegal practice needs a close watch to stop it. Increased and regular patrolling inside the forests and, motivation and engaging fishers against this is under way to stop this practice.
Mangrove forest is the breeding and nursery ground of many fish species. In a prohibitive order, the BFD had banned fishing in all of the canals (around 450) in the Sundarbans for the two months of July and August in 2019 for ensuring safe breeding and for conservation. It has been also observed that the intensity of poison fishing is higher in these two months; however, more studies need to be carried out to accurately identify the canals and creeks where breeding of fish occurs and which fish breed especially in the downstream with their specific breeding seasons. Netting of fish was also banned in the beels (e.g., Andaria beel) and chatals of the Sundarbans from February to March in 2019 for smooth breeding as proposed in IRMP (2010). The beels and chatals are lake-like wetlands with static water but chatals are relatively smaller. Further, a chatal gets totally dried out in the late winter but a beel does not. Some chatals are located between Chandpai and Sharankhola range of the Sundarbans. Both of the wetlands are reservoirs of freshwater. Both waterbodies are the source of many small indigenous freshwater fish species such as Anabas testudineus, Clarias batrachus, Heteropneustes fossilis, Channa sp. etc. The actions taken by the Government of Bangladesh to protect the availability and diversity of fish in Sundarbans have been shown in Table 2. This table has been prepared based on UNESCO (2016) along with different acts and regulations made by Bangladesh Government. Besides, to prevent over fishing, the number of boat license certificates (BLC) provided by BFD to allow fishermen for catching fish inside Sundarbans were limited. The maximum number of annual BLC issuance has been 12,000. The first priority in issuing BLC is given to those boat owners who live within 5km area around the Sundarbans. The maximum limit of permits for a month is given for three times and 5-7 days fishing is allowed under one permit (UNESCO 2016).
This article is primarily aimed to compile the information generated by authors and previous workers on the occurrence of total fish species from the past to the present in the core and adjacent marine areas of the Sundarbans, Bangladesh. This checklist should be considered as a working document and several additions of records of fish species for Sundarbans are added with survey work, particularly in the unique aquatic ecosystem. Ban on fishing of the species Shilon (Silonia silondia), Vola (Johnius argentatus) and Air (Bagarius bagarius) lower than 12 inch.
Ban on fingerling and fish fry collection