Indigenous Freshwater Ichthyofauna in the Dhanu River and Surrounding Wetlands of Bangladesh: Species Diversity, Availability, and Conservation Perspectives

: A checklist of the native ﬁshes of the Dhanu River and adjacent waters at Mithamain Upazila under the Kishoreganj district of Bangladesh is prepared with details on their availability, conservation status, habitat preferences, population trends, and threats. Data were collected monthly by ﬁeld survey, focus group discussions, and personal interviews by using a semi-structured questionnaire and a pictorial check list of ﬁsh species. A total of 91 indigenous ﬁsh species of 59 genera belonging to 29 families under 11 orders were documented where 17.6%, 27.5%, 31.9%, and 23.1% species were abundantly available, commonly available, moderately available, and rarely available, respectively. Among those, 24 ﬁsh species (26.4%) were under the threatened category (3 critically endangered, 11 endangered, and 10 vulnerable) according to the IUCN Bangladesh. Notably, the globally threatened Cirrhinus cirrhosus , Channa orientalis , and Wallago attu were available in the study area. Fish population trends of 24.2% and 59.3% of the entire ﬁsh species were found in decreasing trends at the global and national level, respectively. The leading threats to the ﬁsh diversity were indiscriminate overﬁshing followed by ﬁshing by dewatering of wetlands, the katha ﬁshing method, the use of deprecated ﬁshing gears, etc. Minimizing anthropogenic impacts, enacting ﬁshing laws, installing and managing ﬁsh sanctuaries, and raising public awareness can be effective for the conservation of existing ﬁsheries resources.


Introduction
Bangladesh is a realm of wetlands with a diversified area of open water fisheries resources comprising inland and marine fisheries [1]. The inland share of these resources has an irrevocable contribution in the economy and food habits of the Bangladeshi citizens [2,3]. By position, the country ranked third in inland open water capture fisheries production and attained the fifth position in aquaculture production in the whole world [4]. Fish constitutes the major source of protein, consisting of about 60% of the available animal protein in the Bangladeshi people's diet [1]. Fish, the most consumed fisheries resource, has a wide range of species variation in Bangladesh, and the numbers of fish species in the wetlands differ from each other because of the distinct ecologies of those wetlands [5]. Rahman [5] recorded 265 indigenous freshwater fish species in Bangladesh, which were under 154 genera and 55 families. However, only 253 indigenous fish species were assessed by IUCN Bangladesh [6], where 113 species were floodplain inhabitants, 104 were riverine, and 36 were migratory species (travel floodplains and rivers) [6].

Data Collection Framework
The data were gathered monthly from the study area by field survey and specimen collection [8,9,22]. Information about present condition of existing fishes were collected by means of personal interview with fishers, fishing vessel owners, fish traders, native people, riverside residents, and other forthcoming people from the sampling areas. Focus group discussions were also performed in fish landing centers and fishers' villages near those selected sampling sites with a pictorial check list of indigenous fish species and a semi-structured questionnaire.
Samples of different fish species were collected directly from the catch of fishers, fish landing stations of the selected sites and from retail markets as well. The collected fish

Data Collection Framework
The data were gathered monthly from the study area by field survey and specimen collection [8,9,22]. Information about present condition of existing fishes were collected by means of personal interview with fishers, fishing vessel owners, fish traders, native people, riverside residents, and other forthcoming people from the sampling areas. Focus group discussions were also performed in fish landing centers and fishers' villages near those selected sampling sites with a pictorial check list of indigenous fish species and a semistructured questionnaire.
Samples of different fish species were collected directly from the catch of fishers, fish landing stations of the selected sites and from retail markets as well. The collected fish samples were identified in the field to the species level by the examination of the morpho-metric and meristic attributes, following some previously published literatures [5,6,23,24]. The fish samples, which were found difficult to identify in the field, were preserved in 10% buffered formalin and transported to the laboratory of the Department of Aquatic Resource Management, Sylhet Agricultural University, Sylhet, for appropriate identification. Identified species of fish were grouped into 4 categories based on people's perceptions and frequency of occurrence (percent of surveys in which respective species were documented by the researcher) modified from Myers et al. [25] and Jadhav et al. [26]. The categories are as follows: abundantly available (AA): species plentifully observed throughout the year (frequency of occurrence: 76-100%); commonly available (CA): species generally observed throughout the year but in small numbers (frequency of occurrence: 51-75%); moderately available (MA): species observed infrequently in the study area (frequency of occurrence: 26-50%); and rarely available (RA): species observed occasionally in very small numbers (frequency of occurrence: 1-25%). The national and global conservation status and population trends were determined by following IUCN Bangladesh [6] and IUCN [27].

Diversity of Fish Species
In total, 8 sampling sites represented the existence of 91 indigenous fish species, which were under 59 genera, 29 families, and 11 orders (Table 1). Since no research work on fish biodiversity of the Dhanu River has been done, it deters any kind of comparison of the current findings. A similar limitation was also confronted by several other scientists while assessing fish diversity in different wetlands of Bangladesh [14,15,22,28]. However, the present study can be compared with other rivers of Bangladesh. Almost similar findings were documented by Gain et al. [29], and they reported 95 fish species under 14 orders, 45 families, and 77 genera from the Passur River. The number of species was higher in the Passur River than the Dhanu River because the Passur River is situated in the intertidal coastal region where both freshwater and marine fishes were available. On the other hand, Joadder et al. [30] reported 71 fish species from the Padma River, those were under 10 orders, 26 families, and 54 genera. Arefin et al. [10] found 62 fish species under 23 families after investigating the Rupsa River; Islam et al. [31] recorded 75 fish species under 25 families from the Juri River in the haor basin of the Sylhet district. All the above studies showed lower fish diversity compared to the present study. The possible reasons behind the availability of fish species in higher numbers in the present study areas may be the suitable water quality [13], the geographical location of wetlands in the haor basin, and being a tributary of the biodiversity-rich Meghna River [12]. Pramanik et al. [32] found 107 fish species searching the Meghna River, which are under 36 families and 13 orders. However, Mahalder & Mustafa [33] obtained a better finding in fish species combination, which was 126 fish species contributing to 39 families from the haor region of the Sunamganj district.

Present Status of Indigenous Fish Fauna
The current research showed 16 abundantly available (AA), 25 commonly available (CA), 29 moderately available (MA), and 21 rarely available (RA) fish species in the area of investigation (Figure 4). It is indicated almost a quarter of fish species was in the rarely available category, whose occurrences were sporadic. By observing their declining population trends, local fishers opined that RA species might be disappeared from the river in future.

Present Status of Indigenous Fish Fauna
The current research showed 16 abundantly available (AA), 25 commonly available (CA), 29 moderately available (MA), and 21 rarely available (RA) fish species in the area of investigation (Figure 4). It is indicated almost a quarter of fish species was in the rarely available category, whose occurrences were sporadic. By observing their declining population trends, local fishers opined that RA species might be disappeared from the river in future.

Habitat Based Status of Fish Fauna
In Bangladesh there are three well known categories of freshwater fishes based on their residential status, such as riverine, migratory, and floodplain resident species. For instance, during the dry season when the water levels of the floodplains reach their nadir, several floodplain-dwelling species take shelter in nearby perennial water bodies such as rivers and deeper beels [6], which makes their categorization more complex. However, the maximum lifespan of freshwater fishes is spent in rivers and/or perennial beels. Hence, in the present survey, the total existing fish species were categorized in to two categories where 54.9% were beel residents and the remaining 45.1% were riverine resident species ( Figure 5). Their availability status showed that the highest portion of riverine resident species (31.7%) was found under the rarely available category ( Figure 6).

Habitat Based Status of Fish Fauna
In Bangladesh there are three well known categories of freshwater fishes based on their residential status, such as riverine, migratory, and floodplain resident species. For instance, during the dry season when the water levels of the floodplains reach their nadir, several floodplain-dwelling species take shelter in nearby perennial water bodies such as rivers and deeper beels [6], which makes their categorization more complex. However, the maximum lifespan of freshwater fishes is spent in rivers and/or perennial beels. Hence, in the present survey, the total existing fish species were categorized in to two categories where 54.9% were beel residents and the remaining 45.1% were riverine resident species ( Figure 5). Their availability status showed that the highest portion of riverine resident species (31.7%) was found under the rarely available category ( Figure 6). maximum lifespan of freshwater fishes is spent in rivers and/or perennial beels the present survey, the total existing fish species were categorized in to two where 54.9% were beel residents and the remaining 45.1% were riverine reside ( Figure 5). Their availability status showed that the highest portion of riverin species (31.7%) was found under the rarely available category (Figure 6).

Global and National Conservation Status of Fish Species
In Figure 7 pursuant to the global conservation status, the highest percentage fish species was occupied by the least concern category (84.6%) followed by near threatened (9.9%), vulnerable (3.3%), and not evaluated (2.2%). It might be notable that globally vulnerable fish species, namely, Cirrhinus cirrhosus (Bloch, 1795), Channa orientalis Block & Schneider, 1801, and Wallago attu (Bloch & Schneider, 1801), were found to be MA, CA, and AA, respectively, in the study area. Similar results were found by Galib [15]

Species (%) Present availability status
Beel resident species Riverine resident species

Global and National Conservation Status of Fish Species
In Figure 7 pursuant to the global conservation status, the highest percentage fish species was occupied by the least concern category (84.6%) followed by near threatened (9.9%), vulnerable (3.3%), and not evaluated (2.2%). It might be notable that globally vulnerable fish species, namely, Cirrhinus cirrhosus (Bloch, 1795), Channa orientalis Block & Schneider, 1801, and Wallago attu (Bloch & Schneider, 1801), were found to be MA, CA, and AA, respectively, in the study area. Similar results were found by Galib [15] in the Brahmaputra River, where a majority of the indigenous fish species (82.0%) were under least concern followed by the near threatened (9.0%), not evaluated (5.0%), vulnerable (2.0%), and data deficient (2.0%).

Global and National Conservation Status of Fish Species
In Figure 7 pursuant to the global conservation status, the highest percentage fish species was occupied by the least concern category (84.6%) followed by near threatened (9.9%), vulnerable (3.3%), and not evaluated (2.2%). It might be notable that globally vulnerable fish species, namely, Cirrhinus cirrhosus (Bloch, 1795), Channa orientalis Block & Schneider, 1801, and Wallago attu (Bloch & Schneider, 1801), were found to be MA, CA, and AA, respectively, in the study area. Similar results were found by Galib [15] in the Brahmaputra River, where a majority of the indigenous fish species (82.0%) were under least concern followed by the near threatened (9.0%), not evaluated (5.0%), vulnerable (2.0%), and data deficient (2.0%). Comparing with the national conservation status, the highest position was occupied by the least concern category with 49 species (53.8%), followed by near threatened (17.6%), endangered (12.1%), vulnerable (11.0%), critically endangered (3.3%), and data deficient (2.2%) ( Table 2). Notably, 26.4% of the fish species in the present study were under the threatened category of IUCN Bangladesh [6]. Pramanik et al. [32] found 20.0% of the total identified species in the threatened condition in the Meghna River, where 2.0% species were critically endangered, 7.5% were endangered, and 10.3% were in the vulnerable condition.

Global Fish Population Trend
According to the global population trends, the highest percentage of the fish species of the present study was in the unknown category (57.1%), followed by decreasing (24.2%) and stable (18.7%) (Figure 8). A similar trend was found in the Brahmaputra River by Galib [15], where the highest percentage of the fish species was in the unknown category (57.0%) followed by decreasing (25.0%), stable (13.0%), and not evaluated (5.0%). Joadder et al. [30] mentioned the population trend of 51.0% fish species of the Padma River as unknown, and almost 24.0% were of declining status.

Global Fish Population Trend
According to the global population trends, the highest percentage of the fish species of the present study was in the unknown category (57.1%), followed by decreasing (24.2%) and stable (18.7%) (Figure 8). A similar trend was found in the Brahmaputra River by Galib [15], where the highest percentage of the fish species was in the unknown category (57.0%) followed by decreasing (25.0%), stable (13.0%), and not evaluated (5.0%). Joadder et al. [30] mentioned the population trend of 51.0% fish species of the Padma River as unknown, and almost 24.0% were of declining status.

National Fish Population Trend
The population trend of over half (59.3%) of fish species found in the present study was in decreasing trend at the national level assessed by IUCN Bangladesh [6], and only 2.2% were found to be increasing ( Figure 9). Galib [15] found that the population trend of more than two-thirds of the total native fishes in the Brahmaputra River were showing a decreasing trend. A decreasing trend was recorded for the fish species in the Padma River, too [28].

National Fish Population Trend
The population trend of over half (59.3%) of fish species found in the present study was in decreasing trend at the national level assessed by IUCN Bangladesh [6], and only 2.2% were found to be increasing (Figure 9). Galib [15] found that the population trend of more than two-thirds of the total native fishes in the Brahmaputra River were showing a decreasing trend. A decreasing trend was recorded for the fish species in the Padma River, too [28].

Threats to the Fish Diversity
Despite the fact that the Dhanu River and adjacent beels are rich in fish species, there are concerns regarding the fish biodiversity's long-term sustainability due to several manmade and natural drivers that are responsible for destroying fish biodiversity and habitats (breeding, feeding, and nursing grounds) of the river and adjacent beels. The leading intimidations to the fish diversity of the studied wetlands were indiscriminate overfishing (92.2%), followed by fishing by dewatering of wetlands (87.8%), the katha fishing method (86.7%), the use of deprecated fishing gears (78.9%), climate change impacts (75.6%), siltation and sedimentation (72.2%), intensification of agricultural farming (67.8%), natural drought in winter and summer seasons (66.7%), construction of development and communication infrastructures (58.9%), and water pollution (51.1%) ( Table 3). In addition, the conversion of wetlands to human settlement, lack of alternative incomegenerating activities of fishers, navigation, tourism, non-native fish species, and revenueoriented leasing systems have direct or indirect impacts on fish diversity. Notably, nonnative fish species might have devastating impacts on native species, causing extinctions and affecting natural ecosystems, which were unexplored due to poor knowledge of fishers. However, 10 non-native fish species, Barbonymus gonionotus (Bleeker, 1849), Clarias gariepinus (Burchell, 1822), Ctenopharyngodon idella (Valenciennes, 1844), Cyprinus carpio Linnaeus, 1758, Hypophthalmichthys nobilis (Richardson, 1845), Hypophthalmichthys molitrix (Valenciennes, 1844), Oreochromis mossambicus (Peters, 1852), Oreochromis niloticus (Linnaeus, 1758), Pangasianodon hypophthalmus (Sauvage, 1878), and Pterygoplichthys pardalis (Castelnau, 1855), were recorded in the Dhanu River and the surrounding floodplains. Talukder et al. [22] found similar threats to the fish diversity of the Shari-Goyain River in northeastern Bangladesh. Overfishing, lack of water flow in the winter season, water pollution, siltation, indiscriminate harvesting of brood and juvenile fishes, catching fish with Increasing (2.2%) Decreasing (59.3%) Unknown (38.5%) Figure 9. National fish population trend.

Threats to the Fish Diversity
Despite the fact that the Dhanu River and adjacent beels are rich in fish species, there are concerns regarding the fish biodiversity's long-term sustainability due to several manmade and natural drivers that are responsible for destroying fish biodiversity and habitats (breeding, feeding, and nursing grounds) of the river and adjacent beels. The leading intimidations to the fish diversity of the studied wetlands were indiscriminate overfishing (92.2%), followed by fishing by dewatering of wetlands (87.8%), the katha fishing method (86.7%), the use of deprecated fishing gears (78.9%), climate change impacts (75.6%), siltation and sedimentation (72.2%), intensification of agricultural farming (67.8%), natural drought in winter and summer seasons (66.7%), construction of development and communication infrastructures (58.9%), and water pollution (51.1%) ( Table 3). In addition, the conversion of wetlands to human settlement, lack of alternative income-generating activities of fishers, navigation, tourism, non-native fish species, and revenue-oriented leasing systems have direct or indirect impacts on fish diversity. Notably, non-native fish species might have devastating impacts on native species, causing extinctions and affecting natural ecosystems, which were unexplored due to poor knowledge of fishers. However, 10 non-native fish species, Barbonymus gonionotus (Bleeker, 1849), Clarias gariepinus (Burchell, 1822), Ctenopharyngodon idella (Valenciennes, 1844), Cyprinus carpio Linnaeus, 1758, Hypophthalmichthys nobilis (Richardson, 1845), Hypophthalmichthys molitrix (Valenciennes, 1844), Oreochromis mossambicus (Peters, 1852), Oreochromis niloticus (Linnaeus, 1758), Pangasianodon hypophthalmus (Sauvage, 1878), and Pterygoplichthys pardalis (Castelnau, 1855), were recorded in the Dhanu River and the surrounding floodplains. Talukder et al. [22] found similar threats to the fish diversity of the Shari-Goyain River in northeastern Bangladesh. Overfishing, lack of water flow in the winter season, water pollution, siltation, indiscriminate harvesting of brood and juvenile fishes, catching fish with destructive fine-meshed nets, the katha fishing method, and habitat destruction have negative effects on all aquatic fauna, particularly fish [22]. According to local fishers of the Rupsa River, water depth reduction by siltation and sedimentation (91.0%), overfishing and navigation (90.0%), the use of illegal fishing gear (86.0%), waste pollution (72.5%), urbanization (64.0%), temperature fluctuation (61.8%), and turbidity (56.0%) are the main causes for fish diversity reduction in the Rupsa River [10]. Flowra et al. [37], Pandit et al. [8], Sultana et al. [35], and Islam et al. [31] found similar types of causes responsible for species reduction in the different inland waters of Bangladesh. Based on surveys and direct observations, the current study suggested different measures for conserving fish diversity by minimizing the threats to these wetlands. The reduction of anthropogenic impacts, enactments of fishing laws, installation and management of fish sanctuaries, and raising public awareness can be effective for the conservation of existing fisheries resources in the natural waters of Bangladesh. Due to a lack of funding, the current study contains several minor drawbacks, especially the collection of fish samples from the fishers and fish traders rather than direct sampling through fishing by hired fishers. As a result, the sample size was limited, with the only purpose being proper identification. In addition, threats were recorded based on stakeholder perceptions via a survey and researchers' observations. Therefore, a more in-depth study is required in the future for more specific information in order to develop better management strategies for the wetlands.

Conclusions
This is the first attempt at looking into the fish species diversity of the Dhanu River, and the study focused primarily on documenting the available native (and implicitly to non-native) fish species of the river and adjacent beels. Fish species richness is a good allusion of healthy fish diversity in the waters, which could be conserved. On the contrary, the poor availability status and decreasing trend of many fish species intimates the alarming present situation of the fisheries resources. Furthermore, the current study also observed that the freshwater fish species are affected by a range of anthropogenic and natural threats, and it suggested a number of conservation solutions. Moreover, ecosystem-based fisheries management with local community participation is highly recommended for these water bodies to conserve fish diversity and to prevent any kind of catastrophes in the future.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.