A checklist of indigenous ichthyofauna in the Dhanu River and surrounding wetlands of Bangladesh: current status, population trends and threats

This study was conducted in the Dhanu River and adjacent waters at Mithamain upazila (sub-district) under Kishoreganj district of Bangladesh to prepare a check list of available native fishes with their availability status, conservation perspectives, habitat preferences, population trends and intimidations. Data were collected monthly by direct field survey, focus group discussions, and personal interviews with fishers 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 17.58% species was abundantly available, 27.47% was commonly available, 31.87% was moderately available and 23.08% was rarely available. Cypriniformes was found as the dominant order, consisting 37.36% of the fish species aggregation and Cyprinidae was the most dominant family with 32.97% of the entire species assemblage. Twenty four piscine species (26.37%) were under threatened category in Bangladesh which subsumed 3 critically endangered (3.29%), 11 endangered (12.08%), and 10 vulnerable species (10.99%). Notably globally threatened Cirrhinus cirrhosus, Channa orientalis, and Wallago attu were available there. Fish population trends of 24.18% and 59.34% of the entire fish species was found in decreasing trends in global and national level, respectively. Leading intimidation to the fish diversity was indiscriminately overfishing, followed by fishing by dewatering of wetlands, katha fishing method, use of deprecated fishing gears, climate change, etc. Minimization of anthropogenic impacts, assuring the flux of water round the year, enactments of fish laws, installation and management of 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 colossal area of open water fisheries resources comprising inland and marine fisheries (DoF 2018). Inland share of these resources has an irrevocable contribution in the economy and food habits of the Bangladeshi denizen (Kibria & Ahmed 2005;Hossain et al. 2006). Fish, the most consumed fisheries resource, has a wide range of species variation here and the number of fish species in wetlands differs from each other because of the distinction of ecology of those wetlands (Rahman 2005). Rahman (2005) recorded 265 indigenous freshwater fish species in Bangladesh, which were under 154 genera and 55 families. However, total 253 indigenous fish species were assessed by IUCN Bangladesh (2015) where 113 species were floodplain inhabitants, 104 were riverine and 36 were migratory species (travel floodplains and rivers) (IUCN Bangladesh 2015).
Haors are saucer or bowl shaped shallow depressions distributed along the north-eastern part of Bangladesh which include irrigation canals, large seasonally inundating agricultural lands and coalescence of hundreds of inter-affiliated beels (Hussain & Salam 2007;Pandit et al. 2015a;Pandit et al. 2015bPandit et al. , 2015b. Beels are permanent wetlands and constitute a major habitat for fish production (869 kg ha -1 ) in Bangladesh. Total area of beel in Bangladesh is nearly 114,161 ha, which is a contributor of 2.32% fish of country's yearly fish production (DoF 2018). As good as 700 rivers with their ramifications interspersed in the country, shape a waterway of total length about 24,140 km (Banglapedia 2012; IUCN Bangladesh 2015; Arefin et al. 2018). These rivers are the main perennial freshwater wetlands of Bangladesh which provide habitats for feeding, breeding, and dry season shelter to various fish species Natural resources are limited and their sustainable use is very much important for human being. For ascertaining it, biodiversity conservation is considered as major moves (Arefin et al. 2018). However, wild fish populations are disappearing gradually from the rivers, haors, beels, floodplains, and other wetlands of Bangladesh due to degradation and loss of the natural habitat, overexploitation, climate change, aquatic pollution, and other manmade and natural causes (Galib et al. 2013;IUCN Bangladesh 2015;Pandit et al. 2015a;Pandit et al. 2015b;Sufian et al. 2017). Dhanu River has an auspicious water quality that bolsters variety of aquatic lives (Kabir et al. 2019) but fish biodiversity of the Dhanu River and adjacent beels are at the verge of disappearance. Many of the fishes may already have amid the vulnerable, endangered and critically endangered categories which are unknown due to lack of research work as no research on the fish biodiversity of the Dhanu River has yet been done. For this reason, there is no list of existing fish species. Worldwide events of unprecedented extinction of variety of species have come to light with the help of the study on biodiversity (Baillie et al. 2004) again, Huda et al. (2009) and Pandit et al. (2015a maintained that, the biodiversity of fish should be known before apply a management tool on the fishery of a waterbody, which intimates the necessity of a profound research on fish biodiversity of these wetlands.
At this circumstance, an erudite research work is aspired to get a narrated overview with an updated list of existing fish species of the river Dhanu and its adjacent waters beside, for planning subsequent steps for the efficacious management of these resources and their conservation as well. However, the specific objective of this study was to prepare a checklist of indigenous fish species including their present availability status in the Dhanu River and adjacent wetlands in relation to the national and global conservation statuses, habitat preferences, population trends and intimidations.

Study area and period:
The current study was conducted in the Dhanu River and surrounding beels of Mithamain upazila (sub-district) under Kishoreganj district of Bangladesh.
Mithamain is located between 24°49' and 25°06' north latitudes and 91°27' and 91°49' east longitudes (Fig. 1). To collect primary data on fish availability, the study area was divided into 8 sampling sites and data were collected from each site starting on July 2016 to June 2018. Sampling spots were 4 sites in the Dhanu River, Farua Beel, Nikla Beel, Khunkhuni Beel and Deodhuri Beel (Fig. 1). Data analysis: Microsoft Office Excel, ver. 2010 was used for making simple statistical analysis and graphical demonstrations of accumulated data and for this, obtained data were uploaded in a computer after collection.

Figure 2. Status of fish species percentage against orders in the study area.
In this investigation Cyprinidae was found as the richest family with 30 species (32.97%) followed by Bagridae (8.79%), and others (Fig. 3). Galib (2015) reported Cyprinidae as most diversified family with 15 species in the Brahmaputra River followed by Bagridae (6 species

Present availability status of indigenous fish fauna:
This research showed that, there were 16 abundantly available, 25 commonly available, 29 moderately available and 21 rarely available fish species in the area of investigation (Fig. 4). It is indicated almost a quarter of fish species was in rarely available category whose occurrences were sporadic. Local fishers opined that, these species are going to be disappeared very soon.   Arefin et al. (2018) found 14 species as commonly available (23%), 28 species as moderately available (45%), and 20 species as rarely available (32%). The availability status of fishes in the study area is disparate a bit from the above studies, which might be due to the differences of geographical location, study period, study duration, water quality and differences in the hazards on each water bodies.

Habitat based status of fish fauna:
In Bangladesh there are three lucid and well known categories of freshwater loving fishes based on their residential status, viz. riverine, migratory and floodplain resident species. For instance, during the dry season when water level of the floodplains reaches to its nadir, several floodplain dwelling species take shelter in nearby perennial water bodies like rivers and deeper beels (IUCN Bangladesh 2015) which makes the categorization more complex. However, maximum lifespan of freshwater fishes is spending in rivers and/or perennial beels. Hence, in the present survey, amid the total existing fish species found, 54.95% were beel residents and rest 45.05% were detected as riverine dwellers (Fig. 5). Their availability status showed that highest portion of riverine resident species (31.71%) was found under rarely available category (Fig. 6).

Global conservation status of fish species:
Pursuant to the global conservation status, the highest percentage fish species was occupied by the least concern category (84.62%) followed by near threatened (9.89%), vulnerable (3.30%), and not evaluated (2.20%) (Fig. 7).
It might be notable that 3 globally threatened fish species namely Cirrhinus cirrhosus, Channa orientalis, and Wallago attu were found MA, CA and AA, respectively in the study area. Similar result was found by Galib (2015) in the Brahmaputra River where majority of the indigenous fish species (82.00%) was under least concern where the near threatened (9.00%), not evaluated (5.00%), threatened (2.00%), and data deficient (2.00%).   Global population trend: According to global population trend, highest percentage of the fish species of the present study was unknown category (57.14%) followed by decreasing (24.18%) and stable (18.68%) (Fig. 8). Similar trend was found in the Brahmaputra River by Galib (2015) where highest percentage of the fish species was unknown category (57%) followed by decreasing (25%), stable (13%) and not evaluated (5%). Joadder et al. (2015) mentioned the population trend of 51% fish species of Padma River as unknown and almost 24% were of declining status.   Top ten threats to the fish diversity: According to the respondents (N = 90), many manmade and natural drivers are responsible for destroying fish biodiversity and habitats (breeding, feeding and nursing grounds) of the river and beels. The leading intimidations to the fish Stable Decreasing Unknown Increasing Decreasing Unknown diversity of the studied wetlands was indiscriminately overfishing, followed by fishing by dewatering of wetlands, katha fishing method, use of deprecated fishing gears, climate change, etc. (Table 3)

CONCLUSION
Rich number of fish species during study period is a good allusion of healthy fish diversity in the waters which could be maintained and/or increased through further research and well management. On the contrary, poor availability status and decreasing trend of many fish species intimates the alarming present situation of the fisheries resources of these areas.
Considering these facts, a becoming management is urgent for these water bodies to avert any kind of catastrophes of this natural asset in future. Reduction of anthropogenic impacts, maintaining water flow, enactments of fish laws, fish sanctuary establishment, and raising public awareness can be effective for the conservation of existing fisheries resources.