Ecological Insights, and Fin Fish Diversity in Carps Spawning Grounds: Case Studies from the Surma River and Tanguar Haor, Bangladesh

Tis study aims to provide a thorough ecological understanding of fn fsh diversity within carp spawning grounds in the Surma River and Tanguar haor. Over two spawning seasons, this research investigates ecological factors impacting fn fsh diversity and abundance in carp spawning grounds of the Surma River and Tanguar haor, emphasizing water quality, habitat features, spawn availability, and environmental infuences. Fish spawn samples were collected with eight “Savar nets” at chosen sites and reared in a fberglass tank at the mini hatchery for species identifcation. Te representative spawn samples were examined under a microscope for preliminary species identifcation before rearing. Te study found that both the Surma River and Tanguar haor ofer ideal conditions for carp spawning due to factors such as water quality, natural food availability, habitat suitability, and favorable climatic conditions. Te study identifed 39 fsh species under the 10 fsh groups from both locations, with a higher percentage of carp species (31.42%) in the Surma River in 2021 compared to 2022 (22.50%). In Tanguar haor, the percentage of carp species was 7.55% and 6.50% in 2021 and 2022, respectively. Te Surma River’s ecological indices (2021-2022) indicated decreased diversity, likely due to environmental degradation, while Tanguar haor showed lower diversity possibly attributable to multiple environmental stressors. Te dominant carp species, Labeo calbasu , Labeo bata , and Labeo gonius , were identifed in both the Surma River and Tanguar haor. Te spawning distribution varied among diferent locations, with some sites showing a presence of carp species, such as Hajipur ( T 1 ) in the Surma River and Alam Duar ( T 2 ) in Tanguar haor. Te fndings highlight the importance of hydrological and environmental parameters in shaping carp spawning habitat distribution and abundance, contributing to aquatic biodiversity conservation and resource management.


Introduction
Fish and fsheries play an important part in the economy and livelihoods of rural communities [1,2].Te fsheries industry is one of the most remarkably productive and vibrant sector, contributing extensively to the country's socioeconomic development [3,4].After China and India, Bangladesh has the third-highest fsh diversity in Asia, with over 800 distinct species [5,6].Te freshwater fsheries assets are widely acknowledged for their remarkable abundance and variety, encompassing a minimum of 265 types of fnfsh [7,8].Te Surma River and Tanguar haor are two essential aquatic ecosystems in Bangladesh that support a diversity of fsh species and provide signifcant contributions to the country's fsheries industry [9][10][11].Te Surma River plays an important role in the fsheries of the Sylhet region.As a key drainage system in the region, the Surma River supports a diverse aquatic ecosystem, serving as an ideal habitat for numerous fsh species [12,13].An ecological haven referred to as a sanctuary or safe place of unrivaled signifcance is the Tanguar haor, which is treated as the largest freshwater wetland in Bangladesh.Tis distinctive habitat contains a nexus of rivers, beels (oxbow lakes), and marshes to support diverse forms of an exquisite tapestry of species and is therefore designated as a Ramsar site [11,14].Te fresh waters of the haor are home to a great array of fora and wildlife, including large carp that are essential to the food security and economic well-being of the surrounding areas.
Bangladesh's water bodies are home to the Indian major carp (IMC), encompassing four distinct species: Catla (Catla catla), Roho (Labeo rohita), Mrigel (Cirrhinus mrigala), and Kalibaush (Labeo calbasu) [15,16].Tese prominent carp varieties are highly regarded in the fsheries industry due to their commercial and nutritional value.Remarkably, major carp species contribute a substantial 26% to total fsh production [17], efectively addressing nutritional requirements while bolstering revenue streams.Despite the ecological importance of major carp species and the vital role they play in the region's economy, knowledge of their natural spawning habitats and reproductive behavior is still inadequate.Based on the diferences in spawning locations, spawning periods, and geographical dispersion, Bangladesh's major carp species are conventionally segmented into four stocks: (i) the stocks of the Brahmaputra-Jamuna, (ii) the upper Padma, (iii) the upper Meghna, and (iv) the Halda are listed in that order [18,19].Tere used to be natural carp spawning grounds in several rivers in the larger Sylhet area, including the Surma, Kushiara, and Monu Rivers [20,21].Aquaculture activities during the early 1960s and 1970s mostly involved raising natural carp hatchlings from the Jamuna, Padma, Boral, and old Brahmaputra rivers, as well as fertilized eggs during the monsoon from the Chittagong Halda River [22].Te natural carp seed population was 0.67% in 2012 and 0.39% in 2020, representing a 50% drop over the previous ten years [17,23].On the other hand, highquality fsh seed has been provided to the aquaculture sector by government and private hatcheries.Over time, fsh seeds' quality has deteriorated, specifcally in private hatcheries.Some of the factors infuencing the poor quality and bringing the genetic variety of IMC (Indian Major Carp) natural populations into peril are genetic inbreeding, interspecifc hybridization, and cross-breeding [24].
Owing to an interplay of diverse natural and humaninduced infuences, it is anticipated that spawning habitats may vanish or face extinction shortly [25].As noted by Brander [26], climate change has demonstrated its impact not only directly on physiology, behavior, growth, reproduction, mortality, and distribution but also indirectly on the productivity, structure, and composition of aquatic ecosystems that serve as essential sources of sustenance and refuge for fsh.In a prior study, it was also identifed that fsh have a close connection to both their biotic and abiotic environments.Te life cycle, maturation, and growth of fsh are strongly infuenced by temperature, DO, pH, free CO 2 , alkalinity, and other salts.Small changes in such variables have an impact on fsh growth, development, and maturity [27,28].Te spawning sites used by the carp species are comparable, and they have similar environmental requirements [29].Adult carp begin their spawning migration in early March due to rains, rising water temperatures, and increasing water fow from the Himalayas.Spawning starts in May with the arrival of the southwest monsoon rains and lasts until July [18,30].Very little research has been done on the spawning locations and spawning efciency of major carp in Bangladesh.To enhance natural carp spawn production, it is crucial to investigate spawning sites in the Surma River and Tanguar haor.However, this study aims to provide a thorough ecological understanding of fn fsh diversity within carp spawning grounds in the Surma River and Tanguar haor.Te fndings highlight the importance of hydrological and environmental parameters in shaping carp spawning habitat distribution and abundance, contributing to aquatic biodiversity conservation and resource management.

Study Area and Design.
Te investigation was carried out in the Surma River and Tanguar haor over the two-spawning periods from March to August 2021 and 2022.A large river in northeastern Bangladesh, the Surma River is well-known for having a remarkable impact on the hydrology and ecology of this area.Tis river's topology describes its structural features, including its course, tributaries, general spawning areas, spawning seasons, and geographic distribution.
Tanguar haor, located on the northern outskirts of Bangladesh at 25.1615 °N and 91.0778 °E, is one of the largest river basins, covering 9,727 ha (https://rsis.ramsar.org/ris/1031) in the Surma-Kushiyara river basins.It has a diverse ecosystem, including swamp forest land that plays a significant role in fsh production and serves as a "Mother fshery" for the country [31].Eight sampling transects were chosen from incentive areas along the Surma River and Tanguar haor with the help of the Upazila Fisheries Ofce, the fshing community, frequent visits, and previous research study [32] (Figure 1).Tese transects were divided into groups consisting of reference and treatment for the Savar net setting and sample collection (Table 1).Each station contains one spawning reference location that has been described in prior research.

Water Quality Measurement
. Key parameters such as temperature, dissolved oxygen (DO), pH, turbidity, total dissolved solids (TDS), conductivity, and ammonia play pivotal roles in assessing the overall health of aquatic ecosystems.Temperature, DO, pH, and turbidity were measured using precision instruments including the Hack multimeter, PHC 101 pH probe, DO probe, and Sacchi disk.TDS and conductivity were measured using both the amber bottle and beaker method, as well as a multiparameter device featuring a CDC-401 probe.Te concentration of ammonia, a crucial indicator of water quality, was assessed using the HACH HQ40d multiparameter analyzer.Daily water quality assessments were carried out using a range of instruments during the critical spawning period.Tis systematic approach ensures the thorough monitoring and management of aquatic environments, contributing to the preservation and sustainability of ecosystems.

Natural Food Abundance (Plankton Analysis).
Te plankton samples were thoroughly collected from the study area during the last week of each month, precisely between 8 and 9.30 a.m.Te collection process involved sieving 50 liters of water from the habitat, approximately 8-10 cm below the water's surface.Tis water was then transported through a 25 μm mesh plankton net to capture the plankton.Subsequently, the collected plankton was concentrated to 25 ml.To preserve the samples for further analysis, a 5% formalin solution was used.Te samples were carefully stored in a dark environment for one day to facilitate the precipitation process while minimizing the infuence of sunlight, ensuring accurate measurements.Te qualitative and quantitative analysis of the plankton was conducted using an electric microscope (Olympus Xcam-Alpha, Germany) to provide detailed insights into their characteristics and abundance.Each sample was stirred gently before examination under the microscope.Te identifcation of plankton was performed using keys provided by [33] and APHA [34].To count the plankton number per liter sample using the drop count method, the following formula was applied: Total plankton count per liter where A � number of organisms per drop, L � volume of the original sample (50 liters); n � total volume of the concentrated sample (25 ml), v � volume of one drop (0.04 ml).In this study, n was 25 ml, L was 50 liters, and v was 0.04 ml (equivalent to 25 drops per 1 ml).

Meteorological Condition Analysis and Habitat
Characteristics.Te meteorological and environmental data related to precipitation and water level were collected from the data repository of the Power Data Access Viewer of NASA and https://www.hydrology.bwdb.gov.bd(retrieved on June 21, 2022).In addition, the habitat characteristics of the chosen study area were visually observed through regular visits to the site.

Fish Spawn Collection and Identifcation Process.
Te fsh spawn at the study sites was collected by using a Savar net.Te fsh spawn was sampled at six-hour intervals in each site employing a Savar net in the hapa setting.To determine the presence of carp spawn in the collected samples, a series of further steps were followed, which involved sample collection, transportation, rearing, and eventual identifcation in the laboratory (Figure 2).Subsequently, these samples were transported to the SAU's mini hatchery using oxygen bags for further examination.
Te spawn collection process is outlined below:

Savar Net Preparation and Setting. Te traditional
Indian major carp spawn fshing technique uses a "Savar net," a fxed funnel-shaped net deployed mainly in the Ganges-Padma and Brhamaputra-Jamuna River systems.Tis net is small, anchored to shallow river shores, and equipped with a collection pocket.It catches microscopic fsh eggs during the monsoon season.To prevent escape, the upper tail bag border is kept 4-5 cm above the water.In this research study, eight nets were placed strategically at sampling stations 30-50 meters apart in the Surma River and Tanguar haor.Professional fshermen collected fsh spawn samples from each net monitored four times daily at six-hour intervals.

Spawn Collection.
In 2021, the Savar net on the Surma River captured 1.01 kg of fsh spawn, leading to a 200-gram sample for analysis.By 2022, this amount had surged to 3.90 kg, prompting a larger 400-gram sample collection.In contrast, in 2021, only 0.40 kg of fsh spawn was found in the Tanguar haor Savar net, leading to a 100-gram sample.By 2022, this had increased to 0.60 kg, with a 200-gram sample collected for analysis.

Spawn Transportation to SAU Mini Hatchery, Rearing,
Identifcation, and Preservation.Upon gathering the samples, they were promptly transported and placed in the raising tank at SAU's mini hatchery.After preliminary identifcation, the collected samples were cultivated for 30 days.During this period, the fsh seeds were meticulously reared using appropriate management techniques, including feeding with egg yolk and subsequently foating powder feed from mega feed to facilitate nursing.After 30 days, the fsh spawn was about 1.5 to 2.5 cm in size and easily identifable.During the stocking of spawn in the raising tank, the ocular examination is used to identify fsh species and fnally preserve the fsh.

Data Processing and Analysis.
Before undergoing statistical analysis, the obtained data was organized, processed, and compiled in Microsoft Excel (2010).Statistical analysis and interpretation of the raw data, using Kruskal-Wallis ANOVA, were conducted using SPSS v24.Te Violin box Plot was performed in R-studio, PAST (version 4.03) was used for biodiversity indices, ArcGIS 10.8 and Google Earth Explorer were used for map formation [38].Te normality was tested by using Shapiro-Wilk, and the Kolmogorov-Smirnov Test (Supplementary table 1).As data did not exhibit a normal distribution pattern, the Kruskal-Wallis test was employed at p < 0.05 for comparing means (Supplementary table 2).

Results
3.1.Water Quality Environment.Spawns were collected from the Surma River and Tanguar haor between July 10th and July 14th in 2021 and on June 2nd and 3rd in 2022.During this period, the average temperature of the selected study site was 28 °C, where the maximum temperature (33 °C) was found in March and the lowest temperature (28 °C) was observed in June and July.Te dissolved oxygen was found to be 5.9 to 7.4 ppm during spawning time, and the maximum value was observed at spawning time.Te pH of the spawning period was 7.4, which was satisfactory for spawning (Table 2 and Figure 3).Te TDS ranges from 37 to 39 ppm during the spawning period.Water turbidity is very important for the spawning of carp species, with observed values of 18-20 NTU during the spawning time.Te ammonia level in the study area was ideal in both years (Table 2).Te maximum value of the EC was found to be 89 μS/cm, and the lowest was 80 µS/cm.During the spawn collection time, the EC was observed at 84-87 μS/cm in both stations (Figure 3). 4 Scientifca

Natural Food Abundance.
Carp species rely on plankton as a crucial food source for their growth, survival, and reproduction.Plankton serves as an indicator for assessing the productivity and fshery potential of diferent water bodies due to their importance in the fsh food chain.A total of 37 planktonic taxa were found among them 23 taxa belonged to phytoplankton and 14 taxa were classifed as zooplankton.Chlorophyceae (11 taxa), Bacillariophyceae (4 taxa), Cyanophyaceae (6 taxa), and Euglenophyceae (2 taxa) were further divisions of the phytoplankton group.Rotifer (4 taxa), Cladocera (5 taxa), Copepods (3 taxa), and Protozoans (2 taxa) were all included in the zooplankton group (Tables 3(a) and 3(b), and Figure 4).
Te fndings recorded Chlorophyceae as most abundant, constituting 25% of the population, while protozoans were the least abundant at 2%.Notably, Rotifers, which are essential zooplankton for carp feed, were found at a signifcant abundance of 14%.Overall, the planktonic community's abundance demonstrated that the study area provided a rich natural food source for carp species during the spawning period (Figure 5).

Meteorological Condition During the Spawning Period.
Te minimum rainfall and water level were noted in March, while the maximum values were observed in June and July.Tis pattern suggests a correlation between rainfall and water level fuctuations and the spawning success of carp species at the study site.Tese environmental factors were likely to play a signifcant role in infuencing the conditions for successful carp spawning (Figure 6).

Habitat Characteristics for Carp Spawning. Te Surma
River exhibits distinct sections and habitats, featuring slower-moving segments with gravelly or sandy bottoms, alongside riverbanks adorned with vegetation and submerged objects such as boulders and tree roots.In contrast, the shallow, marshy regions of Tanguar Haor harbor submerged plants ideal for spawning, ofering a conducive environment for safeguarding fry and attaching eggs of carp species.

Fish Spawn Status and Carp
Spawning.After 30 days of cultivation in a small hatchery at SAU, the fsh spawn reached a size of approximately 1.5-2.5 cm and was successfully identifed.Te study yielded a total of 10 groups of fsh species from both the Surma River and Tanguar haor, comprising 39 diferent fsh species (Figure 7).Te analysis of the collected data showed that carp spawn had the highest percentage (31.42and 22.50) among the fsh species in the Surma River, while featherbacks had the lowest percentage (0.29 and 0.26).Conversely, in Tanguar haor, barbs and minnows had the highest percentage, and featherbacks had the lowest.Specifcally, in the year 2021, the carp species accounted for 31.42% of the fsh population in the Surma River and 22.5% in the year 2022.In Tanguar haor, the percentage of carp species was 7.55% in 2021 and decreased to 6.50% in 2022.
Te fndings indicated the presence of minor carp species, namely, Labeo calbasu, Labeo bata, and Labeo gonius, both in the Surma River and Tanguar haor.In the case of the Surma River, the carp species Labeo calbasu exhibited the highest percentage (55.55% in 2021 and 38.33% in 2022), while Labeo bata showed the lowest percentage (6.67% in 2021 and 28.33% in 2022).Conversely, in Tanguar haor, the carp species Labeo gonius displayed the highest percentage (47.63% in 2021 and 48.39% in 2022), whereas Labeo calbasu had the lowest percentage (19.04% in 2021 and 16.13% in 2022) (Figure 8).Te study revealed that most of the carp spawn in the Surma River station, a signifcant portion, 95%, of the carp species was collected from Hajipur (T 1 ), whereas the lowest numbers were observed in Vurvuri Dor (T 2 ) and Chainkhair (T 3 ).Moreover, no carp spawn was found in Muradpur Bazar site.In Tanguar haor was discovered in Alam Duar (T 2 ) accounting for 80% of carp spawn, while the remaining 20% was found in Rowa Beel (T 3 ).However, no carp spawn was found in Gulabari-Joypur and Beshkhali-Gulduba (T 1 ) (Figure 9).6 Scientifca   Scientifca factors, including habitat fragmentation, invasive species, or natural resource exploitation.However, further investigation is needed to understand the specifc drivers behind these patterns and their implications for ecosystem health and management.Eight orders were identifed, with Cypriniformes exhibiting the highest species diversity, primarily dominated by carp species (Table 4 and Figure 11).Upon collecting the fsh sample, the Savar net contained various small indigenous species of fsh (SIS), including Mola (Amblypharyngodon mola), deshi Puti (Puntius sp.), Darkina (Esomus danricus), Chela (Chela cachius), Tengra (Mystus sp.), and Baim (Mastacembelus sp.), among others (Table 4 and Figure 11).In comparison to carp species, the SIS displayed a signifcantly larger size.Te SIS varied in size from 2.5 to 4.0 cm, depending on the specifc type of fsh.

Discussion
Te successful carp spawning in the Surma River and Tanguar haor can be attributed to several key factors.Te favorable water quality, availability of natural food, suitable habitat, and favorable climatic conditions provided an ideal environment for carp reproduction in both ecosystems.Tese fndings emphasize the importance of maintaining and preserving these vital components of the aquatic environment to support healthy fsh populations.In the past, many rivers in greater Sylhet, especially the Surma, Kushiara, and Monu rivers, served as the natural spawning grounds for various carp species [13,22,39].Te selection of the study area was made based on previous research studies and frequent visits.Scientists examined whether the parameters of temperature, turbidity, dissolved oxygen (DO), carbon dioxide (CO 2 ), pH, chloride, nitrite, and acidity at the Hetimganj site adhered to the established standard limits for fsheries.Te levels of total dissolved solids (TDS), alkalinity, and hardness were notably reduced in the rainy season [13,40].During the spawning time, the temperature, DO, pH, TDS, turbidity, and conductivity were 28 °C, 7.4 ppm, 7.4, 34 ppm, 19.5 NTU, and 81 μS/cm, respectively.According to the previous study, the accepted threshold for alkalinity is greater than 100 ppm, while transparency should measure 40 cm or below [41].TDS has a standard limit of 165 ppm and hardness has a standard limit of 123 ppm [42,43], while pH has a standard limit of 6.5 to 8.5 and DO has a standard limit of 5.0 ppm, as referenced by scientists [16,44].Te observed trends in carp species abundance and distribution highlight the signifcant infuence of environmental factors on spawning success.Te higher percentage of carp species in the Surma River compared to Tanguar Haor indicates variations in habitat suitability and water quality between these two locations.
Te study revealed that 23 taxa of phytoplankton were identifed, distributed among four groups: Chlorophyceae (11 taxa), Bacillariophyceae (4 taxa), Cyanophyaceae (6 taxa), and Euglenophyceae (2 taxa).Tese fndings were consistent with previous studies [45][46][47][48].It was also noticed that a total of 14 taxa of zooplankton were identifed, categorized as Rotifer (4 taxa), Cladocera (5 taxa), Copepods (3 taxa), and Protozoans (2 taxa).Tis fnding is aligned with previous research [45] in the beels of the haor region in Bangladesh, where they identifed 15 major taxa of zooplankton belonging to the Rotifer, Cladocera, and Copepod groups.Researchers [46] also conducted a study and identifed 24 diferent taxa of zooplankton, including Rotifer, Cladocera, Copepod, and Ostracoda.However, this Scientifca diverse planktonic community likely serves as a signifcant food source for carp, supporting their spawning success.Te abundance and distribution of phytoplankton, such as Chlorophyceae, Bacillariophyceae, Cyanophyaceae, and Euglenophyceae, along with zooplankton taxa like Rotifer, Cladocera, Copepods, and Protozoans, may directly infuence the availability of natural food for carp during spawning periods.Consequently, variations in planktonic community composition and abundance could afect the reproductive success of carp populations in these aquatic ecosystems.
Te weather was benefcial for rainfall and temperatures during the spawning time.Unpredictable rainfall patterns from late February through the frst week of March led to inadequate water levels in the rivers and beels of haors.However, this phenomenon rapidly triggers fsh breeding.
While some fsh species manage to breed during these unfavorable rainfall conditions, insufcient water levels hinder the larval development of fsh [47].Furthermore, due to the irregularity and scarcity of rainfall, along with reduced water levels in interconnected rivers and canals, the broodfsh struggle to reach the breeding grounds promptly [48].As a result, the climatic fuctuations caused by erratic rainfall disrupt breeding activity and fsh diversity.It was also denoted by the present study that there was a possession of optimal spawning habitat with slower river fow, vegetation, marshy land, and submerged substrate for carp.Shallow, slow-fowing waters with submerged vegetation or plant debris, sandy bottom, and stone were suitable for carp spawning (water.vic.gov.au).
Te comprehensive investigations conducted during this study led to the identifcation of a diverse array of fsh 14 Scientifca species.A total of 39 diferent fsh species belonging to 10 groups were observed in both the Surma River and Tanguar haor.Tis biodiversity showcases the ecological richness of these aquatic ecosystems and highlights their signifcance in sustaining various fsh species, including carp.Te study also revealed interesting trends in the percentage of carp species over the two-year observation period.Notably, in the Surma River, the percentage of carp species was higher in 2021 (31.42%) compared to 2022 (22.5%).Similarly, in Tanguar haor, the percentage of carp species showed a decline from 7.55% in 2021 to 6.50% in 2022.Te variations in carp species percentages over two years suggest potential shifts in fsh populations in the Surma River and Tanguar haor.Various ecological factors, like habitat alterations, water quality fuctuations, and human activities, may infuence these changes.Further investigation through long-term monitoring and ecological assessments is necessary to understand the underlying drivers and develop conservation strategies for sustaining fsh populations in these ecosystems.In a study conducted near Sylhet Sadar, northeastern Bangladesh, the Surma River is known to harbor 51 fsh species, categorized into 16 taxonomic groups [7,10].Te dominant family was Cyprinidae, accounting for 36% of the species which was closely related to the present study.Te absence of carp spawn at the Muradpur bazar site may be attributed to the absence of brood fsh, likely due to the lack of deeper portions in this segment of the Surma River.Over the past two decades, 143 species of fsh (137 native and Tanguar haor emerge as potential hotspots for carp spawning, indicating the need for targeted conservation eforts in these areas.
Researchers found that in the upper Meghna in Bangladesh, there is virtually limited information on carp spawning sites and spawn collection points.Tere are no commercial carp spawn harvesting centers in the upper Meghna River basin, unlike in other river systems [32].Tis stock's spawning could occur hundreds of miles upstream in India, or it could be so scarce that fry/spawn collectors are not interested.However, some authors claim that spawn collection locations can be found around the Surma River's headwaters in Manipur province, as well as in Tripura province in India [22,51].Recent studies have identifed seven carp spawn collecting areas in the larger Sylhet basin based on fshermen's catch data provided by Paul [52].Tese areas include the Juri River in Hakaluki haor near the Fenchugonj Bridge; the Kawani, Boroiya, and Baulai Rivers near Daulatpur and Milonpur in the Dharampasha Upazila; the Baulai River near Alamduarer bank in the Tahirpur Upazila; and the Dhanu river near Ranichapur.Tis study also suggests that potential spawning grounds for carp species could be located around the Surma River and Tanguar haor, where deeper water portions, locally referred to as "Dor," are present.

Conclusions
Te study ofers a comprehensive understanding of the ecological dynamics and fnfsh diversity in the carpspawning grounds of the Surma River and Tanguar haor in Bangladesh.Critical factors such as water quality, food availability, habitat suitability, and climatic conditions affecting carp spawning success are identifed.Tis underscores the need for conserving these ecosystems and implementing efective management strategies to sustain fsh populations, especially carp species.By elucidating relationships between hydrological, environmental, and biological parameters, the research enhances our knowledge of aquatic ecosystem dynamics and emphasizes the importance of informed conservation practices.Te fndings provide insights for future conservation eforts and responsible aquatic resource management.However, further research is recommended to explore the biodiversity of fsh, plankton, and other nonpiscine consumers, as well as catchability, for a more comprehensive ecological understanding.Overall, the study highlights the importance of preserving the ecological balance of these habitats to support fsh populations and their ecosystems.

Figure 1 :
Figure 1: Map displaying the selected study area of the Surma River and Tanguar Haor, created using ArcGIS and Earth explorer.

Figure 2 :Figure 3 :
Figure 2: Flowchart detailing the collection and identifcation process of carp spawns from the Surma River and Tanguar Haor.

Figure 6 :Figure 7 :
Figure 6: Linear regression of monthly rainfall (a, b) and water level (c, d) in the Surma River and Tanguar Haor, respectively.

OsteoglossiformesFigure 11 :
Figure 11: Order of freshwater fshes with representative species from the Surma River and Tanguar Haor.

Table 1 :
Design of the study site for spawn collection and sampling using Savar net.
SUM [Pi × log(Pi)]; Where, H ′ � Shannon-Wiener index; Pi � ni/N; ni � no. of individuals of a species; N � total number of individuals Margalef species richness (d): d � (S − 1)/ log(N); where, S � total species; N � total individuals.Pielou's evenness index (J ′ ): J ′ � (H(s)/H (max)); where, H(s) � the Shannon-Wiener information function; H(max.)� the theoretical maximum value for H(s) if all species in the sample were equally abundant.Simpson dominance index (c):C �  s n�1 ni/N; Where, ni � number of individuals in the "each" species; N � total number of individuals; S � total number of species Fisher's alpha evenness: S � a * ln (1 + n/a) where S is the number of taxa, n is the number of individuals, and a is Fisher's alpha.Berger-Parker dominance: d � N max \N where Nmax is the number of individuals in the most abundant species, and N is the total number of individuals in the sample.

Table 2 :
Mean values of water quality parameters at various sampling stations analyzed by using the Kruskal-Wallis test throughout the study duration.

Table 3 :
(a) Te general status of plankton with their diverse groupings was documented from the Surma River.(b) Te general status of plankton with their diverse groupings was documented from the Tanguar Haor.

Table 4 :
Available fsh species as recorded from the collected spawn of the Surma River and Tanguar Haor.