Diversity pattern of ichthyofaunal assemblage in South-central coastal region of Bangladesh

Abstract

The south-central coastal waters of Bangladesh are recognized as one of the foremost aquatic biodiversity hotspots despite severe anthropogenic and environmental hazards to natural fishery resources. Therefore, this study was undertaken to examine the current status of fish faunal diversity and evaluate the condition and conservation status of fish in the rivers spanning the south-central coastal region of Bangladesh. During the research period, spanning from July 2021 to February 2022, a total of 105 fish faunal species belonging to 84 genera, 50 families, and 14 orders were documented. The order Perciformes (27.62%) was found to be the most diverse fish group, followed by Cypriniformes and Scombriformes (12.38%), Siluriformes and Clupeiformes (11.43%), Synbranchiformes and Pleuronectiformes (3.81%). According to the International Union for Conservation of Nature (IUCN) Red List classification, 52 species recorded from the studied areas were determined to be of Least Concern, while 11 species were Vulnerable, 14 species were classified as Endangered, and 7 species are recognized as Critically Endangered. In order to assess the species diversity, richness, and evenness of the fish assemblage in the sampling areas, four diversity indices, viz. the Shannon-Wiener index (H), Simpson’s dominance index (D), Simpson’s index of diversity (1-D), and Margalef’s index (d), were applied. Their overall values were 3.39–3.59, 0.05–0.07, 0.95–0.94, and 7.67–8.75, respectively. All of the species were more or less evenly distributed throughout the three sites, with Kalapara having the most even distribution. Several anthropogenic and natural attributes that contribute to decline in fish diversity were recorded using a survey questionnaire which revealed that management and conservation measures should be taken with utmost urgency.

Keywords:

• South-central coastal region
• ichthyofaunal diversity
• conservation status
• biodiversity index
• threats

Introduction

Food security and nutrition are put at risk due to biodiversity loss which poses a significant challenge in the 21^st century. Due to the decline of fishing resources, this concern is especially acute in many coastal and inland areas. Overexploitation of fish, climate change and many other anthropogenic activities including siltation, pollution of aquatic environments through industrial chemicals, indiscriminate use of fertilizers and insecticides, and disturbances of breeding, nursery, and feeding grounds all synergistically affect fish biodiversity (Colossi et al. 2019). Diversity of fish and resilient fisheries are deemed two key milestones to meet the ever-growing demand for fish consumption in Bangladesh, but it is difficult to find management strategies that address both food and nutritional security and biodiversity.

Bangladesh is bestowed with abundant and diverse fishery resources due to its enormous riverine network (Rahman 2015). Coastal rivers in Bangladesh are considered an indispensable, balanced ecosystem providing natural breeding, feeding, and nursery grounds for many economically important fisheries resources (Aminul Islam et al. 2015). Fishing has long played a vital part in the country’s national economy, serving as the prime source of animal protein, employment opportunities, food and nutritional security, foreign earnings, and socioeconomic advancement (Siddiq et al. 2013; Ali et al. 2014, 2015). The most productive and dynamic source of income for the population is fishing, which has an impact on the country’s agricultural economy (Shamsuzzaman et al. 2020). Patuakhali is the south-central coastal district of Bangladesh blessed with diversified fisheries resources and considered an important economic hub of the coastal region of Bangladesh (Ali et al. 2020).

The productivity and richness of a water body are greatly influenced by its biodiversity. The diversity of species in a water body is typically influenced by both human activity and natural calamities. Unplanned construction of dams and bridges, changing the river flow course, loss of habitat, water pollution from industrial and agricultural waste, alien species invasions, heavy exploitation of larvae and juvenile fish stocks, and the use of illegal fishing gear are the most impactful anthropogenic causes of biodiversity depletion in coastal ecosystems (Sarkar et al. 2008; Rahman 2015). In addition, natural causes like floods, sudden alteration of meteorological and hydrological parameters, algal blooms, and siltation can have an impact on fish biodiversity (Hossen et al. 2020; Jahan et al. 2020; Pandit et al. 2021). Considering the above-mentioned circumstances, it is urgently necessary to take proper initiatives for the conservation of aquatic biota and maintain sustainable utilization levels (Fu et al. 2003; Eros & Schmera 2010; Rao et al. 2014).

The state of the complex network of biological entities in an ecosystem is evaluated via diversity analysis (Joshi et al. 2017). The diversity index stands as a valuable tool offering crucial insights into the distribution of species, indicating both occurrence and abundance within a community in a specific water body (Tikadar et al. 2021). Many studies regarding biodiversity of fishes have been performed in Bangladesh and around the world (Uttam & Bain 2007; Montaña et al. 2011; Chowdhury et al. 2011; Mukherjee et al. 2013; O’Hara et al. 2019; Akhi et al. 2020), but to date, reports on the composition, distribution and community structure of fish fauna in southern coastal regions of Bangladesh are limited. Consequently, the purposes of the study were to evaluate the present status of the ichthyofaunal assemblage, identify the main causes contributing to the depletion of fish biodiversity, and formulate recommendations for the conservation of fish faunal diversity.

Materials and methods

Study area

The study was conducted for a period of 6 months, from July to September 2021 (wet season) and from December 2021 to February 2022 (dry season), in order to obtain fish faunal diversity data in two south-central coastal regions (Patuakhali and Barguna) of Bangladesh. The survey areas were divided into four sampling stations (Figure 1): Kalapara (S1, 21°98′61″N, 90°24′22″E), Alipur (S2, 22°25′55.0″N, 90°51′16.2″E), Amtoli (S3, 22°12′71.9″N, 90°22′82.5″E), and Tengragiri (S4, 21°96′34.79″N, 89°96′42.68″E).

Figure 1. Map displaying the research area with four sampling sites in the southern coastal region of Bangladesh: Kalapara (S1, 21°98′61″N, 90°24′22″E), Alipur (S2, 22°25′55.0″N, 90°51’16.2″E), Amtoli (S3, 22°12′71.9″N, 90°22′82.5″E), and Tengragiri (S4, 21°96′34.79″N, 89°96′42.68″E).

Data collection

To conduct this research, both primary and secondary data was used. Fish fin samples were procured from local fish landing centers, obtained both during the catch and from fishermen who had been asked beforehand. In the studied area, local fishermen utilize a diverse array of fishing gear, including set gill nets, seine nets, drift gill nets, cast nets, moshari bar jal, hooks, and traps. This gear varies in terms of specific species targeted, size, and overall efficiency. From the study sites, pertinent information was gathered, including the local names of the different fish species that were taken as well as the source, availability, and dispersion of the species. Secondary data was gathered from the Department of Fisheries (DoF), the Statistical Yearbook of Bangladesh, and District Fisheries Offices. In addition to these statistics, relevant documents were also gathered from a number of governmental organizations, independent entities, and non-governmental organizations (NGOs). In order to assemble historical data of abundance and availability for evaluating biodiversity status, research articles on the fish faunal diversity of Bangladesh were also considered. Key informant interviews were conducted in order to understand the threat assessments in the research area (Figure 2).

Figure 2. Flow diagram of the data collection method for ichthyofaunal diversity in the south-central coastal region.

Collected sample identification

Based on the primary physical characteristics, the collected fish species were systematically organized. Those species that posed challenges in identification onsite were transported to the Molecular Biology and Conservation Laboratory at Patuakhali Science and Technology University, Bangladesh. Fish were transported in a 5–10% buffered formalin solution. Fish collected from the sampling areas were identified by analyzing their morphometric and meristic traits according to previously published data (Hussain 2010; Eschmeyer 2022). Following identification, fish species were systematically categorized according to Bell (2006) and Nelson (2006).

Fish abundance and biodiversity status analysis

In this study, fish species abundance and diversity was analyzed using several indices, viz. the Shannon-Wiener diversity index (H), Simpson’s dominance index (D), Simpson’s index of diversity (1-D), Margalef’s index (d) and the evenness index (E).

Shannon Weiner diversity index (H)

This index evaluated both the number of species and the distribution of individuals among species. Values of this index were evaluated on a monthly basis by applying the following equation:

1 $\mathrm{H}=-\mathrm{\Sigma }\mathrm{Pi}\times \mathrm{Ln}\mathrm{Pi}$1

where P_i = n_i/N; n_i = number of individuals of each species in the sample, and N = total number of individuals of all species in the sample.

Simpson’s dominance index (D)

This index is often used to quantify the biodiversity of a habitat, taking into account the number of species as well as the abundance of each species. The equation used for calculating Simpson’s dominance index is:

2 $\mathrm{D}=\frac{\mathrm{\Sigma }\mathrm{ni}\left(\mathrm{ni}\text{minus}1\right)}{\mathrm{N}\left(\mathrm{N}\text{minus}1\right)}$2

Simpson’s index of diversity (1-D)

Simpson’s index of diversity was calculated using the following formula:

3 $1\text{minus}\mathrm{D}=1\text{minus}\frac{\mathrm{\Sigma }\mathrm{ni}\left(\mathrm{ni}\text{minus}1\right)}{\mathrm{N}\left(\mathrm{N}\text{minus}1\right)}$3

Margalef’s diversity index (d)

To measure species richness, Margalef’s diversity index can be assessed using the following formula:

4 $\mathit{d}=\frac{\mathrm{S}\text{minus}1}{\mathrm{Ln}\mathrm{N}}$4

Evenness (E)

The evenness index, or the relative abundance of the different species making up the richness of an area, was calculated using the following equation.

5 $\mathrm{E}=\frac{e^H}{\mathrm{S}}$5

where H = Shannon Weiner diversity and S = total number of fish species.

$\mathit{d}=\frac{\mathrm{S}\text{minus}1}{\mathrm{Ln}\mathrm{N}}$

Analyses of the various diversity indices were carried out using the Paleontological Statistics (PAST) version 4.03 software program. Graph pad Prism 8 and R programming were used to conduct statistical analyses. A dendrogram was constructed using the results of the hierarchical clustering in order to compare the examined stations (Clarke et al. 2014).

Results

The study area represents a natural hub for a diverse array of marine and freshwater species spaning various orders and families. The collection of data involved engaging in discussions with 215 fishermen from the local fishing community and surveying distinct fish landing centers. This comprehensive approach was undertaken with the goal of augmenting the species checklist within the designated study area.

The present study recorded a total of 105 species of fishes belonging to 84 genera, 50 families and 14 orders. Table I displays the conservation status (International Union for Conservation of Nature (IUCN) and Bangladesh Red List status) of the species found in the study area, accompanied by their order, family, scientific name, and common name. Out of 14 orders, Perciformes contributed the most, with a share of 27.62% of the total catch, followed by Cypriniformes and Scombriformes (12.38%), Siluriformes and Clupeiformes (11.43%), Synbranchiformes and Pleuronectiformes (3.81%), Acanthuriformes, Anguilliformes Carangiformes, Mugiliformes, and Tetraodontiformes (2.86%), whereas Myliobatiformes (1.90%) and Aulopiformes (0.95%) were least dominant (Figure 3). According to our study, the dominant family was Cyprinidae, comprising 13 species which accounts for 12.38% of the total number of observed species. Other dominant families were Gobiidae (8.57%) and Scombridae (8.57%), while Clupeidae, Sciaenidae, Bagridae, Ariidae, Cynoglossidae constituted 3.80% species each.

Figure 3. Percentage composition of families, genera and species of ichthyofaunal diversity documented in the south-central coastal regions of Bangladesh.

Table I. Ichthyofaunal abundance and conservation status of recorded individuals at four stations in the south-central coastal regions of Bangladesh.

Order				Conservation status
	Family	Species name	Common name	Bangladesh	Global
Acanthuriformes	Lobotidae	Lebotes surinamensis	Triple tail	NA	LC
Acanthuriformes	Signidae	Siganus javus	Striked spine foot	NA	VU
Acanthuriformes	Leognathidae	Leiognathus equulus	Tak chanda	EN	LC
Aulopiformes	Synodontidae	Harpadon nehereus	Bombay duck	NO	VU
Anguilliformes	Anguillidae	Anguilla bengalensis	Indian mottled eel	VU	NT
Anguiliformes	Muraenesocidae	Congresox talabonoides	Indian pike conger	NO	CR
Anguilliformes	Anguillidae	Monopterus cuchia	Asian swamp eel	VU	LC
Carangiformes	Istiophoridae	Istiompax indica	Black marlin	NA	DD
Carangiformes	Coryphaenidae	Coryphaena equiselis	Pompano dolphin fish	NA	LC
Carangiformes	Carangidae	Parastromateus niger	Black pomfret	NA	LC
Clupeiformes	Clupeidae	Escualosa thoracata	White serdine	NA	LC
Clupeiformes	Clupeidae	Anodontostoma chacunda	Gizard shad	LC	VU
Clupeiformes	Clupeidae	Tenualosa ilisha	Hilsa shad	LC	LC
Clupeiformes	Clupeidae	Tenualosa toli	Toli shad	LC	VU
Clupeiformes	Dussumieriidae	Dussumieria acuta	Rainbow sardine	NA	VU
Clupeiformes	Dussumieriidae	Dussumieria elopsoides	Rainbow serdine	NA	CR
Clupeiformes	Pristigasteridae	Pellona ditchela	Indian pellona	LC	LC
Clupeiformes	Pristigasteridae	Ilisha filigera	Coromendel ilisha	LC	DD
Clupeiformes	Pristigasteridae	Ilisha megaloptera	Bigeye ilisha	LC	LC
Clupeiformes	Engraulidae	Coilia neglecta	Neglected grenadier anchoby	NO	LA
Clupeiformes	Engraulidae	Setipinna phasa	Gangetic hairfin anchoby	DD	LC
Clupeiformes	Engraulidae	Coilia dussumieri	Gold spotted grenadier anchovy	LC	LC
Cypriniformes	Cyprinidae	Esomus danricus	Flying barb	NO	LC
Cypriniformes	Cyprinidae	Esomus lineatus	Striped flying barb	LC	LC
Cypriniformes	Cyprinidae	Osteobrama cotio	Cotio	NT	LC
Cypriniformes	Cyprinidae	Chela cachius	Silver hatchet barb	VU	LC
Cypriniformes	Cyprinidae	Laubuca laubuca	Indian glass barb	LC	CR
Cypriniformes	Cyprinidae	Salmophasia phulo	Fine scale razor belly minnow	NT	LC
Cypriniformes	Cyprinidae	Salmophasia acinaces	Silver razor belly minnow	LC	LC
Cypriniformes	Cyprinidae	Salmophasia bacaila	Large razor belly minnow	LC	LC
Cypriniformes	Cyprinidae	Labeo boga	Boga bata	CR	LC
Cypriniformes	Cyprinidae	Labeo bata	Bata	LC	LC
Cypriniformes	Cyprinidae	Puntius ticto	Ticto barb	VU	LC
Cypriniformes	Cyprinidae	Puntius sophore	Pool barb	LC	LC
Cypriniformes	Cyprinidae	Puntius sarana	Olive barb	NT	LC
Myliobatiformes	Dasyatidae	Dasyatis zugei	Pale edged sting ray	CR	VU
Myliobatiformes	Dasyatidae	Himantura bleekeri	String ray	NA	EN
Mugiliformes	Mugilidae	Chelon subviridis	Greenback mullet	LC	CR
Mugiliformes	Mugilidae	Planiliza parsia	Gold spot mullet	NA	LC
Mugiliformes	Mugilidae	Rhinomugil corsula	Corsula mullet	NO	LC
Perciformes	Gobiidae	Trypauchen vagina	Burrowing goby	LC	VU
Perciformes	Gobiidae	Odontamblyopus rubicundus	Rubicundus eel Goby	LC	CR
Perciformes	Gobiidae	Taenioides cirratus	Bearded worm goby	LC	DD
Perciformes	Gobiidae	Boleophthalmus boddarti	Boddarts goggle-eyed goby	LC	LC
Perciformes	Gobiidae	Pseudapocryptes elongatus	Pointed tail goby	LC	LC
Perciformes	Gobiidae	Taenioides buchanani	Burmese goby eel	LC	EN
Perciformes	Gobiidae	Periophthalmodon schlosseri	Giant mudskipper	LC	NA
Perciformes	Eleotridae	Eleotris fusca	Dusky slipper	LC	LC
Perciformes	Gobiidae	Acentrogobius caninus	Tropical sand goby	NA	LC
Perciformes	Gobiidae	Scartelaos histophorus	Walking goby	LC	NA
Perciformes	Polynemidae	Elutheronema tetradactylum	Four finger thread fin	NA	EN
Perciformes	Polynemidae	Polynemus paradiseus	Paradise threadfin	LC	NA
Perciformes	Sciaenidae	Otolithoides pama	Pama croaker	LC	NA
Perciformes	Sillaginidae	Sillaginopsis panijus	Flathead sillago	LC	EN
Perciformes	Ambassidae	Chanda nama	Elongate glass perch	LC	LC
Perciformes	Ambassidae	Parambassis ranga	Indian glassy fish	LC	LC
Perciformes	Latidae	Lates calcarifer	Sea bass	NA	NA
Perciformes	Lutjanidae	Lutjanus madras	Indian snapper	NA	EN
Perciformes	Nemipteridae	Nemipterus japonicus	Japanese Threadfin Bream	NA	NA
Perciformes	Scaophagidae	Scatophagus argus	Tiger scat	LC	EN
Perciformes	Sciaenidae	Johnius argentatus	Silver jew	NA	LC
Perciformes	Sciaenidae	Johnius carutta	Karut croaker	NA	LC
Perciformes	Sciaenidae	Johnius coitor	Coitor croaker	LC	LC
Perciformes	Spariidae	Pagrus major	Red sea bream	NA	LC
Perciformes	Spariidae	Diplodus sargus	White sea bream	NA	LC
Perciformes	Spariidae	Acanthopagrus latus	Yellow fin sea bream	NA	DD
Perciformes	Platycephalidae	Platycephalus caeruleopunctatus	Blue spotted flathead	NA	EN
Perciformes	Platycephalidae	Platycephalus indicus	Bartail flathead	NA	DD
Perciformes	Serranidae	Epinephelus marginatus	Dusky grouper	NA	VU
Pleuronectiformes	Cynoglossidae	Brachirus pan	Pan sole	LC	NO
Pleuronectiformes	Cynoglossidae	Cynoglossus arel	Largescale tonguesole	LC	EN
Pleuronectiformes	Cynoglossidae	Cynoglossus cynoglossus	Bengal tongue sole	LC	NA
Pleuronectiformes	Cynoglossidae	Cynoglossus lingua	Long tongue sole	LC	VU
Scombriiformes	Scomridae	Scomberomorus guttatus	King Mackerel	NA	DD
Scombriiformes	Scomridae	Euthynnus affinis	Mackerel tuna	NA	LC
Scombriiformes	Scomridae	Sarda lineolata	Pacific bonito	NA	EN
Scombriiformes	Scomridae	Katsuwonus pelamis	Skipjack tuna	NA	LC
Scombriiformes	Scomridae	Thunnus obesus	Big eye tuna	NA	VU
Scombriiformes	Scomridae	Auxis thazard	Frigate tuna	NA	LC
Scombriiformes	Scomridae	Auxis rochei	Bullet tuna	NA	LC
Scombriiformes	Scomridae	Thunnus albacares	Longtail tuna	NA	LC
Scombriiformes	Scomridae	Rastrelliger kanagurta	Indian Mackerel	NA	DD
Scombriiformes	Stromateidae	Pampus argenteus	Silver pomfret	NA	VU
Scombriiformes	Stromateidae	Pampus chinensis	Chinese pomfret	NA	NA
Scombriiformes	Trichiuridae	Lepturacanthus savala	Savalai hairtail	NA	EN
Scombriiformes	Trichiuridae	Trichiurus haumela	Ribbon fish	NA	LC
Siloriformes	Ariidae	Netuma thalassina	Gaint sea catfish	NA	NA
Siloriformes	Ariidae	Hexanematichthys sagor	Sagor catfish	NA	EN
Siloriformes	Ariidae	Arius gagora	Gagora catfish	DD	NT
Siloriformes	Ariidae	Ketengus typus	Big mouth sea catfish	NA	EN
Siloriformes	Plotosidae	Plotosus canius	Grey eel catfish	VU	EN
Siloriformes	Pangasidae	Pangasius Pangasius	Yellow tail catfish	EN	LC
Siloriformes	Bagridae	Mystus gulio	Long whiskered catfish	NO	LC
Siloriformes	Bagridae	Mystus cavasius	Gangetic tengra	NT	LC
Siloriformes	Bagridae	Sperata aor	Long whiskered catfish	VU	LC
Siloriformes	Bagridae	Rita rita	Rita	EN	LC
Siloriformes	Schilbeidae	Clupisoma garua	Garua bacha	EN	CR
Siloriformes	Schilbeidae	Silonia silondia	Silond catfish	LC	LC
Synbranchiformes	Mastacembelidae	Macrognathus aral	One stripe spiny eel	DD	LC
Synbranchiformes	Mastacembelidae	Macrognathus pancalus	Indian spiny eel	LC	CR
Synbranchiformes	Mastacembelidae	Mastacembelus armatus	Zigzag eel	LC	LC
Synbranchiformes	Synbranchidae	Ophisternon bengalense	Bengal eel	VU	LC
Tetraodontiformes	Tetraodontidae	Tetraodon fluviatilis	Green pufferfish	NA	LC
Tetraodontiformes	Tetraodontidae	Tetraodon cutcutia	Ocellated pufferfish	LC	NO
Tetraodontiformes	Balistidae	Abalistes stellaris	Starry trigger fish	NA	EN

CR = Critically Endangered, EN = Endangered, VU = Vulnerable, LC = Least Concern, NT = Near Threatened, DD = Data Deficient, NO = Not Threatened, NA = Not Assessed.

In all of the research regions, the dry season (December 2021 to February 2022) revealed a higher abundance of fish species in contrast to the wet season (Table II). Kalapara station exhibited the highest diversity of 101 species, followed by Alipur (97), Tengragiri (93), and Amtoli stations (85). The maximum number of individuals (91,884) was recorded at sampling station S1, followed by S2, S4, and S3 where 80,442, 61576, and 57,202 individuals were found, respectively (Table II). In terms of recorded species richness and abundance, Perciformes was found to be the dominant fish order, followed by Cypriniformes and Siluriformes, among the four stations. During the study period, 10 of the 105 species were most prevalent, with Tenoalosa illisha taking the highest rank and being followed in abundance by T. toli, Esomus danricus, and Puntius sarana. The least prevalent species in both the dry and wet seasons were Lates calferifer and Chanda nama (Figure 4).

Figure 4. Temporal abundance of the 10 most abundant fish species in the study area.

Table II. Total number of fish species and individuals identified from four stations in the south-central coastal waters of Bangladesh.

Study area	Number of fish species (S)	Number of individuals in dry season (N)	Number of individuals in wet season (N)	Total number of individuals (N)
Kalapara (S1)	101	53,148	38,736	91,884
Alipur (S2)	97	44,512	35,930	80,442
Amtoli (S3)	85	32,804	24,398	57,202
Tengragiri (S4)	93	32,562	29,014	61,576

In accordance with the IUCN Red List categories, 11 species were recorded as Vulnerable, 14 species were identified as Endangered, 7 species were Critically Endangered, and 52 species recorded from the researched areas were found to be of Least Concern. Out of the 105 fish species, 64 Bangladesh (BD) red-listed species were recorded from the study locations, of which 2 species were found to be Critically Endangered, 5 species were identified as Endangered, 7 species were Vulnerable, and 37 species were deemed to be of Least Concern (Figure 5). Our study demonstrated that compared to other stations under examination, Kalapara station recorded the majority of the threatened species as per IUCN.

Figure 5. Conservation status of fish species recorded in our study under International Union for Conservation of Nature (IUCN) and Bangladesh (BD) red list categories.

The Kalapara station had the highest number of critically endangered fish species (7 in total), whereas Amtoli and Tengragiri had the lowest numbers (both 2). In contrast to Amtoli, which had the lowest number of endangered species (8), and Tengragiri, with the fewest vulnerable species (5), Kalapara demonstrated the highest figures of endangered and vulnerable species, at 14 and 11, respectively (Figure 6).

Figure 6. Number of species in various threat categories recorded in the present study.

In Figure 7, the values of several diversity indices for the four research sites are displayed. The highest (3.462) and lowest (1.927) levels of Shannon-Weiner diversity index value were found in Kalapara (S1) and Amtoli (S3), respectively. Simpson’s dominance index data showed that Amtoli (S1) had the highest value, followed by Tengragiri (S4), Alipur (S2), and Kalapara (S1), with values of 0.062, 0.061, and 0.048, respectively. The maximum and minimum values of Margalef’s richness were found in S1 and S3, respectively. The evenness value showed that all of the species were more or less evenly distributed among the four stations, with S1 having the most even distribution (Figure 8).

Figure 7. Various diversity indices measured at sampling locations in the southern coastal region.

Figure 8. Evenness of ichthyofaunal diversity in four sampling stations.

A cluster analysis was performed on the recorded species from the sample sites. The study found that the species were divided into two major clusters and shared 95% similarity, according to the Bray-Curtis similarity matrix. Stations Amtoli and Tengragiri constituted one cluster while Kalapara and Alipur formed another (Figure 9).

Figure 9. The temporal fish assemblage cluster constructed as per the Bray-Curtis similarity matrix.

The factors that contribute to declining ichthyofaunal diversity include indiscriminate fishing, destruction of feeding, breeding, and nursing grounds, pollution, urbanization and modernization, introduction of alien invasive species, salinization, siltation, abrupt changes in climatological parameters, unplanned infrastructure, and a lack of enforcement of fishing regulations. According to the respondents, the main threats to the depletion of fish faunal resources in the research area include overfishing, habitat degradation, the effects of climate change, and a lack of compliance with fishing laws and regulations (Figure 10).

Figure 10. A heat map depicting the reasons for ichthyofaunal diversity in the south-central coastal region.

Discussion

Research into fish biodiversity often reveals the variability of species within a given location. Documentation of the current fish population, alongside its ecosystem and biodiversity status, is crucial for adapting to the changing conditions worldwide. Our study delves into the specifics of species composition, geographical distribution, and temporal variations of fish in the south-central coastal region of Bangladesh. Generally, the susceptibility of different fish species varies in response to anthropogenic interference, natural disasters, and environmental deterioration (Fialho et al. 2008).

A total of 105 fish species belonging to 14 orders and 50 families were found in the current study area. There were a maximum of 27.62% fish species in the order Perciformes and a maximum of 12.38% fish species in the family Cyprinidae (Table I). The species composition indicates a blend of marine, estuarine, and freshwater species, attributed to the convergence of freshwater, brackish, and marine waters in this study area. Hanif et al. (2015) identified Perciformes as the dominant order, accounting for 35.71% of all species observed in southern coastal waters of Bangladesh. According to Ahsan et al. (2014) and Rao et al. (2014), Cyprinidae made a significant contribution, with high species richness in various open water bodies. In this study the highest numbers of species were recorded during the dry season (Table II); the water depth might be lower at this time due to limited rainfall, allowing fishermen to use their fishing gear more efficiently. Similar findings were reported by Rao et al. (2014), who found that the dry season had the highest diversity of ichthyofaunal species. Throughout the study period, S1 exhibited the highest count of individuals (91,884), whereas S3 had the lowest count (57,202). These variations may be attributed to differences in several station parameters, including water temperature, depth, water currents, and scarcity of nutrients (Zare-Shahraki et al. 2022).

In the fish samples that were taken, Kalapara station contained the greatest number of IUCN red-listed species (Figure 5). Hanif et al. (2015) reported the highest number of threatened species in Patuakhali region. This finding is consistent with our research. The Kalapara area represented the highest number of Critically Endangered, Endangered, and Vulnerable species due to low human interference, low fishing pressure, and sound environmental conditions.

Biodiversity index assessment was carried out to determine the fish assemblage of the south-central coastal region of Bangladesh. For ecological data, the Shannon-Wiener index (H) value typically varies from 1.5 to 3.5 and almost never exceeds 4.0, which is mostly supported by our estimated data. The H value can be higher than 3.5 when the water body maintains a sustainable environmental condition (Ali et al. 2020). The present study showed the lowest (3.385) H value in the Amtoli area which, was slightly lower than the value reported by Hanif et al. (2015), due to the variation of environmental factors in this study region.

Typically, Simpson’s dominance index (D) ranges from 0 to 1, with a wider range of values indicating a lower range of biodiversity. In the S3 region, Simpson’s dominance index was higher, whereas stations S1 and S2 exhibited lower values. S1 was found to be the most species rich when taking the D value into account, whereas site S3 was found to be the least enriched. The Simpson’s index of diversity (1-D) similarly ranges from 0 to 1; a higher value denotes a more diverse sample. The maximum value of this index was observed in S2 and represented the highest fish faunal species diversity. Among the other three stations all the recorded species are more or less evenly distributed.

The Margalef index value is used as an indispensable indicator to compare the study sites based on the number of recorded species (Vyas et al. 2012). The highest value of the Margalef index corresponds to the greatest number of individuals in the study area. The greatest Margalef’s index value was found at site S1, which indicated the presence of significantly more individuals than the other studied sites.

Based on the Bray-Curtis similarity index, a dendogram was generated utilizing the recorded fish faunal species at the four sampling stations. Two major clusters were formed among the four sampling stations which commonly shared 95% fish species. In a previous study of the Andharmanik River, there was a 59.36% similarity between the fish samples obtained at the six different sites (Roy et al. 2022). In the present study, one cluster included stations Amtoli and Tengragiri, while Kalapara and Alipur formed another cluster due to variation in geographical location.

Natural and anthropogenic issues are getting worse every day, affecting the spatial and temporal distribution of fish species on a national scale (Hanif et al. 2015). Consequently, many fish species are recorded as endangered in Bangladesh. The southern coastal waters of Bangladesh are used periodically by a huge number of freshwater, marine , migratory fishes as a key breeding, spawning, nursing, feeding, and migration route connecting with the Bay of Bengal (Roy et al. 2022). But in recent years, the southern coastal waters of Bangladesh have undergone significant changes as a result of drastic pollution, human meddling, and abrupt climate change, all of which have ruined the riverine ecosystem (Kunda et al. 2022). Loss of habitat, decreased water flow, and indiscriminate harvesting of fry and fingerlings are also considered significant contributors to the declining fish assemblage in Bangladesh (Mazumder et al. 2015; Mostofa et al. 2022). These cited studies recorded similar declining attributes that represent crucial contributors to declining fish diversity in the study area, signaling the gradual reduction of fish diversity of Bangladesh.

Conclusion and recommendations

There is a growing awareness that the ichthyofaunal assemblage in the southern coastal region of Bangladesh has not been thoroughly assessed, primarily due to a lack of adequate scientific studies. As a result, it becomes imperative to undertake an in-depth study of the species structure as well as the life histories and reproductive biology of fish species in this region. The most crucial part of biodiversity conservation for coastal resources is raising stakeholder awareness through effective communication, collaboration, and education. In order to start new studies, research, monitoring, and awareness-raising among the fishermen for the protection of fish diversity in Bangladesh’s coastal area, financial support from the government and donor organizations is also essential. To sum up, since fish and fisheries in this area provide food and a means of subsistence for millions of marginally poor people, particularly fishermen, the government should take action and develop long-term conservation strategies to maintain sustainable production from southern coastal waters.

Acknowledgments

The authors express their gratitude to the Institute of Fishing Village & Aquaculture at Chonnam National University for providing publication funding.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Ministry of Education, Government of the People’s Republic of Bangladesh [Research grant numbers [LS 20201352 (2)].