Md Mhmudul Hsn Mredul, Md Rushn Alm,b, Antr Bint Akks, Sdi Shrmin,Shib Nth Pttdr,d,＊, Md Lokmn Ali

aFaculty of Fisheries, Patuakhali Science and Technology University, Dumki, Patuakhali, 8602, Bangladesh

bSchool of Environmental and Life Sciences, University of New Castle, Callaghan, NSW, 2308, Australia

cDepartment of Coastal Studies and Disaster Management, University of Barisal, Barisal, 8200, Bangladesh

dSchool of Environment and Natural Resources, The Ohio State University, Columbus, OH, 43210, USA

ABSTRACT

This study reflects the biometric characters of the endangered species Nandus nandus (Hamilton, 1822) including sex ratio, length-length relationships (LLRs), and length-weight relationships (LWRs) utilizing the sum of 14 linear variables, various meristic measurements, reproductive biology, and condition factor from the Borni Baor,a large wetland ecosystem of southwestern Bangladesh. A total of 579 specimens of N. nandus have been periodically collected from the Borni Baor throughout the year (January–December, 2019) using different traditional fishing gears. Highly significant LLRs (r2 >0.90) values indicate that length parameters are profoundly associated with one another. In the LWR formula (W =aLb), higher exponent b values (b >3.0) were demonstrating a positive allometric growth with a high correlation coefficient (r2 >0.94). Moreover, monthly gonadosomatic index and dobriyal index fluctuations suggest the primary breeding season of this fish is from March to July. Data revealed the breeding time was associated strongly with the rainfall and air temperature (p <0.05). The results of our study exposed a cutting-edge biometric feature of this endangered species that would be applicable for fish stock evaluation and making management strategies and conservation policy for this valuable fish resource.

ARTICLEINFO

Keywords:

Sex ratio

Length-length relationship

Length-weight relationship

GSI

Baors

1.Introduction

Bangladesh is a country rich in diverse aquatic resources, only behind China and India in Asia. The total fish production of Bangladesh was assessed at about 4.1 million metric tons, and it ranked fourth and fifth in open water fish production and aquaculture, respectively, in the world (Pattadar et al., 2020; DoF, 2018). Experts projected that open inland water fisheries will contribute about 20% of total fish production in the fiscal year 2020-21, and Boar fisheries will pay a significant role in reaching the target. Boar is a unique inland open water saucer shape lake developed from the dead arm of the river (Abdullah-Bin-Farid et al.,2013). The distinct features of Baors, including stagnant swamp water and high peripheral land support rich biodiversity, which plays an immense role in the social and ecological economics of Bangladesh.However, the Baors fisheries management strategy is frequently constrained due to lack of in-depth knowledge offish reside in this unique habitat.

Nandus nandus (Hamilton, 1822), the leaf fish of the family Nandidae, was once widespread throughout the Indian subcontinent (locally known as Meni or Veda) mainly in wetlands and in flooded areas. It is a highly coveted fish that has tremendous consumer demand for its striking body color, delicious flesh, and high nutritional value (Bogard et al., 2015). Also, it is a benthopelagic fish, commonly occurs in slow-moving or standing waters of rivers, lakes, beels and others (Talwar & Jhingran, 1991; Froese & Pauly, 2019). Nonetheless, N. nandus are such a species that is already listed as an endangered community(IUCN, 2000), which indicates that it is essential to continue to monitor biological data regarding reproduction and other morphometric features to save the fish from becoming extinct. Such information has significant importance since it offers an overview offish growth, general wellbeing,and fitness to the aquatic environment (Alam et al., 2020; Cao et al.,2020). Some studies (Gupta, 2018; Islam et al., 2017) have demonstrated the seasonal morphometric traits and biology of N. nandus from the rivers of Bangladesh and India. However, no comprehensive information is available with accurate biological details such as morphometric features, growth patterns, and spawning time of N. nandus in the focus of massive, unique Baors fishery management.

Therefore, this study aimed to explore the sex ratio, length-length and length-weight relationship, and meristic characteristics of N. nandus in the Borni Baor of Bangladesh. Further, we assessed the Gonadosomatic index (GI), Dobriyal index (DI), and Fulton’s condition factor (K) to identify spawning time of the species. Additionally, the current assessment is envisaged to estimate the relationship between rainfall and temperature and the breeding season of N. nandus as these two ecological factors may remarkably stimulate fish reproduction.

2.Materials and methods

2.1.Study area and sampling

The fish samples were collected from the wild catch of Borni Baor(Latitude: 22570N to 22580N and Longitude: 894930E to 89520E), Gopalganj, Bangladesh (Fig. 1) with the help of local fishermen. Borni is one of the notable Baor in this district, encompassing an area of 110 hectare (DOF, 2018). A total of 569 individuals of N. nandus were intermittently collected from January to December 2019 using traditional fishing techniques including cast net, gill net, and square lift net with mesh sizes of 1.0–2.0 cm, 1.5–2.5 cm, and 1.0 cm, respectively.After collection, the samples were wrapped in polythene containers,placed in iceboxes, and transported to a laboratory for further study.Monthly rainfall and air temperature data of the sampling area were also collected from the Meteorological Department of Bangladesh.

Fig. 1.Study area map for N. nandus in Borni Baor, under Gopalganj District of Southwestern Bangladesh.

2.2.Fish biometric measurement

Digital slide calipers were used to measure length parameters of individual fish in different dimensions such as Total length, Standard length, Head length, Head depth, Pre-orbital length, Orbital length,Post-orbital length, Pre-pectoral length, Pre-anal length, Post-anal length, Pre-dorsal length, and Post-dorsal length. Bodyweight and gonad weight of each specimen were taken with an electronic balance with a precision of 0.01 g. All samples were divided into male and female sex groups by morphometric characteristics and the meristic counts of

N. nandus

in different fins, including pectoral, pelvic, dorsal,anal, and caudal fins were examined using a magnifying glass. The length-weight relationship of

N. nandus

was determined by W =aL,which log-transformed by linear regression equation Ln (W) =Ln (a) +bLn(L), where (a) and (b) are continually representing the body kind and the fish growth type, respectively, with the weight (W) and length (L)(Froese, 2006). Furthermore, the general linear relationships between different length parameters were determined using consolidated data throughout the analysis. The coefficient of determination (r) and b was calculated with 95% confidence levels. The gonadosomatic index (GSI),Dobriyal index (DI), and Fulton’s condition factor (K) were calculated to understand the reproductive physiology and health condition of

N. nandus

as follows: GSI (%) =(GW/BW) × 100; DI = ∛(GW); and K =(BW/SL) ×100, where GW is gonad weight; BW is the body weight, and SL is the standard length.

2.3.Statistical analysis

All data were analyzed with the help of SPSS version 26.0. Before investigating the morphometric data, sex ratio data were tested by performing the Chi-square test to evaluate the statistical significance of the sampling month. We performed one-way analysis of variance(ANOVA) to explore the acceptable difference in means of different measurements among sampling times. At the same time, an independent sample t-test was employed to identify the significant difference of different morphometric features and indexes between sexes. To verify if b of the linear regression of length-weight for the species is statistically significantly different from the predictions assigned for isometric growth (b =3), student one sample t-test mean comparison was assigned. Spearmen’s correlation was employed to understand the effects of rainfall and temperature on the GSI.

3.Results and discussion

3.1.Sex ratio

A sum of 579 specimens of

N. nandus

collected throughout the year from the study area consisted of 48% male and 52% female. The ratio of 0.93:1 between male and female sexes, which did not represent significant variations (d

f

=1, χ=0.762,

p >

0.05) from the anticipated rate of 1:1, except the February sample where significant changes were observed (Table 1). Kalita et al. (2016) divided the sample from Grajan wetland of Assam, India into male and female before estimating length-weight relationship and reported similar sex ratio for

N. nandu

s(0.96:1). Sex ratio is very critical in population dynamics and can be impacted by anthropogenic pressure changes such as selective fishing.Our study results confirmed that gender-biased fishing was not practiced in the study area year-round. However, a significant variation in sex ratio was observed in the month February, which could be explained as either the excessive fishing pressure to gravid female at the end of winter just before the peak reproduction time or the fish were more vulnerable to diseases like epizootic ulcerative syndrome (EUS).

Table 1The sex ratio in the different sampling month of N. nandus caught from Borni Baor, Gopalganj, Bangladesh.

3.2.Length-length relationships (LLRs)

Total length (TL) ranged from 9.0 to 20.0 cm, whereas the body weight ranged from 7.02 to 103.79 g (Table 2). In the current study, the maximum TL of

N. nandus

was 20.0 cm, which is higher than in the Atrai and Brahmaputra rivers (TL =14 cm) (Islam et al., 2017) but similar to the maximum documented TL value of 20.0 cm in West Bangle, India(Goswami & Dasgupta, 2007). Total length 11–12 cm size constitute 27.8% of the population of Borni baors and found dominant (Fig. 2).Whereas Hossain et al. (2017) reported comparatively smaller size group (7–8.99 cm) was dominant in the Ganges river of the northern part of Bangladesh. Thus, it could be argued that new recruitment of the species may be hampered due to fishing pressure in the Borni baors. LLR involves the relationship between TL and other 13 parameters; the formula is Y =bx +a, and Fig. 3 enlisted the regression equation. The results of all correlation coefficient (

r

) are highly correlated at a 95% level of significance. The value of slope close to 1.0 in LLR suggests that the observed other 13 length parameters grow to a similar extent as the total length (TL). Maximum length knowledge is required to approximate growth parameters such as asymptotic length and growth coefficient offish, which is necessary for sustainable fisheries management(Kaka et al., 2019).

3.3.Length-weight relationships

A high correlation coefficient value (

r

>

0.93,

r

>

0.94.

r

>

0.94)in the analysis of length and weight relationships (LWRs) of male, female, and pooled samples respectively were observed over the whole year of study (Table 3 and Fig. 4). The estimated value of b exponent for male, female, and collective sexes was 3.25, 2.97, and 3.11, respectively.However, LWRs allometric co-efficient (b) values may vary from 2.0 to 4.0 (Carlander, 1969) with the most extensive variations are found between 2.5 and 3.5 (Froese, 2006). In this study, the b value was 3.11,which designates positive allometric growth for pooled samples of

N. nandus

in Borni baor, Bangladesh. The relevant result was also mentioned by Islam et al. (2017) in a study of the Atrai and Brahmaputra rivers where authors reported b value for

N. nandus

ranged from 2.544 to 3.305. The b value of the same species can differ from study to study because of the use of distinct fishing gears with varying mesh sizes, the varying number of specimens examined (Hossain et al., 2013), and the preservation technique of the samples. In addition, these differentiations between studies can be a result of the habitat, seasonal, sexual, and health conditions offish (Froese, 2006).

Fig. 2.Length frequency distribution bar chart of N. nandus from Borni Baor,Gopalganj, Bangladesh.

Fig. 3.The relationship between total length with other length-based morphometric indexes of N. nandus caught from Borni Baors, Gopalganj,Bangladesh. a. Total length vs. Standard length; b. Total length vs. Upper jaw length; c. Total length vs. Lower jaw length; d. Total length vs. Head length; e.Total length vs. Head depth; f. Total length vs. Pre-orbital length; g. Total length vs. Post-orbital length; h. Total length vs. Pre-dorsal length; i. Total length vs. Post-dorsal length; j. Total length vs. Pre-pectoral length; k. Total length vs. Post-anal length; l. Total length vs. Pre-anal length.

Table 2The variation in observed length (cm) and weight (gm) of N. nandus caught from Borni Baors, Gopalganj, Bangladesh.

3.4.Meristic characteristics

All meristic measurements are listed in Table 4. The fin formula of

N. nandus

is Dorsal, D. VIII-XII/8–12; Pectoral, Pc. 8–10; Pelvic fin, Pv. 2;Anal, A. III-IV/ 3–4; and Caudal, C. 16–18, respectively. In this study, the total number of fin rays of the dorsal fin is VIII-XII/ 8–12, pectoral fin 8-10, pelvic fin 1, anal fin III-IV/ 3–4 and caudal fin 16-18, which is similar to what was reported by Hossain and Sarker (2013). This feature could be useful for identifying the species in the future as part of an effective conservation plan for the fishes.

3.5.Gonadosomatic index, dobriyal index, and Fulton’s condition factor

The pooled means of GSI, Dobriyal index and Fulton’s condition factor for both male and female are presented in Table 5. The monthly fluctuation of mean GSI (%) with upper and lower values of female and male

N. Nandus

are illustrated in Fig. 5 The highest GSI value in females was 7.44 in May, with the lowest of 1.50 found in December. In the case of the male, the highest value of GSI recorded was 4.78 in May, and the lowest 1.38, also in December. The GSI value started to upsurge in February and remained stable until July with a topmost in May. The mean GSI then fell sharply in August and continued weak in the months that followed. Thus, the spawning season of

N. nandus

was evaluated to be from March to July. The monthly fluctuations in the GSI were closely related to the seasonality of the macroscopically defined maturity stages, as found by Abou Shabana et al. (2012) that the highest peak of GSI for Argyrosomus regius were from March to July. From January to April, a rise in female GSI was observed, which signaled the presence of pre-spawning individuals. The GSI observed in May referred to the first spawning individual measurement. The spawning season persisted from April to September for males, suggesting that possibly female fish become ready to spawn earlier than males. Therefore, our study result provides the determination of the spawning time of this species. This information could be engaged in attention for the Baor fisheries management plan of Bangladesh. Authorities can place sanctions on fishing in the Baors during this period to accelerate the natural propagation of this species.

As such, DI is also used to assess spawning season, sexual maturity,and fish spawning frequency. In the female, the DI was the highest(1.709) in May with a sharp decline in June (Fig. 5). The DI showed a rising increase in males from February through May when it was as high as 1.343. This index gave us a clear understanding of the peak spawning season of the species, which is from March to July. We observed a declining pattern in value between June and November, which indicates an extended spawning season. The changes in Fulton’s condition factor(K) as monthly and size class for both males and females are presented in Fig. 5. The monthly K-values were not significantly differed between sexes, ranging from 2.979 to 3.249 in males, and in the case of females, it was from 2.922 to 3.274. The lowermost K value was observed in males in November and August, while for females in November and April. For both females and males, the highest K was in December. The K value in females was consistently higher than that of males. By size class, males demonstrate a lower K value at 7.5 cm SL and higher at 11.5 cm SL; at the same time, females showed most moderate values at 16.5 cm SL and the highest at 12.5 cm SL. K would, in general, be lower after 11.5 cm and 12.5 cm for males and females, respectively. During the beginning of the spawning season, when more mature eggs were present, the N. nandus condition factor was strong and gradually decreased over the long spawning cycle. This demonstrates the influence of gonad weight on the value K. An increase or decrease in the condition factor may be due to food supply,flooding, heat, temperature fluctuations, pH or water contamination, etc. (Osman et al., 2019).

Fig. 4.The length-weight relationship of a wild catch of N. nandus from Borni Baor, Gopalganj, Bangladesh. a. Male; b. Female; and c. Pooled sample.

Fig. 5.Mean a. GSI, b. Dobriyal index, c. Fulton’s condition factor (size class) and d. Fulton’s condition factor (sampling time) of male and female of wild catch N.nundus from Borni Baor, Gopalganj, Bangladesh.

3.6.Association between spawning season with mean rainfall and temperature

Monthly average air temperature and entire rainfall were correlated with the spawning season predictor mean GSI for both males and females of N. nandus (Table 6). It revealed a profound association between temperature fluctuations and breeding time (r=0.80, p <0.01 for female and r=0.76, p <0.01 for male), and between rainfall and breeding season of N. nandus (r=0.62, p <0.05 for male and r=0.71,p <0.01 for female). Numerous research (e.g. Akter et al., 2019; Hossen et al., 2018; Ohtomi & Ahamed, 2012) has shown that air temperature and rainfall play an effective role in gonad maturation, ovulation,spermiation and spawning. The temperature decreases with rainfall and at the same time raises the dissolved oxygen content, food availability and metabolic rate, thereby supplying the fish with optimal spawning conditions. This also contributes to the endocrine function leads to hydration offish gonads to facilitate normal spawning (Ahamed et al.,2018). This result suggests that combined effect of high temperatures(29C) and high rainfall (272 mm) enhances gonad maturation which may initiate the fish to spawn and decline with low temperature 23.7C and low rainfall (7 mm). There are no prior observations of this species on the gonadosomatic index to equate our results with temperature and rainfall. Thus, this finding suggests Baor Fishery management authorities should take special care after a rain shower to ensure the uninterrupted spawning of this species.

Table 3The estimated parameter of the length-weight relationship of the wild catch of N. Nandus from Borni Baor, Gopalganj, Bangladesh.

Table 4Meristic measurements of the N. nandus (n =569) captured from the Borni Baor,Gopalganj, Bangladesh.

Table 5Variation of GSI, Dobriyal index, and Fulton condition factor between sex of wild catch of N. nandus from Borni Baor, Gopalganj, Bangladesh.

Table 6Relationship of Gonadosomatic index with mean monthly temperature (0C) and mean rainfall (mm).

4.Conclusion

In summary, our analysis is the first to provide comprehensive information on

N. nandus

biometric features in the vast Baors wetland ecosystem of Bangladesh. Length parameters are highly correlated with each other. Approximately all LWRs showed positive allometric growth offish, which might be assigned to environmental factors. Moreover,GSI and DI confirmed the peak spawning time of the species, which was highly associated with mean rain shower and temperature. Eventually,all the findings of the study could be used as a baseline for Baors fishery management. For instant, the knowledge of growth rate, reproductive capacity, size at sexual maturation, spawning time of the species will help to protect this precious fishery resource and imply appropriate fishing regulations to reduce fishing pressure. The reproductive and biometric characteristics of the

N. nandus

population in Borni baor revealed in this study will act as a valuable information for the fisheries management authority to prepare an applicable management rules for sustainable exploitation of this natural resource. However, further extensive research on abundance and gonad development is required to maintain and conserve this species sustainably.

CRediT authorship contribution statement

Md Mahamudul Hasan Mredul: Conceptualization, Methodology,Validation, Formal analysis, Investigation, Writing - original draft. Md Rushna Alam: Conceptualization, Resources, Data curation, Writing -review & editing, Project administration. Antara Binta Akkas:Conceptualization, Resources, Writing - review & editing. Sadia Sharmin: Investigation, Validation. Shib Nath Pattadar: Conceptualization,Investigation, Data curation, Supervision, Writing - review & editing.Md Lokman Ali: Investigation, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no competing interests for publishing this study and eager to supply any clarification if necessary.

Acknowledgments

This study is funded by Patuakhali Science and Technology University research budget supported by University Grants Commission of Bangladesh. The authors are thankful to the local fishermen of Borni baor, Gopalganj, Bangladesh, the Department of Aquaculture, Patuakhali Science and Technology University, and Department of Fisheries,Bangladesh for providing time, suggestions and laboratory facilities to carry out this research.