Length-Weight Relationships and Condition Factor of Four Threatened Riverine Catfish Species in the Meghna River Estuary, Bangladesh

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Introduction
Catfsh, which belong to the order Siluriformes, are a popular and species-rich group of fsh in Bangladesh, ranking third in terms of species diversity [1].Among 55 catfsh species found in Bangladesh, the riverine catfsh species, including yellowtail catfsh (Pangasius pangasius), rita (Rita rita), long-whiskered catfsh (Sperata aor), and Goonch catfsh (Bagarius bagarius), are highly prized in the country's table fsh market due to their large size, taste, and low number of intramuscular bones.Tese species are also rich in nutritional values, with good quality protein and high fat content [2,3], and contribute to the second-highest riverine fsh production in volume (29%) after hilsa in the Meghna River Estuary (MRE) [4].However, the natural populations of these catfsh species have drastically declined in recent years due to various anthropogenic and natural factors, and they are currently facing diferent levels of threats.S. aor is classifed as vulnerable, R. rita and P. pangasius is classifed as endangered, and B. bagarius is classifed as critically endangered in Bangladesh [5].Urgent conservation eforts are required to prevent their extinction.Two primary conservation strategies, safeguarding the fsh species in their native habitat and/or incorporating them into aquaculture practices, could be employed to bring them back to their previous state.Eforts to include these species within the realm of aquaculture have not yielded success so far, and sustainable management practice is supposed to be the only viable approach to rejuvenate the natural populations of these species.However, before efective conservation strategies can be developed, the biology of the species needs to be thoroughly studied.Estimating the length-length relationships (LLRs), length-weight relationships (LWRs), and condition factor (K) of fsh is essential in fsheries' management, as it provides insights into the well-being and growth patterns of the species [6,7], aid in the development "n", number of examined individuals; "TL", total length; "W", body weight; "max", maximum value; "min", minimum value; "a" and "b" are intercept and slope of LWRs; "p" and "q" are intercept and slope of LLRs; "R 2 ", coefcient of determination; "CI", confdence interval; "L CI ", lower bound of CI; "U CI ", upper bound of CI; "K", condition factor; "Avg", average.
2 Journal of Applied Ichthyology of single-species stock assessment models [8], and play a pivotal role in estimating the biomass of fsh populations [9].However, limited research is available [10,11] on these four species, particularly in their largest habitat, the MRE.Terefore, the present study aims to estimate the LLRs, LWRs, and condition factor of the four riverine catfsh species in the MRE.

Materials and Methods
A total of 578 individuals (Table 1) from four riverine catfsh species were collected by directly purchasing from contracted fshermen in Chandpur (23 °14′38″ N; 90 °38′09″ E), Lakshmipur (22 °57′17″ N; 90 °37′43″ E), and Bhola (22 °20′38″ N; 90 °59′12″ E) districts between January and October 2022.Te fshermen employed diferent types of seine nets (referred to locally as "lasa jal" and "doom jal") with mesh size ranging from 2.2 to 6.5 cm for capturing S. aor, R. rita, and B. bagarius, whereas "chai" (a large cylindrical fsh trap having an average length 1.52-2.45m, tapering little at both ends and locally made with bamboo splits; mesh size 1.2 cm) was used to catch juvenile P. pangasius.Te length and weight of the specimens were measured using a digital slide caliper with ±0.01 mm accuracy and a digital balance with ±1.0 g accuracy (BH-124), respectively.Te LWRs were calculated using formula W � aL b , where W � weight of fsh (g), L � total length (cm), a � intercept of the regression, and b � regression coefcient.Te parameters "a" and "b" were obtained from the log-transformed equation as Log W � Log a + b Log L [12].Te LLRs was calculated using a simple linear regression model (Y � p + qX), where Y � total length (cm), X � standard length (cm), p � proportionality constant, and q � regression coefcient [13].Te condition factor (K) was calculated using formula K � 100 × (W/L 3 ), where K � condition factor, W � weight of fsh (g), and L � total length of fsh (cm).Statistical analyses were performed using SPSS version 20.0 statistical software package (IBM ® ) at a 0.05 signifcant level.

Results
Te LWRs, LLRs, and condition factor of the four catfsh species were examined separately.Te mean length and weight of the collected samples were 14.67 cm and 25.77 g for P. pangasius, 24.79 cm and 106.70 g for S. aor, 16.47 cm and 56.48 g for R. rita, and 55.78 cm and 2,147 g for B. bagarius, respectively.Te length and weight of each species exhibited a highly positive correlation (r � 0.85, 0.94, 0.94, and 0.98 for P. pangasius, R. rita, S. aor, and B. bagarius, respectively).Te values of intercept (a), regression coefcient (b), coefcient of determination (R 2 ), and condition factor (K) of LWRs and LLRs are presented in Table 1.

Discussion
Te LWRs parameter "b" of a fsh depends on several factors, including species, seasonality, habitat, feeding intensity, age, sex, and geographic region [14].Te value of the regression coefcient "b" in LWRs remains constant at 3 for an ideal fsh living in ideal conditions [15].However, it is common to observe variations in the "b" parameter under natural conditions.A "b" value less than 3.0 indicates a negative allometric growth pattern of a fsh species, where the fsh becomes slenderer as it grows and value greater than 3.0 indicates positive allometric growth, with the fsh becoming heavier [15].In the present study, negative allometric growth patterns were observed in all species (except B. bagarius) which might be attributed to the age of the collected samples.Te collected samples except B. bagarius were in their juvenile stage, which is defned as having a total length of less than 30 cm.It was reported that P. pangasius, S. aor, and R. rita reach maturity at sizes of 54 cm, 57.3 cm, and 29.5 cm, respectively [16][17][18].During the early stages of life, fsh tend to develop more in length than in weight [19].Te result of the present study is comparable with the fndings of Yusof et al. [20].Besides, several authors have demonstrated diferences in "b" values at diferent life stages of fsh, showing that "b" values increase gradually with age [10,21].Te importance of LLRs in fsheries' management also becomes evident when conducting comparative growth studies [22].Te relationship between TL and SL of P. pangasius, R. rita, S. aor, and B. bagarius fsh in all sexes showed that TL-SL are highly correlated (r > 0.9) to each other.Te condition factor (K) is an important quantitative parameter for determining the relative degree of nourishment and habitat condition in fsh [23].Te observed "K" values were found to be less than 1 for all three species except R. rita (1.18).Te lower "K" value could be attributed to the age of the specimens, given that all of them were in the juvenile stage.Te "K" value of a population does not only depend only on environmental elements such as temperature, rainfall, salinity, and food availability [24] but also the age and maturity stage of fsh, which can signifcantly afect the value [25].
Te vital role played by these species in providing nutrition, livelihoods to local fshers, coupled with their vulnerable states, makes it imperative to adopt a comprehensive sustainable management practice.Te foundational data on LWRs and LLRs for the four riverine catfsh species documented in this work might facilitate fsh biologists and policymakers in evaluating population parameters and guiding the implementation of appropriate conservation measures aimed at restoring these species in the MRE.