Length-weight relationships of three freshwater eel species from the oodplain ecosystems of northeastern Bangladesh

This study reports the Length-weight relationships (LWRs) of three freshwater eel species from the northeastern wetlands of Bangladesh. A total of 2684 specimens of Mastacembelus armatus, Macrognathus aculeatus and Macrongathus pancalus were sampled between in the months of June and December 2019 using different nets and eel traps at every 4 days working schedule per month. The calculated a values of the LWRs ranged from 0.008 to 0.020; b values ranged from 2.35 to 2.70, and r 2 from 0.907 to 0.944. The lowest values of regression coecient found for M. aculeatus and the highest for M. pancalus. All the regression parameters for LWRs found highly signicant (p (cid:0) 0.001). These results will provide baseline information for sustainable management and conservation of the sheries resources in the northeastern oodplain region of Bangladesh.


Introduction
The oodplain ecosystems in northeastern Bangladesh are considered as one of the most promising sectors for inland capture sheries with around 858,460 ha of resource area and at least 143 indigenous sh species (BHWDB, 2012).
The length-weight relationship (LWRs) data of sh species is an important biological parameter that provides information on the condition of sh stock, allows estimation of biomass and to determine whether somatic growth was isometric or allometric (Ricker, 1975;Froese, 1998;Moutopoulos & Stergiou, 2002;Gonzalez Acosta et al., 2004;Duan & Tan, 2016). The LWRs are also important in converting the body length into biomass when direct weight measurement in the eld is di cult and time-consuming (Martin-Smith, 1996;Koutrakis & Tsikliras, 2003). However, the LWRs data for the indigenous sh species in the northeastern wetlands of Bangladesh are still scare. Therefore, the present study aimed to estimate the LWRs of three indigenous freshwater eel species Mastacembelus armatus (Lacepède, 1800), Macrognathus aculeatus (Bloch, 1786) and Macrongathus pancalus (Hamilton, 1822) from the northeastern oodplains in Bangladesh, which have not yet recorded in FishBase (Froese & Pauly, 2016). Mastacembelus armatus is a freshwater spiny eel having great economic value in India and Bangladesh; Macrognathus aculeatus is a near threatened highly valuable small indigenous species in Bangladesh with toothsome nutritive value and notable aquarium importance and Macrognathus pancalus is a commercially important and palatable as a table sh which fetches a good price when sold alive. The wild population of these species has steadily declined mainly due to habitat degradation, introduction of alien invasive species, over-exploitation, diseases, pollution, poisoning, and other destructive shing. The results from this study on the LWRs of these eel species will serve as baseline information for online sh database and accelerate sustainable management and conservation of less studied freshwater shes in northeastern Bangladesh.

Materials And Method
A total of 2684 specimens of three freshwater eel species were collected from different Beels in northeastern Haor region of Bangladesh during June to December 2019 from the shermen catches. The selected species were represented by M. armatus (n = 795), M. aculeatus (n = 823) and M. pancalus (n = 1066). Fish were caught by different types of shing gears including sh nets and eel traps. All the specimens were identi ed at species level in the eld, total length was measured by using measuring scale and weight were measured by using digital balance.
The LWRs of each sh species were calculated using the log-transformed equation W = aL b , to derive a linear relationship: Ln BW = Ln a + b Ln TL where, BW is the total weight of sh (g), TL is the total length of sh (cm), a is the intercept and b is the slope of regression coe cient.
Intercept (a) indicates the body structure of species and slope (b) indicates their growth pattern. All the length-weight data transformed to natural logarithm prior to performing regression analysis using MS Excel and the outliers removed following Froese & Pauly, (2015). The regression parameter b and coe cient of determination r 2 estimated at 95% con dence interval (Froese, 2006).

Results And Discussion
Descriptive statistics and estimated regression parameters a, and b at 95% con dence limits and coe cients of determinations (r 2 ) are presented in Table 1 (Froese, 2006) and was found to be signi cant at 1% signi cance level (p 0.001). The length-weight relationships indicate thessssss negative allometric growth for all the eel species in this study (Figure 1,2,3). Higher values of b (>3) indicate proportionate increase in body weight with increase in total length or positive allometric growth and lower values (< 3) indicates negative allometric growth, means that sh become slimmer as the length increases (Jobling, 2002). The lower b value in this study indicates that these three eel species use more energy in axial growth than biomass (Liu, Ye, & Li, 2016;Vicentin, dos S Costa, & Súarez, 2012). Hossain, Ahmed, Leunda, Islam, Jasmine, et al. (2006) reported the coefficient b of 2.864 for M. aculeatus revealing a negative allometric growth pattern from the Mathabhanga River, southwestern Bangladesh which is similar to this study. However, they also reported positive allometric growth in M. pancalus from the Mathabhanga River, southwestern Bangladesh, which do not comply with this study, may be due to seasonal variation, number of specimens observed, differences in observed length ranges, habitat and shing pressure status (Wootton, 1998). Abujam & Biswas (2016) also reported seasonal variation in positive allometric growth (b>3) and negative allometric growth (b<3) of M. pancalus from the upper Assam in India.
The actual length-weight relationship varies from species to species based on their genetic characteristics. Reproductive development and the availability of su cient food also affect the b value throughout the life cycle of sh. Interspecies LWRs also varies according to the condition of individuals within a species (Borah, Bhattacharjya, Saud, Yadav, Debnath, Yengkokpam, Das, Sharma, Singh, & Sarma, 2016). Sarker, Khan, Dabas, Pathak, Mir, Rebello, Pal & Singh (2013) studied the length weight relationship and condition factor of M. pancalus and M. armatus found in Ganga, Gomti and Rapti Rivers of India and found that the b value ranged between 2.11 to 2.65. Pathak, Zahid & Serajuddin (2013) estimated the length-weight relationship of striped spiny eel, M. pancalus and found the b values to be near about 3 that indicates isometric growth of M. pancalus and the coe cient of determination were highly signi cant (p < 0.001). Qin, Lin, Li, Wang, Guo & Liu (2017) estimated the length-weight relationship of M. aculeatus from a tributary of the Yilou River, China and found that the b value was 3.09. This is the rst report on biological parameters for three indigenous eel species in the northeastern oodplain ecosystems of Bangladesh. The results from this study on the LWRs of three freshwater eels will serve as baseline information for online sh database and accelerate sustainable management and conservation of less studied freshwater shes in northeastern Bangladesh.