Length-weight relationships have been used extensively for the conversion of growth-in-length equations to growth-in-weight for use in stock assessment models to estimate the stock assessment, compare the life histories of certain species and other aspects of fish population dynamics [1-6]. It also allows for the study of the ontogenetic allometric changes in fish growth [7] and possible effects from parasites [8].

This study describes the length–weight relationships (LWRs) of 12 least concern and near threaten species according to IUCN Read list from the Padma River, near Rajshahi city, Bangladesh [9]. These fishes species once abundant in rivers, streams, canals, reservoirs, lakes, ponds and beel, haor and baor swamplands of Bangladesh, India, Nepal and Sri-Lanka [10], the populations are in serious decline due to overexploitation augmented by various ecological changes and degradation of their natural habitats. The Length-Weight Relationships (LWRs) of threatened fishes are the most important biological parameters to provide information on the growth and condition of fish species and the entire fish community, and are highly significant for management and conservation of natural populations [11,12]. Length–weight relationships have been reported for some commercially important fishes from the River Padma, but data for most of the endemic freshwater fish species are still missing. The present study was undertaken with the objective to estimate the length-weight relationships for 12 fish species collected from the River Padma near Rajshahi, Bangladesh.

The Padma is one of the largest rivers of Bangladesh. It is the main tributary of the Ganges which originates in the Gangotri glacier of the Himalayan. The part of the Ganga in Bangladesh is known as the Padma which enters Bangladesh from India (Murshidabad district) at Shibganj Upazila (Manakosha and Durlavpur unions) of Chapai Nawabganj district. Its length in Bangladesh is 366 kilometers.

This study was conducted from the year 2009-2011 on the Padma River near Rajshahi City, (24021, N and 88036,E) Bangladesh. Monthly samples were collected from different fishing spots and fish markets of Rajshahi City, June 2012 to May 2013. The main gear used by commercial fishers included cast nets, gill nets, drag nets, mosquito nets, hand nets as well as traditional fishing traps. The fresh samples were immediately chilled in ice on site and fixed with 10% buffered formalin upon arrival at the laboratory then identified according to [13-15] Jayaram, Rahman, Talwar, Jhingran. All morphometric measurements were taken according to [16] Froese and Pauly. All fishes were individually measured to the nearest 0.01 mm and weighed on a digital electronic balance (Bosch EP 628) to the nearest 0.01 g. The LWR was estimated using the expression W = a L^b (Ricker) [17], where W = total weight (g) and L = total length (mm). The parameters ‘a’ and ‘b’ were estimated by linear regression after logarithmic transformation of weight and length data (logW = log a + b log L), Where,

W = Weight of fish (g)

L = Length of fish (cm)

a = y Intercept or the initial growth coefficient

b = Slope or the growth coefficient

The statistical significance levels of r2 and SE for standard error of b (P<0.001) were calculated for all 12 species.

A total of 2015 specimens from 12 fish species belonging to seven families and 11 genera were used for the LWR calculations. The observed maximum total length among all individuals sampled during the study was 1177 mm, which was a specimen of P. pangasius having total weight 16 kg and minimum total length 17 mm with total weight 0.13 g of specimen G. Youssoufi. The regression coefficients (b) values ranged from 2.813 for G. youssoufi to 3.178 for C. reba, whereas the ‘a’ values ranged from 0.1242 to 0.8315 for L. boga and A. coila respectively.

The calculated ‘b’ value of the LWR indicated positive allometric growth in P pangasius, A. coila, C. nama B. lohachata (b>3.00), but the rest of the species shown negative allometric growth (b<3.00). All LWRs were highly significant (P<0.001), with coefficient of determination (r²) ranged from 0.838 for G. youssoufi to 0.976 for C. garua.

In the present study, value of ‘b’ (slope) close to 3 shows that the fish grow isometrically and other values show allometric growth [18]. Most of the estimates for ‘b’ values obtained in the present work show a similar trend with those of Sani et al. [19] in tributaries of the Yamuna and Ganga, by Sarkar et al. [11] in Ganga basin, Pet et al. [20] in Sri Lankan reservoirs, Ahmed and Saha [2] in Kapati Lake, Bangladesh, and by Sivakami [21] and Ramakrishniah [22] in the Nagarjunasagar reservoirs. However, in the present study, the higher value of b (>3) for some species may be due to the dominance of juveniles and an incomplete coverage of the known size range. Differences in the slopes of the length-weight estimates for the same species in different regions can be affected by environmental conditions or developmental state of the fish [23] or, according to Froese [10], by the range lengths used in the length-weight relationships.

The coefficient of determination (r²) ranged from 0.853 (L. boga) to 0.9741 (C. reba), nine of 12 regressions presented r² values higher than 0.90. All linear regressions were statistically significant (P<0.001). The calculated Standard Error of b (SE) ranged from 0.0068 to 0.0817, thus indicating a tendency towards positive allometry, which is in accordance with the majority of fish species [10].

In conclusion, this study provides an important baseline study on the LWRs of 12 least concern and near threatened fish species from the River Padma near Rajshahi City, Bangladesh [9,24]. These results can be an effective tool for fisheries management and conservation to initiate early management strategies and regulations for conservation of the remaining stocks of the endangered species in the Padma River. This study also provides valuable information for the online database, as well as providing an important baseline for future studies within the Ganga- Brahmaputra basin have almost all been subjected to perturbations of various origins.