Size structure, weight-length relationship and condition factor of the endangered loach Botia dario from Bangladesh

Botia dario is one of the most important small indigenous species in Bangladesh, having both edible and ornamental values. Though once the fish was abundant in the natural water bodies of Bangladesh, now is categorized as "endangered" due to a considerable drop in population over past two decades. The study was conducted to explore the seasonal length frequency distribution, weight-length relationships and condition of B. dario using R programming environment. A total of 226 specimens were collected from August 2018 to July 2019 from open water bodies of northeastern Bangladesh. Four seasons, i.e., pre-monsoon (March-May), monsoon (June-August), post-monsoon (September-November), and winter (December-February) were considered to assess season-wise synthetic fitted model. The total length of B. dario was recorded minimum (4.6 cm) in post-monsoon and a maximum of 12.9 cm in winter. The empirical cumulative distribution function varied significantly between pre and post-monsoon as well as between monsoon and winter (p < 0.001). Gabelhouse length category of the double merged Table (quality- and quality+) showed a bold significant difference between two seasons (p = 4.25e-09). Coefficient of determination (R2) values were recorded higher than 0.8 in all models suggesting that log10(Li) was responsible more than 80% to explain log10(Wi). The interaction term of DVR (Dummy Variables) models fSeason x log10(Li) were insignificant in all combinations (y = 0), indicating negligible difference in slopes between any two seasons. Significant differences were observed between the model intercepts (δ ≠ 0) of monsoon versus winter (p < 0.001) and post-monsoon versus winter (p < 0.01). In terms of weight-length residuals (p = 0.0021) and relative condition (p = 0.01), the conditions of B. dario in winter were found significantly lower than the year-round mean value. The season-specific synthetic models recorded in this study provide insight into the current status of the concerned species, which might attract policy makers and scientists to predict a prospective holistic model to save the species from extinction.