Understanding the reproductive ecology of Macrognathus aculeatus in wetland (Gajner beel) ecosystem: insights into eco-climatic influences in northwestern Bangladesh.

The lesser spiny eel, Macrognathus aculeatus (Bloch, 1786), holds substantial economic importance as a food fish in South Asia, due to its exceptional nutritional value. This study was conducted to investigate the reproductive ecology of M. aculeatus within the Gajner beel wetland ecosystem in northwestern Bangladesh, with a specific focus on size at sexual maturity, spawning season, and fecundity in relation to eco-climatic variables. A total of 649 M. aculeatus female individuals were randomly collected from January to December 2018, with 440 mature selected for this study. The findings of this study revealed that the female's total length (TL) exhibited a range from 10.5 to 28.0 cm. Multiple methods, including maximum length (L), TL vs. gonadosomatic index (GSI), TL-standard length (SL) regression, and logistic model estimated the size at sexual maturity (L) at approximately 15.02 cm, 14.0 cm, 14.4 cm, and 14.0 cm, respectively. The spawning season was observed between May and August, with a peak in June-July. The Fulton's condition factor (K) proved to be the most reliable indicator (p < 0.0001), with 50% and 90% of fish spawning at K values of 0.38 and 0.42, respectively. Relative weight (W) for females showed no significant deviation from 100 (p = 0.143), indicating a well-balanced M. aculeatus population. Fecundity varied from 600 to 7000 with TL and body weight (BW) that ranged from 10.5-28.0 cm, to 3.33-87.05 g, respectively. Fecundity was highly correlated with TL (p < 0.0001) and BW (p < 0.0001). The study also observed significant relationships between GSI and environmental factors, including temperature (p = 0.0016), dissolved oxygen (DO) (p < 0.0001), pH (p < 0.0001), alkalinity (p < 0.001), and climatic variable such as air temperature (p < 0.001) and rainfall (p < 0.001). The optimal range of DO, pH, and temperature for M. aculeatus spawning was found to be 5.82-6.10 mg/l, 7.65-7.85, and 28.55-31.0 °C, respectively. Additionally, our observation highlighted a robust correlation between GSI and rainfall. Moreover, data for 47 years (1971-2018) indicated a gradual increase in average air temperature by 0.029 °C per year and a decrease in annual rainfall by 2.96 mm per year. These trends could potentially lead to a shift in the spawning season of M. aculeatus in the next 15-20 years. This information holds significance for the conservation and sustainable management strategies of wild M. aculeatus stocks in the Gajner beel and its adjacent ecosystems, helping to address potential impacts of climate change.