Influence of modern irrigation, drainage system and water management on spawning migration of mud loach, Misgurnus anguillicaudatus C.

We investigated the influence of human activities on the spawning migration of the mud loach, Misgurnus anguillicaudatus. Mud loaches migrate from rivers to paddy fields for spawning. In this study, the number of adult loaches in paddy fields showed a peak in late May and rapidly decreased in July. In early July, water is discharged from paddy fields in order to allow the fields to dry for a few weeks, and water is again supplied later. The spawning season of the loaches probably spans from mid May to August; this was concluded by observing physiological indicators such as the gonad somatic index, oocyte maturity and serum levels of testosterone, estradiol-17 beta and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one. When the loaches mature, they migrated to paddy fields if they could swim upward through the drainage ditches. However, the drainage ditches of modern paddy fields were reconstructed to deepen the water levels. The artificial drought of the rice culture management led to a discharge of water and loaches from the paddy fields during their spawning season. These results indicate that modern rice cultivation causes obstruction of the migration route and loss of spawning grounds; consequently, loaches use limited space and period of the paddy field for their spawning.