Role of tide and lunar phases on the migration pattern of juvenile Hilsa shad (Tenualosa ilisha) within a meso-macrotidal estuary.

Tide and lunar phases often influence the behaviour and life cycle of different fishes, especially migratory species. In the Hooghly River estuary, Hilsa shad is an anadromous fish species that migrates from the adjacent sea to the estuary and rivers for spawning. After spawning, the juveniles remain in the rivers and estuary for few months then start their downstream migration towards the adjacent sea. However, the pattern of their downstream migration has not been studied in detail so far. This study investigates the role of tide and lunar phases on the juvenile Hilsa shad migration pattern. In this study, we have estimated the rate of juveniles migrating through the river channel (no. m-2  h-1 ) during high tide and low tide in all of the lunar phases. The number of juvenile Hilsa shad fishes is found to be much higher during low tides in most of the observations and there is a significant difference (t = 11.904, P < 0.001) between the high tide and low tide catches in the entire study region. Among the eight lunar phases, the number of juveniles is also observed to be higher during the new moon and full moon, and there is also a significant difference in juvenile catch among the lunar phases (F = 64.372, P < 0.001) in the entire stretch of the study area. These observations enabled us to develop a plausible mechanism of the downstream migration of Hilsa shad juveniles.