It is said to come around every spring. Apparently, it has two tails like bass-drum sticks that it swings around like lightning, beating them on the tight drum-like skin on its back. When the fish hear this they all flock near the shore in shallow water and lay their eggs. Then, after they lay their eggs the fish leave and the creature freely keeps watch until all the fish-spawn have hatched out.
Based on additional text that stated, "they (the fish) wouldn't know it was spawning season if they didn't hear the Dum-Dum", it would mean that the beast emits some sort of noise that activates breeding stimulus in fish and allows them to lay eggs at that point of time. Then again, the thought of it being an actual cryptid or a joke is still being weighed upon.