The incident took place at Mungila village, located about 35 km from Patna, as armed assailants barged into houses of some fishermen and fired at random, killing four on the spot.
Another received injuries, Senior Superintendent of Police (SSP) Jitendra Rana said.
Rana, who is camping in the village since last night, said the incident appeared to be a fallout of dispute between the fishermen and assailants who had demanded fish for free from the fishermen earlier in the day.
The bodies have been sent for postmortem and the injured fisherman admitted to the Patna Medical College and Hospital (PMCH) for treatment, he said.
The victims have been identified as Janardan Bind (19), Manish Bind (25), Uday Bind (34)and Ram Pravesh Bind (60), Paliganj police station's Station House Officer (SHO) Subodh Kumar said.
Meanwhile, the murder sparked off a protest by locals who blocked NH-102 near Imamganj Bazar demanding arrest of the culprits and compensation to the kins of the victims, he said, adding, senior police and civil officials went to the spot to pacify the agitators.