Monmouth County's health department received a report at 5:30 a.m., Monday.
Public health coordinator Michael Meddis tells the Asbury Park Press moss bunker appears to be the affected species and the fish kill is mainly taking place in the boat basin in Belmar.
Meddis says bunker reproduce in large numbers and typically use a lot of oxygen. Officials plan to take some oxygen readings.
Meddis can't say how many fish have died.
The state Environmental Protection Department has been notified.
