Monmouth County's health department received a report at 5:30 a.m., Monday. Officials say thousands of fish have died, but a total count has not been completed.
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.
State environmental officials tell the newspaper that preliminary test results showed acceptable oxygen levels and almost no algae in the water. They say "slightly low oxygen levels" likely resulted in a fish kill.