The wash-up of small menhaden, most about 4 inches long, was discovered Wednesday morning.
It occurred on about eight miles of shoreline from Kimbles Beach in Middle Township south to the Villas in Lower Township, just north of Cape May.
Officials say water sampling conducted Thursday suggest that oxygen levels in the bay were probably very low as a result of very warm air and water temperatures.
If the fish had massed close to shore for some reason, they could have rapidly depleted the available dissolved oxygen.
Removal operations are expected to begin Friday.