Hartford — Connecticut environmental officials are looking into what might have caused the die-off of fish in rivers statewide over the Memorial Day weekend.

The Department of Energy and Environmental Protection said Thursday that natural fish kills of Atlantic menhaden were reported in locations including the lower Connecticut River, the Quinnipiac River and the Thames River. It said hundreds or thousands of menhaden were found dead at each location.

The department said it is common for the menhaden to die off in large numbers where the fish are found in abundance. Other such cases have been traced to diseases that spread quickly through schools of fish.

Atlantic menhaden average about 10 to 14 inches in length. Authorities say they have grown abundant in the last two years because of fish harvest limits.