State wildlife officials say that about 200 fish found dead Sunday in a northwestern Indiana river likely died due to natural causes.

The dead shad were found near a Portage, Indiana, marina along the East Branch of the Little Calumet River.

The Indiana Department of Natural Resources’ communication director, James Brindle, says all indications suggest the fish died from “a natural event.” The (Northwest Indiana) Times reports that the DNR says shad die-offs are common in Indiana winters, especially when the temperature suddenly drops from the 30s and 40s to well below freezing — as happened last week in northwest Indiana.