Something extremely alarming is happening in the San Francisco Bay, and scientists are trying to figure out what is going on. Researchers say that huge numbers of dead leopard sharksk have been found since mid March off the coast of Redwood City, Foster City, Alameda, Hayward, Berkeley, Oakland and San Francisco, according to a San Francisco Chronicle report.

It’s being called the largest death event for leopard sharks in the last six years. The leopard shark is the most common species of shark in the bay, and it could involve thousands of sharks.

The last time something like this happened was in 2011, when authorities found more than 1,000 dead sharks near Marin County. What’s causing these events? Scientists think it may be toxins from stagnant water.

Here’s what the Pelagic Shark Research Foundation says about the leopard shark: “The leopard shark represents a top predator within the Elkhorn Slough and is one of the most common coastal/estuarine species of shark to be found in California and the Eastern Pacific Coast. These sharks range from Oregon to northern Mexico. Born at a length of several inches (20 centimeters), adults are reported to reach lengths of over 2 m (over 6 feet) though the average length for adults is less than 1.6 m (5 feet). After a 12 month oviviparous gestation period, 4 to over 30 pups are born during the spring in shallow coastal bays, and marine estuaries which serve as their nursery and incubation areas.”