Last week, El Salvador’s Ministry of Environment and Natural Resources announced that they had discovered a massive sea turtle die-off in Central America’s Jiquilisco Bay. Between 300 and 400 dead sea turtles were found floating seven nautical miles offshore, reports Rae Paoletta at Inverse.

Locals began noticing the decomposing corpses of turtles in the area as early as October 28, reports Sarah Gibbens at National Geographic, but officials waited to break the news to the public so they could collect more information about the deaths. Many species of sea turtles inhabit the Bay, including green turtles, hawksbill, leatherbacks and olive ridleys. The olive ridleys appear to be the species most affected in this latest event. Gibbens reports that another 300 dead turtles may lie 30 miles west in Isla Tasajara, but authorities have yet to confirm those deaths.

This isn't the first turtle mass death in El Salvador. In 2013, 200 turtles died and in 2006 120 corpses were discovered. In both of those cases, the turtles died from what's known as a "red tide," ​Gibbens reports, in which nutrient or chemical runoff causes an abundance of toxic algae to bloom, releasing deadly compounds into the water.

Authorities are now analyzing samples from the turtles but have not yet determined the cause of death, Paoletta reports. “There is not a lot of information provided about the sea turtles so we can only speculate regarding the cause of this mortality,” David Steen, an assistant research professor at Auburn University tells Paoletta. “Given that officials are conducting laboratory tests, we can probably rule out purposeful killing by predators (including people). However, turtles could be drowned in fishing nets. Other potential causes could be stress [caused] by changing temperatures, a bacteria or virus or even a parasite.”

As Gibbens reports, it's unlikely the die-off will severely impact turtle populations in the region, but it is a cautionary tale. After catastrophic losses in the 20th century, a report released in September showed that decades of conservation efforts have led to a slow rise in sea turtle numbers. But at the time, researchers also warned that those gains could be easily reversed by things like poaching, a return to destructive fishing methods, climate change and habitat degradation.

Sea turtles reproduce slowly, with some species taking up to 50 years to reach reproductive age. So disruptions can quickly turn catastrophic for their population numbers.

Another emerging threat to the turtles—and one that researchers do not know how to successfully deal with—is plastic. A 2015 study suggested that 52 percent of sea turtles analyzed globally had eaten plastic debris. That can lead to issues such as intestinal obstruction or perforation of the intestines, not to mention poorly understood toxic and hormonal effects of long-term exposure to the plastics.

The potential for turtle die-offs is not just limited to El Salvador either. Florida in particular has a long history of red tides. Even if the algae blooms themselves are not powerful enough to kill the turtles outright, research has shown the toxins can impair the animals—as well as other endangered species like manatees—leading to more accidents and collisions with boats.