Millions of small prawns became stranded on a beach in southern Chile this week, carpeting a three-kilometer (two-mile) strand in red, local fishermen and police said.

The fishermen in the Coronel area, some 500 kilometers (300 miles) south of Santiago, began noticing something out of the ordinary at the beginning of the week.

There were giant "red spots" appearing just off the shore, fishermen union president Juan Gutierrez told AFP.

By Tuesday, the red mass had moved to land: "Millions of prawns were stranded on the beach," Gutierrez said, shocked.

"I've been a fisherman 39 years and have never seen anything like it before," he said.

Gutierrez added that he's seen more prawns clumping near the shoreline, indicating another mass stranding could happen.

As residents removed the thick red spread of prawns, each a centimeter or smaller, civil police have also arrived to take samples and investigate the cause of this unusual phenomenon.

They will be looking to see "whether there was any trace or element" in the shrimp that would explain why they stranded en masse.

They are also working to determine conditions in the Coronel Bay, including its "temperature, electrical conductivity, and, especially, oxygen," environmental crimes investigator Victor Casanova told local media.

So far, the cause of the event is unknown. Some fishermen are blaming a change in ocean currents, while others suggest the culprit is a local coal-fired thermoelectric plant that uses seawater as a coolant.