It's a mystery: what's causing fish kill off Myrtle Beach?

Something is killing fish and sting rays in the ocean off Myrtle Beach

's coast

, but exactly what's going on out there is a mystery.

All along the beach from around 28th to 68th Avenues north on Friday, beach-goers were finding mixed species of dead fish floating in the surf or washing up on shore.

"It's sting rays, there's pompano, there's whiting, flounders, little bit of everything," said Kerry Caramanis of Virginia.

The fish did not have any obvious signs of trauma, he said. One man and his family told NewsChannel 15 they found 12 dead sting rays in a small stretch of beach.

"You know it's maybe the warm water, I don't know. Who knows? It's rare though, it's rare," said Carlo Pedone of Burlington, Wisconsin.

Longtime fishermen were baffled.

"We've been coming here for over 20 years and I've never seen anything like that happen in this area before," said Bob Colgan of Indiana, Pennsylvania. "The ocean seemed like it was dead this morning."

Any number of things can cause fish kills. Algae blooms, known as red tide, can kill thousands of fish at a time.

"No bite marks on the fish so I would say that it's some kind of red tide or something of that nature causing it," Caramanis said.

A red tide event would be toxic for humans as well as fish.

Phil Maier, a marine biologist with the SC Department of Natural Resources office in Charleston, told NewsChannel 15 that water conditions are right for Friday's fish kill to be caused by hypoxia, or low dissolved oxygen in the ocean, but the investigation continues.

Maier said officials from Coastal Carolina University, DNR and the Department of Health and Environmental Control were involved in trying to find answers.

This week, researchers from CCU's environmental quality laboratory showed off their latest equipment for monitoring low dissolved oxygen levels in the ocean off the Grand Strand