A mysterious discovery along the Jersey shore is raising questions after nearly 30 juvenile sharks were found dead on a Sea Isle City beach, according to reports from CBS News, Fox News and ABC6.

Beachgoer Michelle Greenling told CBS News that she counted 29 dead sharks during a walk on Wednesday, Aug. 12, finding them scattered across a 10-block stretch. Some of the sharks, known as spiny dogfish, had red markings on their undersides, she said.

“It was definitely concerning. I was concerned about the water. I was concerned about the sharks,” Greenling told CBS News.

While officials don’t yet know the cause, experts have noted the number of beached sharks is unusual. Mike Heithaus, a shark ecologist and dean of Florida International University’s College of Arts, Sciences & Education, told Fox News Digital that it is rare to see so many sharks wash ashore at once.

What happened to the sharks?

The cause of the deaths was still unknown as of Friday, Aug. 14. One possibility is that the sharks may have become stranded by changing tides or accidentally caught by fishermen targeting other species and discarded offshore, Stephen Nagiewicz, an adjunct professor of marine science at Stockton University, told CBS News.

Additionally, the absence of other dead marine animals suggests the incident was likely not caused by pollution or another environmental toxin, Heithaus told Fox News.

The New Jersey Division of Fish and Wildlife said dogfish and other small sharks commonly forage close to shore during the summer and can become trapped near sandbars as the tide recedes, according to ABC6.

The sharks were already gone by the time a CBS News crew arrived at the beach around noon Wednesday. Nagiewicz said they were likely washed back out to sea by the incoming tide.

USA TODAY has reached out to the New Jersey Department of Environmental Protection for additional information about the incident.

What are spiny dogfish?

The sharks were identified as juvenile spiny dogfish, a small species of shark commonly found along the Atlantic coast. Spiny dogfish are known for traveling in large schools and can live for 35 to 40 years, according to the National Oceanic and Atmospheric Administration.

Spiny dogfish have two dorsal fins, each preceded by a sharp spine, according to NOAA. Spiny dogfish typically live near the ocean floor but can also be found in midwater and near the surface. They migrate seasonally as water temperatures change, with some moving closer to shore during the spring and summer.

Reporter Anthony Thompson can be reached at ajthompson@usatodayco.com, or on X @athompsonUSAT