Staring down at the brownish-green, debris-filled waters of Waialae Stream in Waialae Beach Park on Tuesday morning, Ethan Gumm shook his head and pointed to the bloated bodies of dozens of dead fish floating along the banks. A fishy odor hung in the hot air.

“That’s a lot of dead fish, and look at the garbage and the plastic,” said Gumm, a Honolulu resident who had brought his visiting cousin from Atlanta to the park and beach access lying between the affluent Kahala neighborhood and the exclusive Waialae Country Club and Kahala Hotel & Resort.

A frequent park user, Gumm said he was shocked at the condition of the stream. “I’ve never seen dead fish like this.”

All the dead fish, about 100 in total, were tilapia, according to the state Department of Land and Natural Resources Division of Aquatic Resources, which sent a team to investigate. “DAR biologists as well as coordinating agencies are looking into possible causes,” said DLNR communications specialist AJ McWhorter. “Even after samples have been analyzed, we still may not be able to determine the cause.”

The scene at the stream evoked fish kills caused by algae blooms, commonly triggered by nutrient runoff from fertilizers and cesspools.

“The water did appear ‘greener’ than normal but no sheen or evidence of a wastewater spill was observed,” Myron Honda, coordinator of the state Department of Health Clean Water Branch, reported in an email after staff assessed the situation.

Honda said fish kills can be “tied to episodic periods of low oxygen, often seen downstream of wetland habitats following large storm events, as well as following periods of hot dry weather in low-lying canals where algae blooms can occur.”

Waialae Stream in Tuesday morning’s low tide formed a stagnant muliwai, or lagoon, separated from the ocean by sand. This month Honolulu rainfall has totalled a scant 0.12 inch.

Honda said only tilapia appear to have been affected, as live crabs, mullet and papio were observed.

“Based on the presence of other aquatic species, and no visible evidence of a petroleum or wastewater spill, there does not appear to be a clear cause of the fish kill,” he said, adding that sometimes apparent fish kills are actually the illegal dumping of bycatch.

Gumm said that in his opinion a fisherman wouldn’t have caught and discarded so many fish.

Waialae Stream gets regular circulation, said Kris Arguin, a chef at Arancino restaurant, as he prepared for a dive.

“At every high tide the ocean washes over the sand,” Arguin said. And, he added, pollution washes out to sea from the stream, which traverses Waialae Golf Course.

“At high tide in the ocean, you can’t see your hand in front of your face.”

When the ocean is clear, “(divers) see everything from mattresses to hundreds of golf balls, which hurts the reef,” he said, adding that he participates in reef and beach cleanups.