A fishy odor wafted from the murky waters of Waialae Stream in Waialae Beach Park on Tuesday morning, and no wonder: All up and down the banks, the decomposing bodies of dozens of fish floated amid leaf litter and trash.

“That’s a lot of dead fish, and look at the garbage and the plastic,” said Ethan Gunn, a Honolulu resident who had brought his visiting cousin from Atlanta to enjoy this rare swath of public open space and beach access, which lies between the Kahala neighborhood of multi-million-dollar mansions and the exclusive Waialae Country Club and ritzy Kahala Hotel & Resort.

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

The corpses were those of tilapia, a hardy, introduced freshwater species that inhabits Honolulu’s streams and canals and is also farmed in ponds.

The scattering of lifeless bodies evoked fish kills caused by algae blooms, 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 Hawaii Dept. of Health Clean Water Branch, reported in an email after staff were dispatched to assess the situation in response to a Star-Advertiser’s inquiry.

Honda said fish kills can be “tied to episodic periods of low oxygen in certain waterbodies, 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.”

Indeed, Waialae Stream in Tuesday’s low tide formed a stagnant muliwai, or lagoon, separated from the ocean by sand. This month, Honolulu rainfall has totaled a scant .12 inch.

Honda added that 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 concluded.

Waialae Stream actually gets regular circulation, said Kris Arguin, a chef at Arancino restaurant, who sat on the beach preparing for a dive.

“At every high tide, the ocean washes over the sand and meets the waters of the stream,” Arguin said.

But the problem with that, he added, is polluted water and solid trash from the stream, which flows through Waialae golf Course, wash out to sea.

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

When the ocean is clear, “we see everything from mattresses to hundreds of golf balls, and it’s all hurting the reef.”

As for now, a team from the Hawaii Dept. of Land & Natural Resources Division of Aquatic Resources is also investigating the dead fish, said Dan Dennison, senior communications manager.