State Department of Health signs posted along the canal warn people not to eat fish or shellfish found in the Ala Wai because of possible contamination from urban runoff into the Waikiki waterway. But that didn't stop Keith Harvey, a barge mate working on the Ala Wai dredging project. Harvey cooked one of several Mantis Shrimp (Odontodactylus Scyllarus) pulled from the mud at the bottom of the canal. The largest shrimp weighed in at 1.35 pounds and 15 inches.

Warning signs along the Ala Wai Canal promenade advise people not to eat fish or shellfish from the canal because of a risk of contaminants. Bruce Asato • The Honolulu Advertiser

"It was big. Like your arm," said Karen Ah Mai, executive director of the Ala Wai Watershed Association. "They do find them, but generally not that big. Maybe it was the super nutrients. That was supposed to be a world record."

Mantis Shrimp are crustaceans that live in shallow waters and normally grow to about a foot long.

Bottom feeders and fish such as tilapia — found in large schools in the canal — do well in the Ala Wai because food from runoff is plentiful, said Eric De Carlo, associate professor of oceanography at the University of Hawai'i-Manoa.

"They are delighted down there because there is so much algae and plant detritus that it is like a smorgasbord," he said.

However, the runoff contains pollutants such as copper, zinc and chlordane that can be a health risk, De Carlo warned.

Mantis Shrimp

"In Hawai'i our storm drains go right into our waterways," De Carlo said. "There is a bunch of heavy metals that have accumulated in the canal, primarily from road runoff, the most notorious being lead from all our gasoline use."

The creatures in the Ala Wai absorb the contaminants and, if eaten, pass them on up the food chain, said De Carlo, who is working on a city study on the efficiency of storm drain filters in reducing nonpoint source pollution from street runoff.

The plentiful shrimp, fish and crabs in the Ala Wai are tempting to some people, he said, but eating them and the pollutants they contain can increase your risk of getting cancer.

"If it weren't in the Ala Wai I'd say pass it over, I'll cook it," De Carlo said. "Most of us know better, but you see a lot of first-generation immigrants who may not speak English very well and they go to the Ala Wai Canal and catch crab. It isn't for fun; it's to go home and put on the dinner table for their families who are struggling. Our immigrant populations are at much greater risk because they don't know of these hazards."

The canal collects and drains water from Manoa, Palolo, Makiki and surrounding areas. It acts as a catchment basin, trapping sediments and other pollutants that flow into the canal, but without ocean circulation, it has slowly filled and in some sections is only inches deep at low tide.

Dan Mahnke, project superintendent for American Marine Corp., the company dredging the canal, said that as crews scoop up mud and silt from the bottom, many things have been found, from grocery carts to tires and sometimes ocean animals.

"We dug up two of (the shrimp) that beat the current state record and one that eliminated the record," Mahnke said. "He was enormous. His tail was bigger around than my forearm and about the same length."

Harvey said now that he tasted the Mantis Shrimp, it is unlikely he will eat anything else from the canal. "I heard they are sweet; that is why I tried it," he said. "It was sweeter than lobster."