PHOENIX (AP) — Fish that primarily feed on algae and weeds in canals that deliver water to the Phoenix area are overeating and getting even bigger.

The Salt River Project made the discovery as crews move fish to different parts of the canal system for maintenance. The white amur fish that typically measure around 30 inches (76 centimeters) are up to 15 inches (38 centimeters) longer, said SRP spokesman Jeff Lane.

The Salt River Project has been using the sterile fish native to China to help clear the canals since 1989 and avoid treating the water with chemicals, The Arizona Republic reported . The largest of them were found near Mesa's Granite Reef Diversion Dam, an area thick with vegetation.

The utility looks for a certain type of fish for clearing canals — ones with heads big enough that they cannot wiggle through grates. Wide-headed fish also mean the grates can have fewer bars and allow water to flow through more freely, said Brian Moorhead, an environmental scientist and engineer with SRP.

The crews drain portions of the major canals annually to repair cracks, remove trash and clear debris. A crane with a net scoops the fish out of the canal.

While they're coveted for the amount of vegetation they can eat — up to two-thirds of their body weight daily — the cold can halt the fish's work.

Moorhead said the fish stop eating when the temperatures dip to 65 degrees, but vegetation has a lower threshold for growing at 60 degrees.

"We want them to eat all the vegetation," he said.

The cleanup of the canal system is scheduled to run through Feb. 5. The maintenance also gives local governments an opportunity to do roadwork or improve bridges over the canals, the Salt River Project said.

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Information from: The Arizona Republic, http://www.azcentral.com