Lake Cuyamaca Sprayed to Curb Weed That Killed Hundreds of Fish

Lake Cuyamaca has been treated with a herbicide to reduce the weed responsible for killing hundreds of rainbow trout in the past month.

No new trout will be stocked in the lake until the water’s alkaline level--raised unusually high by uncontrolled growth of the sago pond weed--is reduced, said Hugh Marx, general manager of the Lake Cuyamaca Recreation and Park District, which regulates recreation at the reservoir.

The lake’s pH level--a measure of acidity--is expected to return to normal by the middle of next week. Shipments of hatchery-bred trout normally are put in the lake every 10 days.

“Everything is under control,” Marx said. “It (the fish kill) was not as bad as it sounds because it was only a small percentage.”

About 700 of the 6,500 trout--or 15% of the lake’s last two shipments--died, but the lake still has as many as 10,000 trout and plenty of healthy bass, catfish, crappie and bluegill, Marx said.

The aquatic herbicide Diquat, classified as moderately toxic, was sprayed on the lake last Wednesday and again Monday. Marx said the chemical application will not affect the fish or the edibility of catches, and fishing remains open as usual.

The state and county health departments and the Helix Water District, which owns the reservoir, approved the use of Diquat on the lake.

The original permit required a two-week ban on fishing after the herbicide was applied, but the state Department of Health Services amended the permit to allow continued fishing after realizing it had “screwed up,” Marx said.

He said the waiting period was based on outdated toxicological data and that more recent research showed that the chemical is not stored in fishes’ edible muscles.

State health department and Helix district officials were not available for comment.

“Believe me, I would not in good conscience use anything that would hurt anybody,” Marx said. “I would not put anybody at risk. I’m the one who handled the stuff on the barge.”

The state health department plans to monitor the lake’s water quality.

Marx described Diquat as one of the safer aquatic herbicides and said it is commonly used to check the growth of weeds on recreational fishing lakes.

Mike Dorsey, deputy agricultural commissioner of the San Diego County Agriculture Department, said Diquat is a common herbicide that has been used on other area lakes to control weeds.

Lake Cuyamaca’s depth has dropped from its normal 12 to 13 feet to less than 10 feet during the drought, allowing the sago pond weed, or duckweed, to proliferate.

A fresh-water version of kelp, the weed grows from the bottom and reaches about 11 feet in length, spreading on the water’s surface. It is common along the edges of deep-water lakes throughout the Western United States.

Lake Cuyamaca’s lowered depth has allowed the weed to grow and cover most of the lake, elevating the water’s pH--the degree of alkalinity--from a normal level of 8 to about 9.7, Marx said. Trout, already stressed by four-hour truck rides from hatcheries, begin getting sick at about 9 pH.

Trout that survive in the lake for two days can adapt to their new environment and are ready to bite after that, Marx said.

Rainbow trout are highly susceptible to changes in their environment and other factors, such as pH, dissolved oxygen and water temperature, Marx said.

“They’re really fragile. You look at them cross-eyed and they turn belly up on you,” he said. “You know how miners used to take canaries into the mines to detect poison gas? Well, rainbow trout are like that, real sensitive.”

Fifty surface acres--or about half the existing surface acreage--was sprayed with the herbicide, leaving plenty of weeds for fish to hide in and ducks to eat, Marx said. At normal depth, the lake has 110 surface acres.

The Lake Cuyamaca Recreation and Park District and the California Fish and Game Department each put 20,000 pounds of rainbow trout into the lake annually. The state’s fish, which weigh about 7 ounces each, or half the size of the district’s fish, make good catches for children, Marx said.

These smaller trout, however, often die after being caught and tossed back by fishermen who want only the bigger trout in fulfilling their daily limit of five fish per person. Trout is by far the most popular target.

District regulations prohibit the throwing back of any fish--trout, especially, are susceptible to the trauma of being hooked and handled--but fishermen commonly break the rule, knowing that the district lacks the manpower to patrol the lake for violators, Marx said.

“There are a lot of greedy people out there,” Marx said. “We try to enforce the rules, but we don’t have enough people.”

The recent fish kills have not caused fishermen to stay away, Marx said. Fishing is allowed year-round. The peak season is from mid-May to mid-September.

The lake has had fish kills in recent years but never with two consecutive restockings and nothing as extensive as the past two, Marx said. The causes of the previous kills were never determined.

The reservoir was created in 1888 in the oak and pine forest along Sunrise Highway in the Laguna Mountains.