Trout have been dying at Mammoth Pool reservoir on the upper San Joaquin River due to a lack of oxygen, and the California Department of Fish and Wildlife is blaming the electric utility that owns the reservoir.
Oxygen readings are in the killing zone because Southern California Edison released too much water from the lake, said Peter Tira, spokesman for the state agency.
“We have a very unfortunate, discouraging situation with Mammoth Pool,” he said.
The lake is a popular fishing spot, but unless the utility company restocks it with fish big enough to catch, anglers at Mammoth Pool will be disappointed next summer, Tira said.
Southern California Edison said it had no choice but to almost completely drain the lake.
“The U.S. Geological Survey and the Federal Energy Regulatory Commission required Southern California Edison to install devices to measure the water levels at Mammoth Pool Reservoir,” the company said in a statement.
In addition, the state agency complaining about the fish die-off was among the entities that signed off on the project, the company said.
“SCE received approval for construction of the project from the U.S. Forest Service, Regional Water Quality Control Board, the U.S. Army Corps of Engineers, and the California Department of Fish and Wildlife,” the statement said.
The utility company started lowering the reservoir water level about a month ago but later learned that Fish and Wildlife had raised concerns about a possible fish die-off, so it contacted the state agency for more information but has not heard back, the company said Wednesday.
As the measuring devices are installed, the reservoir level is slowly rising, it said.
The reservoir was built in 1958 to generate hydroelectric power as part of the company’s Big Creek system. The lake is northeast of Fresno on the border of Fresno and Madera counties.
Fish and Wildlife said a truck from a fish hatchery arrived about Oct. 9 with a tanker carrying 25,000 small rainbow trout to restock the lake for next season.
“The truck driver drove up there and saw the lake was so low and muddy there was no way he could put fish in there and they would survive,” Tira said. He then drove off with the fish.
Soon, fishermen started complaining to the agency about fish dying, he said.
Fish and Wildlife biologists drove to Mammoth Pool three times to figure out what was going on, Tira said.
The dissolved oxygen in the water tested at 2.2 milligrams per liter, Tira said.
“Anything below 4 milligrams per liter is considered lethal for trout and even 4 can put trout under serious stress,” he said. The lake was at 4 percent capacity – too low for trout to survive, he said.
On Oct. 20, biologists paddled a kayak along the shoreline and found dead fish. Oxygen levels averaged 2.5 milligrams per liter. The levels were 3.0 in the north end of the reservoir and 5.0 where the river enters the lake.
Biologist also fear for the native brown trout and hope they’re surviving in the tributaries that flow into the lake, he said.
Fish and Wildlife officials contacted Edison and learned the utility had lowered the lake for a project, Tira said.
“We are in the process of drafting a letter to the Federal Energy Regulatory Commission asking that Southern California Edison rectify the situation and stock catchable trout,” he said.
Without restocking with bigger trout – it’s too late to put in small trout that would grow over the winter and spring – there won’t be recreational fishing in 2018, and that would hurt the local economy, he said.
Fisherman Casey Graham, 32, of Coarsegold said he was shocked when he went to the lake in mid-October and saw how low the lake was.
“It’s a tragedy,” he said. “My dad is from North Fork, and he said he’s never seen it that low.”
He and a friend went out in a boat. “It’s just brown,” he said of the lake.
They saw many fish. At first, “we thought they were actively feeding,” he said. “What they were doing is coming to the surface and gulping. They were trying to get the dirt out of their gills. There were so many you could have scooped them up with a net … The bank was littered with fish, dead fish floating all over the place. It’ll take years to recover.”
David Woolley, 70, of Fresno said he’s been fishing at Mammoth Pool for decades.
About Oct. 20, he stopped at an overlook to take a photo. “I couldn’t believe it, I couldn’t see the water” because it was so low, he said. At the lake, “the water was really, really dirty,” he said.
There were so many fish at the surface trying to get oxygen, eagles and ospreys were feasting on those that were still alive, he said.
Maybe Mother Nature left a way for some to survive, he said: “I hope the fish moved up (to the north end of the lake).”