LIVINGSTON, Mont.-Montana Fish, Wildlife and Parks crews are still searching for answers to what killed more than 700 mountain whitefish in the Yellowstone River south of Livingston last week.

The fish kill was detected Friday. Since then crews have documented more than 1,000 dead fish, but the figure could be more than 10 times that with the possibility of scavengers eating the remains or fish sinking to the bottom of the river.

Although the dead zone initially was noted between Loch Leven and Pine Creek fishing access sites in the Paradise Valley, by Monday FWP had received reports of dead whitefish as far upstream as Point of Rocks (south of Emigrant), to as far downstream as Grey Bear (east of Springdale), according to Andrea Jones, FWP information and education manager in Bozeman.

"Crews will be out again today floating farther downstream," from the Highway 89 bridge to Springdale, Jones said.

On Saturday, FWP took tissue samples from the dead fish to the U.S. Fish and Wildlife Service Fish Health Center in Bozeman, but Jones said it would be at least three days until results would be available.

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"I do know they're working on five of the samples today," she said.

"Tomorrow we're hoping to do some water quality testing, but to this point we don't know what's causing the fish kill," Jones added.

The fish kill was reported to FWP by fishing outfitters and guides.

Whitefish are an indicator species when it comes to the health of a waterway and are more susceptible to "stressors," Jones said.

A large whitefish die-off was documented in Idaho's Upper Snake River drainage - including the Henry's Fork, South Fork of the Snake and Teton rivers - in 2012. That incident affected mostly young whitefish and was possibly caused by a liver parasite, Tetracapsuloides bryosalmonae, also called PKD or Proliferative Kidney Disease, according to an Idaho Department of Fish and Game statement.

"This rare parasite has not been documented in Idaho's wild fish before, although it has been reported in both wild and hatchery trout and salmon in North America and in Europe," IDFG reported. "This may be the first time the parasite has been detected in any whitefish species. The life cycle of the parasite is not well understood, but involves a freshwater sponge as well as a fish."

Transmission and disease signs are linked to elevated water temperatures, IDFG said. But cold weather and water does not necessarily kill the parasite.

In June 1988, warm water was blamed for the death of an estimated 2,000 to 5,000 fish on the lower Madison River, 90 percent of which were whitefish. At the time of the die-off water temperatures were estimated to have ranged from 76 to 83 degrees - lethal temperatures for many coldwater fish species.

According to the U.S. Geological Survey's water gauge at Livingston, temperatures on the Yellowstone River did not exceed 68 degrees between last Wednesday and Sunday and were even a couple of degrees cooler at the next gauge upstream at Corwin Springs.

Mountain whitefish are native to Montana, inhabiting cold-water streams mostly in the western half of the state. Although some anglers don't value the silvery member of the trout family, others prize them as prime smoked fish fare in the winter.

A decline in juvenile whitefish prompted a cooperative research project on the Madison River between 2012 and 2014 in an attempt to understand if the fish are struggling with problems with reproduction.