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9/8/2003
Division of Wildlife

KOKANEE KILL AT VALLECITO RESERVOIR



The Colorado Division of Wildlife’s (DOW) kokanee salmon spawning operation suffered a major blow earlier this month due to a fish kill at Vallecito Reservoir. The kill, affecting only kokanee is due to a variety of conditions perpetuated by the ongoing drought conditions in Southwest Colorado.

The first reports of dead kokanee being found at the reservoir were taken in late August. A conservative estimate of dead fish numbers in the thousands. Vallecito is one of the major contributors to kokanee spawning operations that help provide kokanee for 25 lakes and reservoirs across the state.

The good news for anglers who frequent the reservoir is that the other sport fish species that inhabit the shallower waters of the reservoir were not affected by the die-off.

“The fish kill primarily affected kokanee salmon and it was not a complete kill,” said Mike Japhet, DOW aquatic biologist in the Durango office. “Catchable rainbow trout, brown trout and northern pike are also present in Vallecito and have not been affected by the depleted oxygen conditions found in the deeper part of the lake.”

The life cycle of the Kokanee salmon is relatively short. Depending upon conditions, kokanee spend three to five years in a lake or reservoir before swimming upstream to spawn at the same location from where they were released as fry. The egg-swollen females and the brick-red hook-jawed males average 16 to 18 inches. Kokanee die after spawning. Some of Colorado’s better kokanee reservoirs include Blue Mesa, Elevenmile, Granby, Green Mountain and Vallecito.

At Vallecito, spawning kokanee head to Vallecito Creek to do their yearly ritual. The DOW builds a fish trap to catch the spawning kokanee. Eggs and milt are extracted from mature salmon fertilized eggs are transferred to the Durango Hatchery where they are hatched and reared to 2 inches in length. Kokanee salmon fry are released back into the creek the following spring as the next year class of fish. Over the last 34 years, Vallecito has produced 1.2 million kokanee eggs on average. Even with the kokanee kill, the operation will continue in hopes of making the most from the remaining fish.

“We have low expectations,” said Japhet.

The die off is due to low dissolved oxygen content in the deep water that is inhabited by kokanee. It was a natural phenomenon, which could not have been prevented.

“This is the first instance of a mass fish kill in the history of Vallecito Reservoir,” said Japhet. “Kokanee are known to inhabit deep water just below the thermocline, which at Vallecito presently contains only 3.8 parts per million dissolved oxygen.”

The ongoing drought in Southwest Colorado and continuing effects from last summer’s Missionary Ridge fire led to extreme reservoir draw down, warmer than normal water temperatures and lower than normal dissolved oxygen content of the water.

“High amounts of organic debris that washed into Vallecito from last year’s fire probably contributed to high biological oxygen demand, leading to oxygen depletion in the deeper parts of the reservoir,” said Japhet.

The conditions have been hard on bodies of water throughout the region. A fish kill was also recently reported at Lower Four-mile Lake--a high mountain lake home to cutthroat trout.

The fishery can be, and will be re-established. With help from other kokanee spawning operations, DOW officials are optimistic that loss due to the kill at Vallecito can be covered. The stocking of Vallecito with kokanee will take place next spring in hopes for better conditions.

“If we see more snowpack and more overall precipitation this winter things will change for the better,” said Japhet. “In this business, just like a farmer, you have to be an optimist and the only way to guarantee that there wont be kokanee in the lake is to not put them there.”
 

 

 

 

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