Whirling Disease Parasite Found In Utah Reservoir

Evidence of the whirling disease parasite has been found in Utah's Strawberry Reservoir.

Fish pathologists with the Division of Wildlife Resources (DWR) found spores from the parasite in two of 60 tested kokanee salmon collected by biologists at the reservoir this fall.

The parasite was first found several years ago in the West Fork of the Duchesne River. Water from the river eventually empties into Strawberry through the Strawberry aqueduct and collector system, a Central Utah Project feature. This is the most likely way the parasite made it into the reservoir.

The DWR made attempts to contain the parasite in the West Fork drainage. "We're disappointed that WD is in Strawberry," said Roger Wilson, chief of the DWR's Aquatic Section, "but we're not surprised. We knew it was only a matter of time.

Whirling disease (WD) doesn't harm people, but it can be fatal to small trout and salmon. Some of the fish that contract the disease swim in a whirling motion before they die.

Despite the discovery, DWR biologists say Strawberry Reservoir should remain one of the country's best trout fishing waters.

"There is life after whirling disease," Wilson said. "The disease is not a fatal blow to the trout fishery at Strawberry."

The DWR cheif is optimistic about the future of the fishery for three reasons:

  • WD mostly affects small trout. "As trout grow from fry to fingerling size," he says, "the chances of developing clinical signs of the disease diminish rapidly." Most of the rainbow and cutthroat trout currently stocked into Strawberry are a minimum of 8 inches long. "WD should not affect these fish,"he says.
  • Most of the rainbows currently stocked into Strawberry are a WD-resistant strain known as the Harrison-Hofer strain. Because of the recent WD find, Wilson says the DWR will expand the use of this strain at Strawberry in the future. "Harrison-Hofer rainbows have been shown to develop much lower infection levels than other rainbow strains," Wilson says.
  • Studies have shown that Bear Lake cutthroat trout -- the type of cutthroats that are in Strawberry -- are more resistant to WD than other cutthroat strains.

Of all affected species, kokanee salmon has fish bioligists worried the most because they are the most susceptible to whirling disease. However, Wilson has hopes that what happened with kokanee salmon at Porcupine and Causey reservoirs in northern Utah will also happen at Strawberry.

"Whirling disease has been in the two reservoirs for several years," he says, "but the kokanee salmon in the reservoirs haven't been greatly affected by it."

Another area of concern is what effect the parasite will have on the natural recruitment of cutthroat trout and kokanee salmon in the reservoir.

Biologists currently stock rainbow trout, cutthroat trout, and kokanee salmon in the reservoir, but they also rely on the cutthroats and kokanees to reproduce naturally.

"Current levels of natural reproduction may be affected by the presence and the potential expansion of the parasite," Wilson says. "However, current WD research has identified some tools that may help reduce WD spore loads.

"Rest assured that we'll continue to take the appropriate steps to ensure the continued quality of Utah's most important cold water fishery."

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