STEAMBOAT SPRINGS, Colo. After discovering more than 300 dead mountain whitefish in a lower stretch of the Elk River on Sunday, Colorado Parks and Wildlife (CPW) has implemented a full-day voluntary fishing closure on the river near Steamboat Springs.The closure, which was put in place Monday, runs from the Christina State Wildlife Area downstream to the confluence with the Yampa River, CPW said.The mountain whitefish die-off is "attributed to severe drought conditions," CPW reported, adding that the water temperature was 83 degrees Fahrenheit during the investigation on Sunday. The days prior likely saw highs above 80 degrees as well, CPW said. In addition to the 300-plus dead fish found in the lower section of the Elk River, additional dead fish were discovered downstream. "While difficult to see, this type of event was expected given the extreme drought conditions this year," explained CPW Northwest Region Senior Aquatic Biologist Ben Felt. "Mountain whitefish are especially vulnerable to high temperatures and low dissolved oxygen levels. Implementing this closure allows CPW to continue protecting this important native species by reducing angler pressure in this area."Mountain whitefish and cutthroat trout are the only two native sport fish species in Colorado. In the past few years, mountain whitefish have struggled to survive in healthy numbers in the Yampa River due to non-native predatory fish and environmental conditions, CPW said. The agency also recorded a dramatic decrease in the species due to a severe drought in 2002.Several additional mandatory and voluntary closures are in place along the Yampa River and in other places around Colorado to protect stressed fish. Click here for a full list. It is not yet clear when these closures may lift.In late June, Denver7 reported on another voluntary closure on the Yampa River that one runs from the top of the Sarvis Creek State Wildlife Area down through the Bureau of Land Management stretch of the river. CPW Aquatic Biologist Marisa Eley explained that the agency was particularly concerned about the mountain whitefish here as well.During voluntary closures, anglers are asked to find another location to fish, though they are not required to stop fishing in that spot. However, CPW asks that they not fish on that stretch of river to protect the species.Voluntary closures are only put in place when a river's conditions meet a certain threshold and fish health has been impacted. The conditions include a daily maximum water temperature above 71 degrees, streamflow that is 50% or less of the daily average, visible signs of stress and fungus on fish, and a low level of daily minimum dissolved oxygen, CPW said. These conditions already take a toll on fish and they will struggle to recover if they are caught and released.Anybody who notices multiple dead or distressed fish is asked to contact their local CPW office and report the specific location (including landmarks or GPS coordinates), and the number and species of fish impacted. ...read more read less