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Our CSI analysis showed that of the 26 subwatersheds historically occupied by mountain whitefish, only 11 are currently occupied. They are separated into two metapopulations - one above and one below Mackay Dam. There is no evidence that genetic differences exist between populations separated by Mackay Dam. All populations score highly for genetic purity, as there is no evidence of mountain whitefish from other populations being introduced into the Big Lost River.
Although the mountain whitefish in the Big Lost River appears to be genetically intact, population densities are low in some areas and the metapopulations are fragmented to some degree by diversions or dewatered stream segments during low flows. Population density scored high above the furthest upstream diversion (Neilson) where there is a large extent of connected habitat. The metapopulation downstream of Mackay Dam received a moderate score for extent of connected habitat, which reflects an increase in connectivity due to recent fish passage projects on water diversion structures. The moderate score for disease vulnerability reflects the presence of whirling disease in the basin but there is no evidence of mountain whitefish in the Big Lost River being susceptible to the disease. High scores for life history diversity reflect that there is no evidence that a life history strategy has been lost because they did not historically occupy any natural lakes within the basin.
Habitat integrity indicators scored high to moderate in upstream tributaries of the Big Lost River Basin but low along the mainstem. Tributary habitats are primarily managed by the U.S. Forest Service (Salmon-Challis National Forest), while the Bureau of Land Management is responsible for the lower elevation shrublands along larger streams. Watersheds of headwater tributary streams are largely protected as roadless areas except along stream corridors. Several tributaries are listed as 303(d) impaired by the State of Idaho because of temperature, sediments, nutrients, flow alteration, or habitat alteration. Watersheds along the mainstem Big Lost River scored low because they are privately owned, have extensive networks of canals that divert water, have water diversion structures that disconnect habitat, have land that has been converted to pasture and agricultural fields, and have streamflows that are disrupted by Mackay Reservoir. These conditions, as reflected by low habitat integrity scores, suggest that restoration, reconnection, and protection efforts should be focused on the area upstream of the confluence of Antelope Creek. Nonetheless, the absence of fine-scale instream habitat condition measurements limits our ability to precisely map habitat quality across the basin. As a result, some habitat integrity scores may not reflect conditions in larger streams: the protected uplands of the upper Big Lost River may inflate the habitat integrity scores of degraded subwatersheds in need of restoration from grazing impacts, such as the East Fork of the Big Lost River.
The lower Big Lost River below the Moore Diversion is dry most of the year because water is lost to sinks and is withdrawn for irrigation. Water users in the basin are increasingly switching from flood irrigation to sprinkler irrigation. Because return flows are less with sprinkler irrigation, surface and groundwater losses have increased with this transition.
The mountain whitefish historically occurred in larger tributaries and the mainstem of the Big Lost River. Use of tributaries likely fluctuated as habitat changed with natural precipitation and runoff cycles. However, construction of water diversion structures impassable to upstream movement has prohibited mountain whitefish from recolonizing tributary streams during favorable conditions. In the upper basin, the current drought cycle that peaked in 2005 may explain why mountain whitefish do not currently occupy all of the tributaries were they have been found historically; however, the exact reason for the current contracted distribution where habitat is intact remains unknown because 2007 surveys show recent recolonization of Wildhorse Creek and Fall Creek.
Most subwatersheds scored high for future security. There is low potential for existing unconverted lands to be converted for agriculture. Only one watershed scored low for resource extraction, and only two subwatersheds have been identified for future hydropower development. Risk to mountain whitefish from increased winter flooding and summer temperatures due to climate change was low throughout the basin; however, several mid-elevation subwatersheds were identified as having a high risk to future wildfires caused by climate change.
Trout Unlimited strongly supports current efforts by Idaho Department of Fish and Game to manage mountain whitefish as two metapopulations, one above and one below Mackay Dam and to provide fish passage across water diversion structures to maintain connected metapopulations. Since 2004, passage has been provided on or around six structures. Acquisitions of non-consumptive water rights are also encouraged to improve streamflows reconnecting stream segments and populations; Trout Unlimited recently acquired a 5 cubic feet per second water right for a fish ladder on the Chilly Diversion. Providing large interconnected habitats will increase the persistence of existing metapopulations in the face of both natural and anthropogenic disturbances. Ongoing habitat restoration on impaired streams should also improve mountain whitefish habitat.
Although the mountain whitefish in the Big Lost River has not been listed under the Endangered Species Act, several government and private organizations, including Trout Unlimited, realize that its genetic identity is unique, should be conserved, and cannot be replaced with individuals outside of Big Lost River Basin. Hence, conserving its existing genetic diversity and improving populations within the Big Lost River should remain a future priority.
Prepared by Dan Dauwalter and Kurt Fesenmyer, TU, 2/23/2009
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