Changes in the trout population probably relate to changes 

 in physical habitat and recruitment sources. This type of 

 habitat is typically occupied by rainbow/cutthroat trout 

 fisheries in western Montana (e.g. Rock Creek and upper 

 Bitterroot River) . Their low abundance in the Raymond Bridge 

 Section appears to be due to poor recruitment levels. The only 

 rainbow or cutthroat YOY observed in this reach were found near 

 the mouth of the North Fork where their numbers were limited. 

 The North Fork is the only tributary entering this reach which 

 has significant potential to be used for spawning. Brown trout, 

 which tend be more successful spawners in the raainstems of 

 western Montana rivers, probably dominate this reach because 

 there is at least some source of recruitment available. 



Despite the relatively high gradient in this reach, fine 

 sediment accumulations were easily visible in 1988. These 

 accumulations are probably a result of the low streamflows and 

 the lack of flushing flows during each of the last five years. 

 This buildup of sediment is accompanied by extensive growths of 

 algae and rooted aquatic plants, which according to local 

 residents, are not typically present in this river segment. 

 Increased oxygen demands by this vegetation, in conjunction with 

 low summer flows and elevated water temperatures, probably poses 

 additional stresses on the trout fishery. Entrance of the cooler 

 waters of the North Fork in the lower portion of this reach, 

 however, improves water quality and reduces stress on the 

 fishery. 



This reach also appears to be an unusually harsh environment 

 for trout during the winter. Formation of anchor ice on the 

 stream bottom in the Raymond Bridge Section was extensive during 

 late fall, 1988. Local residents have also commented that this 

 area is particularly prone to anchor ice formation. 



Trout adapt to winter conditions in streams by seeking 

 refuge within open spaces of the stream bottom (provided that 

 they are not filled with sediment) and moving to areas without 

 anchor ice. Peters (1988) documented movement of cutthroat trout 

 from summer habitat in the West Fork of Rock Creek as a probable 

 response to anchor ice conditions in the fall. There appeared to 

 be a similar pattern of trout movement out of the Raymond Bridge 

 Section during successive runs of the fall population surveys, 

 indicating a possible response to winter conditions. 

 Changes in seasonal abundance of cutthroat trout in the North 

 Fork have also been observed by local fishermen. 



If seasonal movements are indeed common in the Blackfoot 

 River, there may be concentrations of trout in overwintering 

 areas and summer population densities may be influenced by 

 availability of these wintering areas. Future management 

 strategies should, therefore, take into account the potential 



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