YCT Multi-state Assessment February 1 0, 2003 



Population Risk - risk of deterministic or stochastic declines in a population that could lead to a 

 reduced probability of viability for that population. Linked to temporal, population 

 size, production considerations and degree of isolation. 



Genetic and abundance information will be provided for each population mapping segment. 

 Genetic and population risk assessments will be done for each conservation population. 



Historical Range 



Tlie historically occupied range of YCT will be assessed based on their believed distribution at 

 the time Europeans first entered the Rocky Mountain West. This assessment was done at a 

 relatively coarse level (Fourth-code level Hydrologic Unit Code; HUC) and assumed the entire 

 hydrography within 4' code HUC was either occupied or unoccupied. While these distributions 

 will be used as the starting point for assessing historical range, the historical range within each 

 HUC will be refined. Fishery professionals familiar with each major drainage basin (Fourth- 

 code HUC) will define historical range for stream mapping segments within each 4'*^ code HUC 

 by comparing the historical range with their personal knowledge of the area, known anecdotal 

 information, known habitat restrictions, known geologic barriers, and historical fisheries data and 

 reports to edit TNC YCT historical range maps at the 1 : 1 00,000 scale. YCT will be assumed to 

 have occupied all streams within their broad known historical distribufion unless information or 

 professional judgment indicates YCT likely did not occupy a mapping segment of stream based 

 on a documented rationale (Table 3). Data sources used to determine whether stream segments 

 were historically occupied, or not occupied, will be provided (Table 2), along with a reference 

 documenting why each stream segment was included or excluded, when applicable. 



Table 3. Look-up table for reasons to exclude or include a stream segment as historical YCT 

 habitat. 



Description 



|Habitat limited - gradient, elevation, temperature 

 [Known geologic barrier 

 [Anecdotal information 

 [Historical scientific survey data 



Judgment only 



Since barriers to upstream fish movement (either long-term geologic, natural short-term, or 

 anthropogenic barriers) will be used to assess whether individual stream segments were likely 

 historically occupied by YCT, or for assessing risk of genefic introgression to exisfing YCT 

 populations, or whether existing YCT populations are connected with other populations; during 

 the effort to describe the historical distribution identity those barriers that represent long-term 

 geologic features that would serve to influence historical distributions. These barrier locations 

 will be located (as points in ARCVIEW) on the population mapping segments. During current 

 population distribution mapping identify other significant barrier locafions (as points in 

 ARCVIEW), including barrier type (Table 4), blockage extent (Table 5), and barrier significance 

 (Table 6). Identify only those barriers that contribute to the maintenance of populafion isolation. 



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