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Native Trout Program - Bull Trout Effective Population SizeThe Wallowa Valley Improvement Canal (WVIC) is a water diversion in
northeastern Oregon that has impacted bull trout and their habitat.
The canal was constructed in the 1880s and diverts water from several
Imnaha River subbasin streams between Big Sheep Creek and McCully Creek
to Prairie Creek in the Wallowa River basin. The diverted water is primarily
used for irrigation purposes. |
Big
Sheep Creek is the most upstream tributary that is diverted by the WVIC.
Barriers that prevent migration into upper Big Sheep Creek (at the WVIC
diversion) and the WVIC (at the Salt Creek summit spillway) have potentially
created an isolated population of bull trout in upper Big Sheep Creek.
There is a culvert on Little Sheep Creek approximately 200 m above where
the WVIC diverts the stream that could impact upstream migration of
bull trout and isolate a population above the culvert. The WVIC does
not divert McCully Creek. Instead, the WVIC is carried over McCully
Creek and some water from the canal is diverted into the creek. It is
not likely that much, if any, immigration into McCully Creek is occurring
through this diversion given the physical structure being used. In addition,
McCully Creek no longer drains into the Imnaha subbasin. The stream
bed was shifted in the past so that the creek now drains directly into
the Wallowa Valley and provides another water source for irrigation.
Therefore, the only potential source of bull trout immigration into
McCully Creek would be from the Grande Ronde River subbasin, through
a series of irrigation canals that most likely act as temperature barriers
for bull trout. Thus, it is reasonable to speculate that the bull trout
population in McCully Creek is isolated. Historically, these populations
could have been connected by migratory individuals. However, the construction
of the WVIC has potentially prevented gene flow or allowed only unidirectional
movement for over a century. Despite the existence of these potential
isolating mechanisms, small bull trout populations persist in all of
these streams above the WVIC. 
Effective
population size (Ne) is simply the number of breeding individuals in
a population given the assumptions of a randomly mating population.
Effective population size is typically smaller than the absolute population
size. Population genetic theory indicates that Ne = 50 is necessary
to prevent inbreeding depression, and an Ne = 500 is necessary for sustainability
over ecological time (Franklin 1980, Soulé 1980, Allendorf and Ryman
2002). It is likely that this theory holds true for bull trout, although
exceptions do exist (Rieman et al. 1997, Whitesel et al. 2004). Departures
from the 50/500 concept associated with Ne should be supported by empirical
data that is robust and well described (Whitesel et al. 2004). Potential
information that relates effective population size theory to absolute
abundance and population genetic variability would provide information
toward defining minimum viable population objectives. 