Walla Walla River Bull Trout Ten Year Retrospective Analysis and Implications for Recovery Planning

We completed a multi-year synthesis of the data and analyses for the Walla Walla River

to help broadly prioritize conservation actions and inform the conservation of bull trout.

• The assessment provides fundamental and critical information on bull trout growth,

movement patterns, and survival rates. At the population level we assess abundance,

structure structure
Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.

Learn more about structure , and growth rate. We characterized habitat quality, suitability, and availability

for the Walla Walla River and Mill Creek that was formerly lacking.

• Protection of South Fork Walla Walla River spawning and rearing habitats and

improvement of the lower Walla Walla River migratory and foraging corridor will allow

bull trout to complete their lifecycle, express life-history variability, potentially serving as

a donor population for other local populations (e.g., Touchet River and Mill Creek

populations), or core areas (e.g., Umatilla Core Area) in less desirable habitat, and

improve the resiliency of the entire Walla Walla River Core Area.

• Walla Walla River migratory fish appear to reach larger sizes and approach their maxima

faster than do residents (i.e., migrants exhibit faster growth rates), although considerable

overlap between the two life-history expressions appears to occur. Growth at juvenile

life stages before emigration may be slightly higher for migratory fish than for resident

fish.

• Generally, fish that migrated as sub-adults and small adults moved farther downstream

and remained in lower parts of the watershed longer than juveniles and large adults. It

appears that environmental factors and/or individual intrinsic growth influence transition

to a migratory life-history. The consequences of the migratory life-history expression

appear to involve complex tradeoffs between the benefits of increased growth and

fecundity, but at a cost of lower survival.

• Larger bull trout size classes showed the greatest tendency to migrate downstream out

of the headwater area. Since the lower river demonstrates a longitudinal trajectory of

habitat degradation, migratory bull trout in the sub-adult and small adult size classes

may be the most susceptible to lower river mainstem mortality. If this is the case,

reduced survival for the sub-adult and small adult size categories could reduce the

potential reproductive contribution of the migratory component of the population and the

opportunity for dispersal.

• Several lines of evidence demonstrate that bull trout in the Walla Walla River Core Area

still attempt to disperse among the local populations and between core areas (e.g.,

genetic and movement data). Providing for dispersal, by improving habitat conditions

that restore connectivity among local populations and between core areas, is vital to

maintaining and enhancing viability of the Walla Walla River Core Area local populations

of bull trout and could be vital to long term maintenance of adjacent core area

populations