Flow and temperature effects on life history diversity of Oncorhynchus
mykiss in the Yakima River basin
Authors: Ian Courter, Casey Justice, and Steve Cramer
Abstract
Oncorhynchus mykiss populations with ocean access display considerable life history
plasticity. Resident (rainbow trout) and anadromous (steelhead) adults commonly
produce offspring of the alternate ecotype, but environmental drivers of this life history
variability are not well understood. To explore potential environmental factors
influencing the distribution of resident and anadromous O. mykiss in the Yakima River
basin, we used a life-cycle modeling approach to simulate flow and temperature effects
on relative reproductive success of each ecotype. As a supplement to more traditional
hypotheses about declining steelhead abundance, we propose a theory that more closely
resembles evidence from pristine rivers. We hypothesize that flow regimes providing
cool temperatures and maintaining depth and velocities necessary to sustain adult O.
mykiss throughout the summer and fall seasons result in increased resident rainbow trout
abundance and decreased steelhead abundance. Model results indicated a correlation
between flow and temperature conditions and relative reproductive success of the two
ecotypes and appear to explain in part why the upper Yakima Basin supports renowned
resident rainbow trout populations, while tributaries in the lower basin continue to
produce predominantly steelhead. Location within the basin also played a central role in
determining the life history composition of O. mykiss. Upper basin sites, having a higher
mortality cost for migration, favored a resident life history even when local
environmental conditions promoted a migratory life history strategy. Channel type was
also an important determinant of ecotypic dominance, with higher relative reproductive
success of anadromous spawners occurring in tributary habitats. Alteration of flow
conditions in mainstem habitats had little effect on the relative reproductive success of
anadromous O. mykiss. Our modeling demonstrated that tributary habitats were most
likely to support an anadromous ecotype, and management actions that improve tributary
habitats have the greatest potential to increase abundance of steelhead in the Yakima
Basin.
1
Table of Contents
Introduction ....................................................................................................................... 3
Modeling Methods ............................................................................................................ 5
Study Area Description ................................................................................................ 7
Spatial Structure ........................................................................................................... 8
Carrying Capacity ...................................................................................................... 10
Growth ......................................................................................................................... 14
Smoltification............................................................................................................... 16
Survival ........................................................................................................................ 16
Maturity, Sex Composition, and Fecundity.............................................................. 19
Mate Selectivity ........................................................................................................... 20
Cross-Ecotype Production.......................................................................................... 21
Dispersal....................................................................................................................... 22
Model Sensitivity ......................................................................................................... 22
Results .............................................................................................................................. 23
Model Sensitivity ......................................................................................................... 26
Discussion and Conclusions ........................................................................................... 28
References ........................................................................................................................ 32
Appendices ....................................................................................................................... 38
2
Introduction
Naturally spawning populations of both resident and anadromous Oncorhynchus mykiss
populations coexist in sympatry throughout the Columbia and Snake River basins. The
anadromous life history form (steelhead) is listed as threatened under the Endangered
Species Act (ESA) while the resident form (rainbow trout) is not. This divergence in
listing status sustains the lingering confusion surrounding the biology of the species and
has proven problematic when attempting to evaluate anadromous O. mykiss population
viability. Better explanation of the biological mechanisms that lead to predominance of
anadromy, residency, or a balanced mixture is needed.
As in other parts of the Columbia and Snake River Basin, Yakima River steelhead are
listed as threatened. A recovery plan, as required for species listed under the ESA, has
been developed for Middle Columbia River (MCR) Steelhead, which includes
populations in the Yakima, Walla Walla, Deschutes, John Day and Umatilla basins.
According to the Plan, three populations in the MCR Evolutionarily Significant Unit
(ESU) were determined to be at high risk of extinction, and two of those are in the
Yakima Basin. The population of steelhead that spawns in the upper Yakima River is
reportedly the most jeopardized independent steelhead population in the Columbia Basin,
with a 10-year geometric mean abundance of only 85 adult spawners (Conley et al.
2008). The upper Yakima Basin is routinely cited as an example of imperiled steelhead
habitat (Good et al. 2007).
Although few steelhead return to the upper Yakima River, resident rainbow trout are
abundant there and support one of the most popular wild trout fisheries in Washington.
Hatchery trout have not been stocked since 1991, and genetic sampling indicates the
resident trout are similar to native steelhead, and quite distinct from the hatchery stocks
formerly planted (Busack et al. 2005). Both mainstem and tributary rainbow trout
populations in the upper basin have been stable in abundance and size since annual
monitoring began in 1990 (Ham and Pearsons 2000; Temple et al. 2004). In contrast, O.
mykiss populations in the lower basin, particularly Satus and Toppenish Creeks, are
predominantly composed of anadromous fish (Conley et al. 2008).
Resident rainbow trout and steelhead are known to interbreed (Pearsons et al. 2007 and
McMillan et al. 2007), and recent genetic analysis indicates a significant amount of
genetic overlap between rainbow trout and steelhead in the upper Yakima River
(Pearsons et al. 2007 and Blankenship pers. comm.). This evidence supports the
conclusion that different life history forms of O. mykiss within a basin are likely
interbreeding when spatial and temporal overlap in spawn timing occurs, congruent with
genetic analyses conducted throughout the Pacific Rim (Busby et al. 1996, Docker and
Heath 2003, and McPhee et al. 2007).
Several studies have confirmed that in addition to interbreeding, both ecotypes are
capable of producing offspring of the alternate ecotype. For example, recent otolith
microchemistry studies revealed that both resident and anadromous O. mykiss in the
Sacramento-San Joaquin Basin can have maternal origins of the alternate ecotype
(Zimmerman et al. In Press); a population of stocked rainbow trout from California has
given rise to an anadromous ecotype in the Santa Cruz River, Argentina that are
3
genetically indistinguishable from the resident population (Pascual et al. 2001); and
breeding studies from the Grande Ronde River, Oregon and Sashin Creek, Alaska found
that pure resident, pure anadromous and mixed spawner crosses all produced both
resident and anadromous offspring (Carmichael pers. comm. and Thrower and Joyce
2004). Moreover, evidence suggests that while these life history traits are heritable
(Thrower et al. 2004), the capacity of O. mykiss populations to produce steelhead is not
lost after many generations of residency. For example, rainbow trout from Sashin Lake,
Alaska maintain the capacity to produce smolts after over 70 years of isolation from
anadromous recruitment (Thrower and Joyce 2004).
The need to understand the factors driving the relative abundance of anadromous and
resident O. mykiss is particularly relevant in the Yakima Basin, where flows in mainstem
reaches are largely controlled by releases from upstream storage reservoirs. Both the
MCR Recovery Plan and ESA consultations over water operations have identified an
urgent need to understand how the production of anadromous and resident O. mykiss is
affected by flow management. Thus, a strong practical need exists to understand if
changes in flow can be used to achieve desired abundances of steelhead.
Three primary mechanisms have been posed as explanations for the observed
distributions of residency and anadromy in O. mykiss populations: genetic control,
growth effects on conditional switching of life-histories, and differences in life history
survival driven by environmental factors. Firstly, breeding experiments have established
that the tendency to follow either a resident or anadromous life history is heritable in O.
mykiss (Thrower et al. 2004); however, numerous studies point out that genetics is only
one of several factors influencing life history response (Jonsson and Jonsson 1993;
Hendry et al. 2004; Nussey et al. 2007; Nichols et al. 2008). Secondly, species of the
family Salmonidae appear to have the ability to respond to environmental conditions by
adopting either a resident or an anadromous life history. This ability is know as a statedependent or conditional life history strategy (Houston and McNamara 1992 and Jonsson
and Jonsson 1993), and allows individuals to maximize their fitness dependent upon their
phenotypic condition (Gross 1996 and Jonsson and Jonsson 1993). Independent of the
conditional ability to switch life histories, a third possibility is that environmental
circumstances simply result in greater survival and reproduction of one ecotype over the
other. Pavlov et al. (2008) concluded this was the explanation for differences in
frequency of the two ecotypes between adjacent, pristine rivers in the Kamchatka Russia.
We hypothesize that the observed predominance of rainbow trout in the upper Yakima
River main stem and steelhead in lower basin tributaries is controlled by environmental
conditions. This same trend can be observed in numerous other regulated rivers
throughout the Pacific Northwest and California. The typical explanation for this trend is
centered on the higher cost of migration due to water storage and hydroelectric projects,
predation, and commercial and sport fisheries. As a supplement to the more traditional
hypotheses about declining steelhead abundance, our theory more closely resembles
evidence from rivers with minimal anthropogenic effects, such as the Kamchatka
Peninsula where unregulated rivers support thriving populations of six distinct O. mykiss
life history types, including the traditional resident and anadromous forms as well as a
range of estuarine and amphidromous ecotypes.
4
More explicitly, we theorize that flow regimes providing cool temperatures and
maintaining depth and velocities necessary to sustain adult O. mykiss throughout the
summer and fall seasons will result in increased resident rainbow trout populations and
decreased steelhead abundance. This is consistent with a commonly referenced ecological
principle that “when the animal’s needs are being met, it stays where it is; when they are
not, it moves until it finds appropriate conditions for its current demands (Thorpe 1994).”
Furthermore, this hypothesis may explain why basins like the upper Yakima support
renowned resident rainbow trout populations and dwindling steelhead populations.
The shift of an O. mykiss population from residents to predominantly anadromous occurs
when the benefits of freshwater survival no longer outweigh the increased fecundity
associated with ocean migration. We expect non-anadromous individuals to experience
the greatest survival pinch when flows drop to levels that would not sustain the depth and
velocity requirements of adult rainbow trout and/or summer temperatures exceeded
approximately 18oC (Todd et al. 2008). In these environments, migratory life-histories
were expected to predominant. Both survival and fecundity are affected by body size, and
growth is often hypothesized as a potential predictor of life history response in
facultatively anadromous salmonids (Jonsson and Jonsson 1993; Hendry et al. 2004;
Thorpe et al. 1998; Mangel and Satterthwaite 2008). In this paper, we examine the
influence of environmental conditions (i.e. temperature and flow) on growth and survival,
and simulate resident and anadromous relative reproductive success in O. mykiss
populations throughout the Yakima Basin.
In this paper we describe an approach that integrated existing information into a lifecycle simulation model to predict resident and anadromous relative reproductive success
in different portions of the Basin. The modeling framework utilized mathematical
functions to account for temperature effects on growth, flow effects on juvenile and adult
survival, and effects of both body size and life history type on fecundity. We then
compared the relative reproductive success of steelhead and resident rainbow trout
predicted by the Life History Response Model (LHRM) to the observed distribution of
steelhead and rainbow trout in the Yakima River basin. Specifically, we address the
following questions: (i) What factors best account for the observed distribution of the two
ecotypes in the Yakima Basin? (ii) How are variation in flow and temperature likely to
influence those relative distributions?
Modeling Methods
There are likely numerous valid modeling approaches for quantifying the effects of
environmental conditions on life history traits. Roff (2002) presents an extensive review
of quantitative approaches to account for life history variation, and in regard to
determining relative fitness between life-histories, he concludes, “In circumstances in
which there is both density-dependence and stochastic variation, a simulation approach is
perhaps the only presently viable approach.” Life-cycle modeling is our preferred method
because drivers can be appropriately quantified spatially and temporally. This approach
allows one to examine the true balance of life history drivers operating in succession
between life stages and across multiple generations with environmental conditions
specific to a river segment of interest. Using data collected in the Yakima River basin, we
5
developed a deterministic life-cycle model (LHRM) that combines survival and fecundity
tradeoffs into a dynamic framework to predict the balance between resident and
anadromous O. mykiss relative reproductive success under different environmental
conditions.
A simple depiction of the linkages between primary functions of the LHRM is provided
in Figure 1. The model operates on a daily time step. Flow and temperature conditions,
specified for a specific river reach of interest, influence growth and available habitat area
for fry, juvenile and resident adult O. mykiss. The model evaluates fish size and
calculates the average territory size requirements of fish within each age-class. Based on
availability of habitat, the model calculates carrying capacity for fry, juvenile and
resident adults. Density dependent mortality acts on the fish during the summer and fall
seasons when habitat area is limited and fish growth is at its highest. Winter mortality is
fixed because fish metabolism is slowed and competition for space is no longer expected
to limit survival. We assume no competition for space between the three freshwater life
stages because they tend to occupy habitat with different depths and velocities. By the
time juveniles have reached a sufficient size for smoltification, they have already made a
life history decision to be either anadromous or resident. These decisions were
predetermined by fixed rates of resident and anadromous offspring production estimated
for each of the possible spawner crosses. Smolt-to-adult survival is dependent on smolt
size at emigration and population location within the Yakima Basin. Fecundity of adult
resident and anadromous female spawners is size dependent and ecotype dependent,
resulting in residents that produce fewer eggs per female at size than anadromous
females. A detailed description of various model functions and parameter values is
provided in Appendix A.
Model results are expressed in terms of relative reproductive success. We define relative
reproductive success as the ratio of total anadromous egg production to total resident egg
production. Values of relative reproductive success greater than one indicate conditions
that favor an anadromous life history. Each time a simulation is carried out under a
specified set of flow and temperature conditions, the model quickly approaches an
equilibrium ratio of mature resident and anadromous individuals with a measure of total
egg production unique to the size distribution of fish within each life history category. As
defined by Hendry et al. 2004, “the change in relative fitness conferred by a given
behavior (e.g. migration) is determined by the influence of that behavior on survival to
maturity, age at maturity, and reproductive output at maturity.” In our model, the ideal
metric for rolling up all three of these parameters into a single measure, which allows us
to make relative comparisons between fitness’s of the two life-histories, is egg production
at equilibrium. For simplicity and consistency, we evaluated reproductive success at
simulation year-ten because the model reached equilibrium within this timeframe under
all specified flow and temperature conditions.
We compared predictions of relative reproductive success across model reaches to
determine if expected patterns in the composition of anadromous and resident life history
types were similar to the observed distribution of O. mykiss ecotypes within the Yakima
Basin. Additionally, we examined the effects of environmental conditions on relative
reproductive success of anadromous O. mykiss by simulating incremental changes in flow
and temperature during the summer base-flow period (June – October) in three different
6
reaches (Kittitas, Taneum, and Toppenish Mid). Our analysis of flow and temperature
effects was limited to summer-fall because flow and temperature constraints on rearing
capacity are likely to be greatest during this period.
Figure 1. Model flow chart showing key model inputs and relationships between model components.
Study Area Description
The Yakima Basin collects runoff from a network of streams draining the eastern slopes
of the Cascade Mountain Range in the southern half of Washington. Most of these
streams enter the two main branches off the basin, the Naches and the upper Yakima
rivers, which join at rkm 183. The Yakima River flows southward through the arid
Yakima Valley to eventually join the Columbia River 541 km upstream of the Pacific
Ocean (340 ft msl). Precipitation varies from approximately 128 inches along the crest of
the Cascades to less than 8 inches on the eastern valley floor. Much of the precipitation
falls as snow in the upper basin, and flows generally peak as snowmelt in the spring.
Flows in the upper Yakima Basin (above the Naches confluence) are augmented during
7
summer by releases from three headwater storage reservoirs (~2,500 ft msl with
~840,000 acre-ft active storage), much of which is eventually diverted to irrigation
networks downstream. Two additional storage reservoirs (~230,000 acre-ft active
storage) modify the flow regime in the Naches branch.
Spawning distribution of steelhead in the Yakima Basin for brood years 1990-1992, as
determined through radio-telemetry studies, was: 46% in the Satus Basin; 31% in the
Naches Basin; 11% in the Toppenish Basin; 2% in the Marion Drain; 4% in the Yakima
River main stem below Roza Dam; and 6% in the Yakima River or tributaries above
Roza Dam (rkm 201). Anadromous adults can be counted passing through fish ladders at
two irrigation diversions; Roza Dam and Prosser Dam (rkm 74). A fish facility near
Prosser Dam also samples juveniles migrating downstream. Steelhead spawning is less
widely distributed than rainbow trout, but is within the geographic range of rainbow trout
spawning. The spawning time of rainbow trout and steelhead is similar, and peaks
progressively later as elevation increases. Steelhead represent less than 1% of the O.
mykiss spawners in the upper Yakima River above Roza Dam (Pearsons et al. 1998), but
nearly 100% of the O. mykiss found in Satus and Toppenish creeks in the lower basin
(Hubble 1992).
Spatial Structure
To study environmental drivers of anadromy and residency at an appropriate spatial
scale, we selected spatial units (reaches) within each of the four Yakima Basin Steelhead
Independent Populations identified in the steelhead Recovery Plan: Upper Yakima,
Naches, Toppenish Creek and Satus Creek (Figure 2; Table 1). Reaches were chosen for
which hydraulic modeling of fish habitat had been completed; either Physical Habitat
Simulation (PHABSIM) (Bovee et al. 1998) or two-dimensional (2D) modeling. This
information was necessary to quantify the effects of flow on habitat area. Additionally,
daily temperature and flow data for each spatial unit were necessary as environmental
data inputs to evaluate how the balance between residency and anadromy was influenced
by environmental conditions. We simulated a total of nine model reaches—four tributary
(Satus Creek, Toppenish Creek Mid and Upper, and Taneum Creek) and five mainstem
reaches (Easton, Kittitas, Union Gap, Wapato and Naches).
Flow conditions differed dramatically between mainstem and tributary sites. Figure 3
shows the contrast in typical hydrographs between Toppenish Creek (lower basin
tributary) and the main stem of the upper Yakima River. Flows in Toppenish Creek drop
to critically low levels in the summer and fall, while flows in the main-stem, augmented
by reservoir releases, actually increase slightly through the summer until delivery of
irrigation water drops sharply in late August.
8
Figure 2. Map of the Yakima Basin showing the four independent steelhead population boundaries
and model reaches within each population (highlighted in pink). Pie charts indicate the proportion of
total egg production of anadromous and resident spawners at equilibrium predicted by the LHRM
for each model reach.
Table 1. General characteristics of study reaches represented in the model.
Model reach
Yakima (Easton)
Yakima (Kittitas)
Taneum Cr.
Naches
Yakima (Union Gap)
Yakima (Wapato)
Toppenish Cr. (upper)
Toppenish Cr. (mid)
Satus Cr.
Description
Cle Elum river to Kachess Dam
canyon toTaneum Cr.
bridge at Rkm 0.7 to Diversion Canal
mouth and Tieton R.
Union Gap to Naches R.
Satus Cr. to Union Gap
Diversion Canal to NF Toppenish Cr.
Diversion Canal to Simcoe Cr.
Logy Cr. To Bull Cr.
Years
2000-2007
2000-2007
2005-2008
2000-2007
1998-2007
1998-2007
2005-2008
2005-2008
1993-1998
Drainage
Area (km²)
1,253
--199
3,312
------1,301
1,117
Reach
Length (km)
16.4
7
2.03
15.5
6.2
16
20
10.1
19.9
Reach Boundary (rkm)
lower
upper
309.7
326.1
239.2
246.2
0.7
2.7
6.0
21.5
177.2
183.4
155.2
171.2
76.2
96.2
47.2
57.3
38.0
57.9
July-Sept Mean
Temp (°C) Flow(cfs)
15.2
422
15.9
2,656
14.8
11
16.0
984
18.1
3,112
18.1
584
16.1
16
18.5
18
18.1
13
9
600
500
3000
400
2000
300
200
1000
100
Flow (cfs) at Toppenish Cr.
Flow (cfs) at Upper Yakima
4000
0
0
3/1/2007 4/1/2007 5/1/2007 6/1/2007 7/1/2007 8/1/2007 9/1/2007 10/1/2007
Figure 3. Spring through fall 2007 discharge for the Upper Yakima and Toppenish Creek subbasins.
Carrying Capacity
Numerous factors influence the carrying capacity of rearing O. mykiss in river
environments including food availability, temperature, inter- and intraspecific
competition, water depth and velocity, mesohabitat composition (i.e. pool, riffle, glide)
and habitat complexity (e.g., instream cover, channel sinuosity). The model provides
daily estimates of carrying capacity for each age class of rearing O. mykiss as a function
of flow, temperature, and fish territory size (i.e., measure of intraspecific competition).
Carrying capacity is then used in the model to estimate density-dependent survival rates
for each age class via a hockey-stick relationship.
Using site-specific analyses that quantified habitat area as a function of stream discharge
(Figure 4), we developed relationships between carrying capacity and stream flow for fry
(age 0), juveniles (age 1), and resident adult (ages 2-5) O. mykiss (Figure 5) for each
modeled reach. Habitat area curves were derived from PHABSIM and 2D modeling
conducted throughout the Yakima Basin (Frederiksen pers. comm.; Pacheco pers. comm.;
Bovee et al. 2008). Depth and velocity suitability criteria for fry, juvenile, and adult O.
mykiss were based on data from the upper Klamath River (T.R. Payne and Associates
2004) and were modified into a binary format for input into the 2D model following a
collaborative Delphi process with biologists and stakeholders working in the Yakima
Basin (Appendix B). Habitat area was converted to an index of carrying capacity for each
life stage using a relationship between fish size and territory size developed by Grant and
Kramer (1990) (Figure 6; Appendix A).
Habitat area predicted from PHABSIM and 2D modeling was adjusted using a
temperature suitability index to account for the effects of excessive stream temperatures
on rearing capacity. The temperature suitability index was based on temperature criteria
for juvenile steelhead developed by Sullivan et al. (2000) and is described by a hockey-
10
stick relationship with values ranging from 1.0 at temperatures less than 17°C to 0 at
temperatures exceeding the incipient lethal temperature of 26°C. After adjusting for
temperature effects, habitat area was divided by territory size to determine the daily
carrying capacity for a given age class. We then calculated a moving average of the daily
capacity estimates over a period of 30 days based on the assumption that changes in
capacity resulting from differences in flow, temperature, or territory size would not occur
instantaneously, but instead would occur gradually over an extended period.
Estimates of suitable habitat area, such as Weighted Useable Area (WUA), are species
and life-stage-specific measures of suitable depth, velocity and substrate area commonly
used to quantify flow effects on fish habitat availability. It is not customary to convert
habitat area estimates from PHABSIM and 2D modeling into estimates of carrying
capacity because these indices of stream hydrodynamics are designed to examine relative
effects of flow changes on habitat area—they are not predictions of true habitat carrying
capacity. There are numerous other factors in addition to depth, velocity and substrate
that influence production of fish (i.e. food availability, temperature, cover, etc.). Because
of this, we caution readers not to draw conclusions about actual fish abundance or
production potential from our carrying capacity estimates, and note that LHRM results
are expressed in relative terms to avoid misinterpretation. Actual fish carrying capacity
was not estimated in any of the spatial units analyzed, but it was necessary to convert
habitat area estimates into a metric suitable for population modeling.
11
6000
75000
Satus
4800
60000
16000
3600
45000
12000
2400
30000
8000
1200
15000
4000
0
0
0
-1
Habitat Area (m² km )
20000
Wapato
10000
100
200
300
400
0
0
40000
Toppenish Mid
3500
7000 10500 14000
0
32000
1600
6000
24000
1200
4000
16000
800
2000
8000
400
0
0
6000
100
200
300
400
2500
5000
7500
10000
10000
3600
18000
7500
2400
12000
5000
1200
6000
2500
0
50
100
150
200
25
12500
Naches
24000
0
7500
10000
75
100
1500
2000
Taneum
0
4800
0
5000
0
0
30000
Toppenish Upper
2500
2000
Unioun Gap
8000
0
Kittitas
50
Easton
0
0
2000
4000
Flow (cfs)
6000
8000
0
500
1000
Fry
Juvenile
Adult
Figure 4. Modeled habitat area as a function of flow for fry, juvenile and resident adult life stages in
the nine LHRM reaches. Tributary reaches derived from PHABSIM studies conducted by the
Yakima Nation and Washington Department of Fish and Wildlife (Frederiksen pers. comm.;
Pacheco pers. comm.). Mainstem Yakima and Naches River reaches derived from 2D modeling
conducted by the U.S. Geological Survey (Bovee et al. 2008).
12
600
Fry (x100)
Juveniles (x10)
Adults
Carrying Capacity km
-1
500
400
300
200
100
0
0
100
200
300
400
Flow (cfs)
Figure 5. Fry, juvenile and adult O. mykiss carrying capacity as a function of flow in Satus Creek.
Capacities derived from habitat area estimates using average territory sizes for fry (0.08 m2),
juveniles (1.48 m2) and adults (10.34 m2). Y-axis scaled down by 100x for fry and 10x for juveniles.
16
14
Territory Size (m²)
12
10
8
6
4
2
0
0
50
100
150
200
250
300
350
400
Fork Length (mm)
Figure 6. Relationship between stream-dwelling salmonid size and territory size (Grant and Kramer
1990) used to convert habitat area estimates to estimates of carrying capacity.
13
Growth
The effects of environmental conditions on fish growth in freshwater is captured through
a bioenergetic model, whereby growth is influenced by food availability and stream
temperature. Fish size information is utilized by numerous functions in the life cycle
model including carrying capacity, overwinter survival, smoltification, marine survival,
and fecundity. We developed a generalized growth model following the methods
described by Mangel and Satterthwaite (2008) and Thorpe et al. (1998). According to this
model, growth is defined as the difference between anabolic gains (i.e. consumption) and
catabolic losses (i.e. energy lost through respiration). Daily growth for an average-sized
fish on day t is given by:
dW
dt
 qi qe (t ) (T (t ))W a   i e xT (t )W b ;
Where Wi denotes fish mass in grams and T denotes daily average stream temperature
(°C). The left-hand side of the equation represents anabolic gains, where qi denotes
individual variation in food finding and processing ability and qe denotes environmental
variation in food availability. The function  (T (t )) describes the influence of stream
temperature on food finding and processing ability and was based on the Thornton and
Lessem algorithm described in Hanson (1997) and parameterized using data for juvenile
steelhead described in Sullivan et al. (2000). According to this function, food finding and
processing ability at full ration peaks between about 12 and18°C and declines rapidly at
temperatures exceeding 20°C. An optimal temperature for consumption between 12 and
18°C is consistent with values reported in other salmonid studies (Elliott 1994; Tyler and
Bolduc 2008). We assumed growth is described by the von Bertalanffy formula by setting
the weight exponents to a = 2/3 and b = 1. The assumption that metabolic costs grow
exponentially with a coefficient of x = 0.06818 was based on the work of Stewart (1980).
We calibrated the growth model to conditions in the Yakima River basin using size at age
data provided by the Washington Department of Fish and Wildlife and the Yakama
Nation Fisheries Program. We grouped data from the main stem Yakima River into sites
between Roza Dam and the Cle Elum River confluence and sites between the Cle Elum
confluence and Easton Dam to correspond with broad-scale differences in growth rates.
Additionally, we modeled growth in three tributary sites including Taneum Creek, Satus
Creek, and Toppenish Creek. The growth model was calibrated separately for each site
using daily average temperature data obtained from the U.S. Bureau of Reclamation’s
Hydromet database and from temperature data provided by the Yakama Nation Fisheries
Program. For mainstem sites, we averaged the temperature data from 1990 to 1993 to
correspond with the years that size at age data was collected. With the exception of
Toppenish Creek, temperature data from the tributaries was not available for the years
during which size-at-age data was collected. Therefore, we used average temperatures for
all available years for model calibrations, assuming that the observed growth data was
reflective of average stream temperatures.
The growth model was calibrated by solving for the values of qi and αi which minimized
the variation between the model prediction and the observed size at age data. The qe
parameter was held fixed at 1.0 for all sites to provide a consistent benchmark for food
availability across sites and to aid in evaluation of model sensitivity to changes in food
14
availability. We assumed a constant coefficient of variation of 16% based on the average
variation in size-at-age data observed in mainstem Yakima River sites.
The growth model appeared to fit the data reasonably well, explaining approximately
76% of the variation in individual fish length in mainstem sites between Roza Dam and
the Cle Elum confluence (Figure 7). According to the growth model, fish length increases
rapidly from spring through summer, and is followed by periods of low or no growth
during the winter months. Growth rate in mainstem sites peaked at approximately 19°C
and then declined rapidly to values below zero at temperatures exceeding 23°C (Figure
8). Fish growth was considerably slower in the tributaries, with the growth model
explaining approximately 50% of the variation in individual fish length on average.
600
Age-0
Age-1
Fork length (mm)
500
Age-2
Age-3
Age-4
Age-5
Spawning
27-Mar
400
Emergence
26-Jun
300
200
100
29-Aug
27-Nov
31-May
2-Dec
2-Mar
5-Jun
3-Sep
7-Mar
8-Sep
7-Dec
10-Jun
12-Mar
12-Dec
15-Jun
13-Sep
18-Dec
17-Mar
19-Sep
23-Mar
21-Jun
24-Sep
23-Dec
26-Jun
0
Time since emergence
Figure 7. Observed size-at-age data (circles) from mainstem Yakima River sites between Roza Dam
and the Cle Elum River confluence (1990-1993) and the model-predicted growth curve with 95%
confidence intervals (black lines).
15
0.05
Growth rate (g  day -1)
0.04
0.03
0.02
0.01
0.00
-0.01
0
5
10
15
20
25
Average daily stream temperature (°C)
Figure 8. Predicted growth rate (g ∙ day -1) in the main stem Yakima River between Roza Dam and
the Cle Elum River as a function of stream temperature.
Smoltification
The age at which juvenile anadromous fish migrate to the ocean is estimated in the model
as a function of fish size at emigration. For simplicity, we assume that all fish of
sufficient size migrate to the ocean on May 1st, which was estimated as the median date
of emigration for all smolts captured at the Chandler Juvenile Fish Facilities from 19992008. We assume that juvenile fish must achieve a length of at least 150 mm in order to
smolt. This threshold length for smoltification was calculated as the fifth percentile of O.
mykiss smolt lengths collected at the Chandler Juvenile Fish Facility (David Lind, pers.
comm.) and is consistent with estimates of minimum O. mykiss smolt size at other sites
sampled in the Columbia and Snake River Basins (Yanke et al. 2007; White et al. 2007).
For each age class of anadromous juveniles, the model calculates the proportion of fish
that exceed 150 mm on May 1st, assuming a growth trajectory as described above and a
coefficient of variation of 16%. All anadromous juveniles are assumed to emigrate by the
spring of their third year in freshwater because age-4 smolts are rarely observed in the
Yakima Basin.
Survival
Survival of rearing O. mykiss in freshwater was estimated using two different methods
depending on the time of year. During winter (November-February), when growth is very
limited due to cold river temperatures and activity is expected to slow considerably,
survival was assumed to be density-independent and size-dependent. Numerous studies
have indicated that survival of juvenile salmonids during winter is positively related to
fish size (Quinn and Peterson 1996; Ebersole et al. 2006; Smith and Griffith 1994).
Lacking specific data from the Yakima River basin, we used the results from a study of
rainbow trout survival in the Henrys Fork of the Snake River, Idaho (Smith and Griffith
16
1994) to develop a size-dependent logistic function for over-winter survival (Figure 9).
We assumed an additional 30% mortality due to predation and other mortality factors that
were not accounted for in the Smith and Griffith study. This additional mortality was
determined by selecting the value which, when combined with other sources of mortality
in the model such as post-spawning mortality and mortality from spring to fall, roughly
corresponded with observed annual survival rates of resident O. mykiss in the upper
Yakima River.
During spring through fall (March – October), daily survival rates were modeled as a
function of carrying capacity and abundance. As fish grow, territory size increases
(Figure 6) and habitat carrying capacity declines. In addition, changes in river discharge
influence rearing capacity via changes in the quantity of available rearing habitat (Figure
4). Such changes in capacity directly influence freshwater survival via a hockey-stick
function (Appendix A). When the estimated abundance for a given age class was less
than the predicted rearing capacity for that age class, we assumed that fish survive at a
maximum annual rate of 40% for fry, and 80% for all other age classes. Lacking specific
data from the Yakima Basin on maximum survival rates of resident O. mykiss at low
abundance, we selected these values based on professional judgment and evaluated model
sensitivity to these values to determine their relative influence on key model results.
When abundance for a given age class exceeded the predicted rearing capacity, we
assumed survival would decline in a density-dependent manner such that the abundance
would quickly approach capacity. Specifically, survival S of fish of age m on day t is
calculated as the ratio of capacity to abundance expressed over a period of d days and is
given by:
Sm,t = (Km,t/Nm,t)(1/d),
where K is the estimated capacity, N is abundance, and d is an assumed lag period for
survival. We apply a lag period of 14 days to density-dependent survival, such that, when
abundance exceeds capacity, the mortality associated with this capacity limitation is
expressed over an extended period of time (e.g. 14 days) instead of occurring
instantaneously.
Significant mortality of resident trout occurs shortly after spawning as a result of
increased energy expenditure associated with competition for mates and spawning
territories and disproportionate allocation of resources for gonadal development
(Schroeder and Smith 1989). In the model, we assume a post-spawning survival rate of
55% based on a estimates from Umtanum Creek, a tributary to the Yakima River
(Wydoski and Whitney 2003). Egg-to-fry survival was assumed to be 20% for both
resident and anadromous offspring based on average egg-to-fry survival estimates for
steelhead in Snow Creek, Washington (Bley and Morning (1989)).
Survival of migrating smolts was based on smolt-to-adult return rates (SAR) estimated
from smolt and adult steelhead counts at Prosser and Roza dams on the Yakima River
and an assumed relationship between smolt size at emigration and marine survival. We
developed a logistic relationship between smolt size at emigration and ocean survival
based on data presented in Ward et al. 1989. We then scaled the logistic relationship to
17
better reflect the lower average SAR rates observed for Yakima River steelhead
compared with the coastal Keogh River population from which the logistic relationship
was developed. Scaled relationships were developed separately for the lower Yakima
Basin (below Roza Dam) and upper Yakima Basin (above Roza Dam) such that the SAR
for an average-sized smolt (i.e., 175 mm) was equal to the geometric mean of the
estimated SAR for steelhead smolts migrating from the Yakima River from 1985-2002
(Figure 10). Assuming a migration survival of 43.3% from the Upper Yakima River to
Roza Dam, the geometric mean SAR for smolts originating from the upper basin was
estimated to be 1.25% compared with a geometric mean of 2.88% for smolts originating
from the lower basin (Chris Frederiksen Pers. Comm.).
Over-winter survival Nov-Feb (%)
100
Smith and Griffith (1994)
80
60
Adjusted curve
40
20
0
60
80
100
120
140
160
180
200
220
Fork length (mm)
Figure 9. Length-dependent over-winter survival function showing the original
18
Smolt-to-adult return rate (%)
10
8
Upper Yakima Basin
Lower Yakima Basin
6
4
2
0
100
150
200
250
300
Length at emmigration (mm)
Figure 10. Relationships between length at emigration (mm) and smolt-to-adult return rates for
steelhead smolts originating from the upper Yakima Basin (above Roza Dam) and the lower Yakima
Basin (below Roza Dam).
Maturity, Sex Composition, and Fecundity
Age-at-maturity for resident female spawners was estimated from age and maturation
data collected in the main stem Yakima River above Roza Dam between 1990 and 1993
(WDFW, Gabriel Temple Pers. Comm.) (Appendix A). According to these data, the
probability of a resident female attaining sexual maturity is roughly 8% at age 1, 22% at
age 2, 47% at age 3, 73% at age 4, and 90% at age 5. We assumed that the resident O.
mykiss population is composed of 60% males and 40% females based on average sex
ratios observed in the upper Yakima Basin from 1990 to 1993 (Pearsons et al. 1993).
The age distribution and sex composition for returning anadromous spawners was based
on fish sampled at Prosser and Roza Dams between 2002 and 2005 (Conley et al. 2008;
Appendix A). The majority of the run (i.e., roughly 60-80%) consisted of fish that
migrated to the ocean as two year old smolts and returned to spawn after 1 or 2 years in
the ocean. The sex composition of anadromous spawners is heavily skewed towards
females, with the percentage of females averaging 68 and 78% at Prosser and Roza dams
respectively. To capture the observed differences in age and sex composition of spawners
sampled at upper basin (Roza) and lower basin (Prosser) locations, we applied values
based on Prosser data to lower basin reaches (i.e., Satus Creek, Toppenish Creek,
Wapato, Union Gap, and Naches) and values based on Roza data to upper basin reaches
(i.e., Kittitas, Easton, and Taneum Creek).
We used length-fecundity relationships developed from steelhead spawners captured at
Prosser Dam (Conley et al. 2008) and resident O. mykiss sampled in tributaries and
mainstem habitats in the upper Yakima Basin (Pearsons et al. 1993) to estimate the
number of eggs produced by each spawning female (Appendix A; Figure 11). With an
19
average fecundity of about 5,100 eggs per female (Fast and Berg 2001), steelhead in the
Yakima Basin are substantially more fecund than their resident counterparts, which
average about 800 eggs per female. Though steelhead produce considerably more eggs
per female, resident trout have a higher rate of iteroparity, roughly 25-90% (Schroeder
and Smith 1989 and Buchanan et al. 1990) versus roughly 5% for steelhead (Schroeder
and Smith 1989 and Chilcote 2001), which slightly offsets the fecundity advantage of
anadromous O. mykiss. In the model, we assumed a repeat spawning rate for resident fish
of 50% based on the average value observed for rainbow trout in the Deschutes River
(Schroeder and Smith 1989). We used a repeat spawning rate of 5.4% for anadromous
fish based on the average value observed for steelhead returning to the Yakima River
(Conley et al. 2008).
8000
7000
Eggs per female
6000
Resident (mainstem)
Resident (tributaries)
Anadromous
5000
4000
3000
2000
1000
0
100
200
300
400
500
600
700
800
Fork length (mm)
Figure 11. Fork length (mm) versus eggs per female for resident and anadromous O. mykiss spawners
in the Yakima Basin
Mate Selectivity
Spawning between resident and anadromous O. mykiss has been well documented
(Pearsons et al. 2007 and McMillan et al. 2007), and we expect mate selection to be
affected by fish size and abundance. We estimated a spawner fidelity rate based on
anadromous and resident male abundance to account for the higher probability of
anadromous females spawning with resident males when anadromous male abundance is
low. We calculated a baseline anadromous spawner fidelity rate of 0.76 when the ratio of
anadromous to resident male abundance was 1:1. Anadromous spawner fidelity is defined
as the probability that an anadromous female with mate with an anadromous male. Our
baseline estimate was derived from studies of O. mykiss mating systems on the Olympic
Peninsula, Washington (McMillan et al. 2007; John McMillan Pers. comm.). The
baseline fidelity rate was then scaled between zero and one by assuming that the fidelity
20
rate was proportional to the ratio of anadromous male to resident male spawners (Figure
12).
Anadromous spawner fidelity
1.0
Baseline fidelity = 0.76
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Proportion anadromous males
(anadromous male spawners / total male spawners)
Figure 12: Estimated anadromous female spawner fidelity as a function of the proportion of the
population comprised of anadromous males.
Cross-Ecotype Production
Evidence of rainbow trout producing anadromous offspring and steelhead producing
resident offspring (Pascual et al. 2001; Zimmerman et al. In Press; Thrower and Joyce
2004) made it necessary to accommodate cross-ecotype production within the LHRM.
Juvenile fish size or growth has been hypothesized as an indicator of which life history,
residency or anadromy, a fish will adopt. However, state-dependent modeling approaches
explaining resident and anadromous salmonid production remain largely theoretical and
there is little data available to parameterize such a model. We took a simplified,
empirically-based approach by assuming a fixed proportion of juveniles from each
parental cross adopt anadromous and resident life history strategies. Barring data specific
to Yakima stocks, we assumed O. mykiss adopt life history strategies proportional to
observed values from the Grande Ronde River, a tributary to the Snake River
(Carmichael pers. comm.) (Figure 13). Model sensitivity to changes in estimates of crossecotype production was explored.
21
Proportion Smolting and Residualizing
1.0
0.10
0.38
0.8
0.52
0.6
0.82
Smolting
Residualizing
0.90
0.4
0.62
0.48
0.2
0.18
0.0
StF x StM
StF x RbM
RbF x StM
RbF x RbM
Parent Spawners
Figure 13. Proportion of offspring smolting and residualizing from each of the four possible parent
crosses in the LHRM. St=steelhead; Rb=resident rainbow; F=female; and M=male
Dispersal
Juvenile dispersal in the fall from tributary to mainstem habitats was built into the model.
Dispersal was a significant uncertainty in the LHRM because data limitations prevent
adequate modeling of this behavior; however, anecdotal and fall trapping data (Conley et
al. 2008) suggest that a portion of juvenile O. mykiss leave tributary habitats prior to
smoltification and continue rearing in the Yakima River main stem. To test the
importance of this behavior for determining the balance between anadromy and
residency, we built mainstem holding reaches into the model and allowed a fixed
proportion of age-1 and age-2 anadromous juveniles to leave tributary habitats and
continue rearing in the main stem. Fixed percentages of fall migrants (Appendix A) were
approximated based on estimates from the Grande Ronde Basin (Carmichael pers.
comm.), and model sensitivity to dispersal rates was explored. Flow and temperature
conditions in mainstem holding reaches was assumed to be the same as conditions in
mainstem sites nearest the tributary of interest. Survival in holding reaches was assumed
to be density-independent and was modeled as a function of stream temperature using the
same temperature suitability index that was used to estimate temperature effects on
rearing capacity.
Model Sensitivity
We evaluated model sensitivity in terms of relative reproductive success in year 10 to
changes in a variety of key model parameters including smolt-to-adult survival rates, fallwinter dispersal rates, anadromous spawner fidelity, cross-ecotype smolt production,
growth (i.e., food availability), age-at-maturity, length threshold for smoltification, and
22
maximum freshwater survival (Mar-Oct). We selected a range of reasonable values for
each input parameter based on available empirical data, literature review, and/or
professional judgment (Appendix C). For simplicity, we limited the sensitivity analysis to
three model reaches (Kittitas, Taneum, and Toppenish Mid), representing different
channel types, hydrologic regimes, and locations within the basin.
To evaluate model sensitivity to assumptions about cross-ecotype smolt production, we
adjusted the proportion of offspring from the anadromous female and resident male cross
that smolt, and examined resulting changes in relative reproductive success in simulation
year 10. We focused specifically on mating pairs involving anadromous females and
resident males because the frequency of interbreeding between different ecotypes, and the
fate of their offspring represents one of the critical uncertainties in the life history
dynamics of O. mykiss in the Yakima Basin.
Results
After ten years of simulation using average flow and temperature conditions,
comparisons of total egg production for resident and anadromous populations suggested
that an anadromous life history strategy is favored in lower basin tributary reaches (e.g.
Satus and Toppenish Creeks), while a resident strategy was most successful in all other
reaches (Figure 2). Total anadromous egg production in lower tributary sites exceeded
resident egg production by approximately 2-5 times, with relative reproductive success
for anadromous fish being highest in the upper Toppenish and Satus Creek sites (Figure
14).
Model predictions of total resident spawner abundance at equilibrium exceeded
anadromous spawner abundance in all model reaches (Table 2). Resident spawner
abundance (fish per km) ranged from 47 in Taneum Creek to 1,246 in the Wapato reach
(mean = 491), while anadromous spawner abundance ranged from only 0.4 to 61 (mean =
18). Resident spawners mature at an earlier age and much smaller size than their
anadromous counterparts, such that a typical anadromous female deposits several times
more eggs than a resident female. Sex ratios are also quite different with females
composing approximately 75% of anadromous spawners, but only 40% of resident
spawners. As a result of the difference in fecundity and sex ratios, the different metrics
used to express the relative production of anadromous to resident offspring (egg ratios
and spawner ratios) are not directly comparable. However, the metrics show the same
trend in response of a given population to changes in environmental factors such as flow
or smolt-to-adult survival.
23
5
4
3
2
1
Satus Cr.
Toppenish Cr. (mid)
Toppenish Cr. (upper)
Yakima (Wapato)
Yakima (Union Gap)
Naches
Taneum Cr.
Yakima (Kittitas)
0
Yakima (Easton)
Relative reproductive success in year 10
(anadromous eggs / resident eggs)
6
Figure 14. Relative reproductive success in
simulation
Model
reach year 10 for each model reach using baseline
environmental conditions and default model parameters. The dashed line indicates the value at
which resident and anadromous reproductive success is equal.
Table 2. Summary of key model results for each reach in simulation year 10 including anadromous
and resident spawner abundance, egg production, and relative reproductive success. Model results
are standardized by reach length to aid in comparisons.
Spawners ∙ km-1
Eggs ∙ km-1
Rel. Repr. Success
Reside
Anadromo
Reside
Anadromo
Anad Eggs / Res
Reach
nt
us
nt
us
Eggs
198,43
Yakima (Easton)
626
3.7
9
19,681
0.10
134,91
Yakima (Kittitas)
394
6.0
3
31,264
0.23
Taneum Cr.
47
0.4
3,437
2,240
0.65
Naches
250
8.5
84,357
36,855
0.44
Yakima (Union
409,98
Gap)
1,199
58.7
6
249,420
0.61
427,34
Yakima (Wapato)
1,246
60.7
7
257,188
0.60
Toppenish Cr.
(upper)
116
4.5
3,884
18,639
4.80
Toppenish Cr. (mid)
251
4.2
8,431
17,227
2.04
Satus Cr.
290
13.9
11,598
57,211
4.93
24
Simulated changes in flow and temperature substantially influenced relative reproductive
success in Taneum and Toppenish Mid sites, but had little effect on populations in the
Kittitas site (Figure 15). Reducing summer-fall flow in the Taneum tributary site by
approximately 3.5 cfs resulted in a substantial shift in the balance of total egg production
towards anadromy. Additional reductions in flow beyond approximately 4 cfs had little
effect on relative reproductive success at this site. Similarly, steady increases in relative
reproductive success resulted from moderate decreases in summer base flow in the
Toppenish Mid reach. In contrast, reproductive success in the main stem Kittitas site was
insensitive to simulated changes in flow of up to 600 cfs above and below summer base
flow conditions.
Relative Reproductive Success in Year 10
(Anadromous eggs /Resident eggs)
3.0
Kittitas (flow x100)
Taneum
Toppenish Mid
2.5
2.0
1.5
1.0
0.5
0.0
-6
-4
-2
0
2
4
6
Hypothetical Flow Change from Observed, Summer-Fall (cfs)
Figure 15. Changes in relative reproductive success in year 10 in response to simulated changes in
flow (cfs) during summer and fall in three different sites (Kittitas, Taneum, and Toppenish Mid).
Temperature changes did not appear to affect relative reproductive success of
anadromous O. mykiss in the main stem Kittitas site, but did affect results in the Taneum
and Toppenish Mid sites (Figure 16). Increasing temperature in the Taneum site reduced
resident abundance and increased anadromous relative reproductive success up to about
2oC warmer than baseline, at which point anadromous relative reproductive success
begins to decline. Surprisingly, the same effect was not observed in the Toppenish Mid
site across the range of temperatures tested. This result was likely caused by the warmer
baseline temperatures in Toppenish Mid and subsequent reduction in growth at
temperatures that exceeded the optimum of 17oC. Growth reduction in the Toppenish
Mid site resulted in a shift in age-at-smoltification toward older age smolts and a
reduction in smolt-to-adult survival.
25
Relative Reproductive Success in Year 10
(Anadromous eggs /Resident eggs)
3.0
2.5
Kittitas
Taneum
Toppenish Mid
2.0
1.5
1.0
0.5
0.0
-2
-1
0
1
2
3
4
o
Hypothetical Temperature Change from Observed, Summer-Fall ( C)
Figure 16. Changes in relative reproductive success in year 10 in response to simulated changes in
stream temperature (°C) during summer and fall in three different sites (Kittitas, Taneum, and
Toppenish Mid).
Model Sensitivity
Model results were highly sensitive to changes in smolt-to-adult survival, moderately
sensitive to changes in cross-ecotype production, spawner fidelity, and growth rate (i.e.
food availability), and minimally sensitive to changes in winter dispersal rates, smolt size
threshold, age-at-maturity, and maximum freshwater survival during spring through fall
(Appendix D). Sensitivity results generally varied by channel type, with changes in
relative reproductive success being greatest in tributary reaches and less pronounced in
mainstem reaches.
Model estimates of relative reproductive success were most sensitive to changes in smoltto-adult survival, particularly for tributary reaches. Relative reproductive success in year
10 for Taneum and Toppenish Mid reaches increased by approximately six fold as SAR
was increased from 0 to 6%, compared with only a 2-fold increase for the main stem
Kittitas site (Figure 17). These results suggest that small to moderate increases in SAR
can tip the balance of total egg production in favor of anadromy, especially in tributary
reaches. For example, relative reproductive success in the Taneum Creek site was
estimated to be 0.65 under baseline conditions (i.e., mean SAR = 1.25%). However,
increasing the mean SAR for this population to only 2.88%, as was estimated for lower
basin populations, resulted in a relative reproductive success value greater than one.
26
Relative reproductive success in year 10
(anadromous eggs / resident eggs)
7
Kittitas
Taneum
Toppenish Mid
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
Smolt-to-adult return rate (%)
Figure 17. Relative reproductive success in simulation year 10 as a function of smolt-to-adult
survival.
Relative reproductive success was moderately sensitive to changes in cross-ecotype smolt
production. For Toppenish Mid, a lower basin tributary reach, relative reproductive
success in simulation year 10 increased considerably in response to increases in the
proportion of offspring from anadromous female and resident male crosses that smolted
(Figure 18). Specifically, relative reproductive success increased from approximately
0.80 to 7.5 as the proportion of offspring that smolted was increased from 30 to 90%. In
contrast, increases in cross-ecotype smolt production had little effect on relative
reproductive success in Kittitas and Taneum reaches. The difference in sensitivity among
reaches is likely due to location within the basin and associated differences in smolt-toadult survival. For the Toppenish Mid population, where SAR is relatively high,
moderate increases in smolt production can yield substantial gains in anadromous adult
returns and associated egg production. On the other hand, increasing smolt production
from upper basin sites, where SAR is quite low, has less of an effect on the reproductive
success of anadromous fish.
27
Relative reproductive success in year 10
(anadromous eggs / resident eggs)
8
Kittitas
Taneum
Toppenish Mid
7
6
5
4
3
2
1
0
20
30
40
50
60
70
80
90
100
Percentage of offspring that smolt
Figure 18. Relative reproductive success in simulation year 10 as a function of the percentage of
offspring that smolt from mating crosses between anadromous females and resident males.
Discussion and Conclusions
Model results were generally consistent with observed patterns of O. mykiss ecotype
distribution in the Yakima Basin and suggest that the spatial distribution of different
ecotypes is largely determined by environmental conditions such as flow and
temperature. The migratory life history type (steelhead) was dominant in tributary sites in
the lower Yakima Basin where summer flows were relatively low and stream
temperatures were comparatively warm. In contrast, residents (rainbow trout) were the
predominate life history form in the upper Yakima Basin and other mainstem locations
where stream channels generally maintained higher summer flows and cooler
temperatures. Important mechanisms influencing life history diversity in the Yakima
Basin identified by the model included rearing capacity (as moderated by flow,
temperature, and channel type), smolt-to-adult survival (as influenced by migration
distance), and growth conditions in freshwater.
Available rearing capacity for adult-sized resident fish (e.g., > 150 mm) appeared to be
an important bottleneck for production of resident O. mykiss, particularly in tributary
sites. Due to the requirements of adult fish for habitats with greater depth and velocity,
sharp declines in flow during summer resulted in substantial reductions in rearing
capacity for adult resident fish, favoring a migratory life history strategy. Tributary sites
in the lower Yakima Basin continued to favor an anadromous life history under a variety
of simulated flow conditions, although the relative reproductive success of anadromous
fish decreased somewhat as simulated summer flows increased. In contrast, the volume of
28
water in mainstem sites tended to remain high enough to sustain adult resident carrying
capacity year-round, even after simulating large summer flow reductions.
Model simulations for the Taneum Creek tributary site in the upper Yakima Basin
highlighted the important influence of population location within the Basin on life history
diversity. Reproductive success was slightly higher for the resident population in Taneum
Creek under baseline environmental conditions. However, increasing the smolt-to-adult
survival rate for smolts originating from this site to a value equal to that of lower Yakima
Basin smolts yielded a relative reproductive success ratio (A:R) greater than one. These
results suggest that mortality costs associated with greater migration distance can have an
important influence on the balance of O. mykiss ecotype distribution within a basin. That
is, habitats located further from the ocean are less likely to support anadromy, even when
local environmental conditions promote a migratory life history strategy. These results
are supported by a recent study examining the influence of landscape on O. mykiss life
history diversity in the Klickitat River basin which demonstrated that genetic diversity
was significantly negatively correlated with elevation and upstream distance (Narum et
al. 2008).
Variability in growth conditions between mainstem and tributary habitats had important
implications for the balance between resident and anadromous O. mykiss in the Yakima
Basin. In mainstem locations, where growth was relatively fast and resident adults were
able to achieve lengths of 400 mm and greater, reproductive success for resident
spawners substantially outweighed that of anadromous spawners. Resident rainbow trout
grow to a larger size in mainstem reaches than tributary reaches (Figure 8), which
resulted in higher fecundity of resident females in the main stem compared to tributaries.
In contrast, growth of resident spawners in tributary sites was predicted to plateau
between 160 and 200 mm, resulting in much lower average fecundity for tributary
spawners.
Studies throughout the Pacific Rim corroborate our findings. O. mykiss populations in the
Kamchatka Peninsula, far eastern Russia provide an excellent opportunity to compare
ecotype distribution patterns predicted by the LHRM to those in pristine river systems.
For example, the Kol and Sedanka Rivers support predominantly resident rainbow trout
in spite of no migration barriers and few anthropogenic influences. Flow and temperature
regimes in the spring-fed Kol and Sedanka Rivers also differ from neighboring rivers that
support predominantly anadromous O. mykiss (Augerot and Foley 2005). They have
cooler temperatures and more consistent, year-round flow conditions with less variability
between wet and dry seasons. More stable environmental conditions are likely to improve
survival and maintain adult resident carrying capacity, particularly through the summer
and fall. Genetic analysis of pristine populations throughout the Kamchatka Peninsula
confirm observations in the Pacific Northwest that resident and anadromous O. mykiss
from the same basin typically function as an interdependent population, and genetic
differences are more closely correlated to geographic separation than ecotype (McPhee et
al. 2007).
Our model results were consistent with broad scale patterns in flow variability and
observed distributions of adult steelhead in the Yakima Basin. Calculating an index of
seasonal flow variability for each population provided a course method of stratifying the
Yakima Basin into spatial units that were likely to provide habitat conditions favorable to
29
either residency or anadromy (Figure 19). Flow variability for the Metolius River,
Oregon, a spring-fed stream with consistent year-round flows, was provided for
reference. Accounting for watershed area, populations in the Yakima Basin experiencing
the most variable flows, Satus and Toppenish Creeks, sustain the largest abundance of
anadromous O. mykiss, and the upper Yakima main stem, having the most stable flows,
produces the fewest steelhead but is known to have a very large population of resident O.
mykiss (Figure 19; Pearsons et al. 2008). This simple approach suggests that flow is
strongly correlated to O. mykiss ecotypic distribution, and that LHRM is parameterized in
a manner that produces results consistent with empirical observation.
14
140
13
Flow variability index
(CV flow Mar-Sep (%))
120
10
100
80
2
60
40
20
0
Metolius Upper Yakima
Naches
Satus
Toppenish
Figure 19. Index of flow variability for the four Yakima independent steelhead populations. The
spring-fed Metolius River, central Oregon is provided as an example of a stream with extremely low
flow variability. Numbers above each box indicate the geometric mean of estimated steelhead
spawner abundance from 1985-2004 expressed in numbers of fish per 100 square km of watershed
area.
The rainbow-steelhead typology has obstructed adequate evaluation of O. mykiss
population performance and restrained proper management of the species. Our findings
are consistent with the conclusion of McPhee et al. (2007) that there is sufficient
scientific evidence to, “…abandon the typological thinking (‘steelhead’ and ‘rainbow
trout’ as biologically independent units) that has pervaded the biology and management
of this species...” Given clear evidence of genetically stable life history plasticity and
demonstrable environmental influence on life history response, a biologically justifiable
method of examining viability or developing restoration goals for steelhead populations
must include efforts to quantify the effects of codependent resident rainbow trout
populations. We recommend using approaches that quantify abundance and productivity
of both anadromous and non-anadromous O. mykiss when evaluating long term viability
of either ecotype.
30
Inadequate understanding of the factors driving ecotype abundance in facultatively
anadromous fish populations continues to deter researchers from properly evaluating
effects of watershed management on steelhead viability. Federal regulations prepared by
the National Oceanic and Atmospheric administration, categorically excluded resident O.
mykiss from steelhead Distinct Population Segments (DPSs) (71 FR 848); however, in
most cases, these two ecotypes are expressions of different life history strategies within a
single population and function in a manner consistent the definition of an Independent
Population described by McElhany et al. (2000).
Observation of codependent anadromous and non-anadromous O. mykiss ecotypes
throughout the Pacific Northwest should not be regarded as unique or unusual. The
overwhelming conclusion from studies of rainbow trout, cutthroat trout, brown trout,
sockeye salmon, Atlantic salmon and charr life history diversity is that partial anadromy,
a strategy beneficial to adaptability and versatility in uncertain river environments, is
common among species of the family Salmonidae (Rounsefell 1958, Jonsson and Jonsson
1993 and Hendry et al. 2004).
Our analysis suggests that alterations to river discharge regimes due to irrigation and
hydropower projects changes the flow and temperature conditions throughout the summer
and fall, making the habitat more suitable for a resident life history strategy. Regulated
river hydrographs throughout Washington, Oregon and California more closely resemble
spring-fed systems that traditionally support large populations of resident rainbow trout.
We suspect this is because adult carrying capacity is maintained throughout the dry
season. Therefore, it is reasonable to conclude that reduction in steelhead abundance and
increase in resident rainbow trout abundance in regulated rivers is in part a response to
environmental conditions.
One of the primary benefits of a life-cycle modeling project is the ability to identify key
data gaps and study needs. Assembly of the complete life cycle forces researchers to
synthesize the best available information. Key functions and parameters driving
population performance are identified, and components of the life cycle that were
developed with uncertain data can be examined through sensitivity analysis. We
identified several studies that would improve our understanding of O. mykiss life history
drivers:

The spatial structure of the model is limited to specific reaches within each
Independent Population. Expanding the spatial structure of the model to the
population level would greatly improve the utility of the model for evaluating
population responses to environmental conditions and would likely yield
important and potentially unanticipated insights into the factors driving life
history diversity. A population level analysis would likely require an alternative
approach to modeling the relationship between discharge and habitat area, as
PHABSIM and 2D modeling methods are costly and highly site specific. On
alternative is an empirically based estimate of flow effects on carrying capacity
similar to the Unit Characteristic Method described in Cramer and Ackerman
(In Press). A combination of fish abundance surveys and measurements of
stream channel type, mesohabitat composition, substrate, wood complexity,
temperature and flow would be required for this type of analysis.
31

Migratory juveniles may residualize upon entry into suitable habitat for
continued rearing, such as conditions found in the upper Yakima River main
stem. Our model did not account for life history switching after initiation of
migration, but the likelihood of that behavior could have a significant effect on
ecotype distribution. Laboratory experiments could provide important insights
into the factors influencing residualization behavior.

Movement of juveniles and adults between mainstem and tributary habitats was
not adequately accounted for in the LHRM due to data limitations. Field studies
of these behaviors are needed. We also recommend an individual based
modeling approach to address these dynamics.

Hydrodynamic modeling would create a link between flow and temperature
making it possible to construct a life-cycle model with continuous spatial and
temporal structure throughout the main stem Yakima River.

Juvenile tagging studies or outmigrant sampling at Roza Dam and the Chandler
Juvenile Fish Facility accompanied by otolith microchemistry or equivalent
technique to link outmigrants back to resident and anadromous parents would
address important uncertainties regarding the contribution of resident spawners
to the anadromous populations.
The LHRM provided a framework that linked together life history parameters and
environmental drivers of O. mykiss ecotype distribution. Moreover, the model offered a
sufficiently detailed reconstruction of the mechanistic relationships between
environmental conditions and ecotypic dominance such that water management impacts
could be evaluated and alternatives tested. High summer flow conditions typical of
regulated rivers improve survival of residents and increase reproductive success of
resident O. mykiss relative to anadromous spawners; however, the ability of water
managers to alter flow conditions in mainstem habitats in a manner that results in greater
reproductive success of anadromous individuals relative to residents appears limited. Our
modeling demonstrated that tributary habitats were most likely to support an anadromous
ecotype, and management actions that protect or improve tributary habitats have the
greatest potential to increase abundance of steelhead in the Yakima Basin. We
recommend controlled field experiments or adaptive management to examine O. mykiss
life history response to altered flow regimes.
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Appendices
Appendix A. Summary of model functions, parameters, and literature sources.
Model Function
Functional Relationship and
Parameter Definitions
Notation
Freshwater Growth
dW/dt = Change in fish mass W in
dW
 qi qe f (T )t W a   i e xTt W b
grams per day t,
dt
Tt = daily average stream
temperature (°C)
qi = individual variation in food
finding and processing ability,
qe = environmental variation in food
abundance,
αi = metabolic rate,
a & b = Von Bertalanffy growth
parameters
x = coefficient for temperature effect
on metabolism.
Temperature
dependence
function for food
finding and
processing ability
Length/Weight
Relationship
Freshwater Capacity
1
Parameter Values
Source
Starting W = 0.40g.
qi = (0.070, 0.052,
0.108, 0.052,
0.052, 0.052,
0.106, 0.106,
0.116), for reaches
1, …, 9).
qe = 1.0,
αi = (0.004, 0.002,
0.009, 0.002,
0.002, 0.002,
0.011, 0.011,
0.011), for reaches
1, …, 9),
a = 0.667
b = 1.0
x = 0.068
See references for
parameters defining
KA and KB.
Mangel and Sattherthwaite
(2008); Thorpe et al. (1998)
f(T)t = KA∙KB (Thornton and Lessem
algorithm)
KA and KB represent the increasing
and decreasing portions of the
temperature dependence function
respectively.
L  e aln(W ) b
L = Fork length (mm)
a = 0.35
b = 3.72
K = freshwater rearing capacity for
age class m on day t
See below
K mt 
HAm ,t  g (T ) t
Functional relationship from
Thornton and Lessem (1978)
cited in Hanson et al. (1997);
Parameters from Sullivan et
al. (2000).
Upper Yakima rainbow trout
data (Washington
Department of Fish and
Wildlife (WDFW), Gabriel
Temple)
TS m ,t
38
Habitat Area
HAm,t= Function of discharge and agespecific suitability criteria.
HAm,t = habitat area (m²) for age
class m on day t
See Table ? for
suitability criteria and
Figure ? for
discharge-habitat
relationships.
Temperature
suitability index
for capacity
If Tt <= Tcrit, Then g(T)t = 1,
Else g(T)t = a∙Tt + b
Tcrit = 17 °C
Tmax = 26 °C
a = 1/(Tcrit-Tmax) = 0.111
b = 1-(a∙Tcrit)= 2.887
Territory Size
TS m,t  10
g(T)t = temperature suitability index
on day t.
Tcrit = upper critical temperature
(°C)
Tmax = incipient lethal temperature
(°C)
TSm,t = territory size (m²) for age
class m on day t,
Lm,t = Fork length (mm).
Sm,t = Survival rate for fish of age
class m on day t,
Smaxm,t = maximum daily survival
rate.
d = lag time for survival (days)
a & b are intercept and slope
coefficients for a logistic scalar for
overwinter survival.
z = additional mortality scalar used
to adjust the survival rates to match
the assumed maximum overwinter
survival rate in the Yakima River.
Nm,t = Abundance of fish of age
class m on day t.
Pdism,t = proportion of fish of age
class m dispersing at time t.
Pdism=0, t = 0.50,
Pdism=1, t = 0.30,
Pdism=2,…,5, t = 0
Freshwater survival
(March – October)2
alog10 ( Lm , t / 10) b
If (Km,t/Nm,t)(1/d) > Smaxm,t, Then Sm,t =
Smaxm,t,
Else Sm,t = (Km,t/Nm,t)(1/d)
( aLm , t b )
Freshwater survival
(November – April)
S m ,t 
Abundance3
Nm,t = Nm,t-1 ∙ Sm,t-1
Winter dispersal4
Pdism,t
e
1 e
( a Lm , t b )
 1  z 
a = 2.61
b = -2.90
Smaxm=0,t = 0.40(1/(365emergence day))
= 0.995,
Smaxm=1,…,5,t =
0.80(1/365)= 0.999
d = 14
a = 0.10,
b = -10.54,
z = 0.30
Nm = 0,…,5, t = 1 =
(61230, 3640,
2310, 780, 150,
50).
2-D modeling results
modified from Bovee et al.
(2008) and RHABSIM results
provided by Yakama Nation
Fisheries Program (YKFP),
Chris Fredericksen.
Sullivan (2000)
Grant and Kramer (1990)
Smith and Griffith (1994)
Starting values based on
average resident O. mykiss
abundance for a 20 km reach
from the Upper Yakima
River main stem (WDFW,
Gabriel Temple Pers.
Comm.)
Based roughly on
downstream migrant trapping
data from the Grand Rhonde
39
Winter dispersal date
Ddis
Ddis = median date of downstream
dispersal of pre-smolts during fall
and winter.
= January 5 (Julian
Day 5)
Smolt emigration
date
Dsmt
Dsmt = median date of smolt
emigration
= May 1 (Julian Day
121)
Smolt length
threshold
Lmin
Lmin = minimum length threshold for
smoltification (in millimeters)
Lmin = 150mm
SARm,t = Smolt-to-adult return rate
for fish of age class m emigrating at
time t at length L.
For average-size
smolts (i.e., L =
175mm) SAR (above
Roza Dam) = 1.25%,
SAR (below Roza
Dam) = 2.88%.
Pmatrm=1,…,5 = 8, 22,
47, 73, 90.
Smolt-to-adult return
rate
SARm,t 
e
a Lm , t b
1 e
aLm , t b
Age at maturity
(resident spawners)
Pmatrm
Pmatrm = percentage of female fish
of age class m that are sexually
mature.
Age at maturity
(anadromous
spawners)5
Pmataf,s
Pmataf,s = percentage of adult fish
that emigrated after f years in
freshwater and returned to spawn
after s years in saltwater.
Populations above
Roza:
Pmataf,s:
(1,1) = 26%
(1,2) = 74%
(2,1) = 45%
(2,2) = 55%
(3,1) = 52%
(3,2) = 48%
Populations below
River (Oregon Department of
Fish and Wildlife (ODFW),
Richard Carmichael)
Estimated from smolt
trapping data from Satus and
Toppenish Creeks 2005-2007
(YNFP, Chris Fredericksen)
Estimated from smolt
trapping data at the Chandler
trapping facility, Yakima
River, 1999-2008 (YNFP,
David Lind)
5th percentile of length
distribution for fish captured
at the Chandler trapping
facility during spring (April –
June), 1999-2008.
Functional relationship based
on Ward et al. (1989),
Average SAR rates from
YNFP, Chris Fredericksen,
Pers. Comm.
Estimated from mainstem
Upper Yakima River data,
1990-1993 (WDFW, Gabriel
Temple Pers. Comm.).
From Yakima Steelhead
Recovery Plan, Conley et al.
(2008).
40
Repeat spawning rate
(resident spawners)
Repeat spawning rate
(anadromous
spawners)
Percent females
(resident spawners)
Rsr
Percent females
(anadromous
spawners)
Pfemaf,s
Fecundity (resident
spawners)
Frm,t = a∙Lm,t+b
Rsa
Pfemr
Rsr = probability of repeat spawning
for resident spawners.
Rsa = probability of repeat spawning
for anadromous spawners.
Pfemr = percentage of the resident
spawner population composed of
females.
Pfemaf,s = percentage of anadromous
spawners that emigrated after f years
in freshwater and returned to spawn
after s years in saltwater that are
composed of females.
Frm,t = number of eggs per female
for fish of age class m on spawning
day t and fork length L (in mm),
Roza:
Pmataf,s:
(1,1) = 68%
(1,2) = 32%
(2,1) = 76%
(2,2) = 24%
(3,1) = 83%
(3,2) = 17%
Rsr = 50%
Rsa = 5.4%
Pfemr = 40%
Populations above
Roza:
Pfemaf,s:
(1,1) = 60%
(1,2) = 93%
(2,1) = 68%
(2,2) = 85%
(3,1) = 94%
(3,2) = 98%
Populations below
Roza:
Pfemaf,s:
(1,1) = 58%
(1,2) = 78%
(2,1) = 68%
(2,2) = 86%
(3,1) = 73%
(3,2) = 93%
For mainstem
reaches: a = 1.4, b =
500,
Deschutes River data,
Schroeder and Smith (1989).
From Yakima Steelhead
Recovery Plan, Conley et al.
(2008).
Pearsons et al. (1993).
From Yakima Steelhead
Recovery Plan, Conley et al.
(2008).
Pearsons et al. (1993).
41
a & b = linear slope and intercept
parameters.
Fas = number of eggs per female for
anadromous fish returning to spawn
after s years in saltwater at fork
length L,
a & b = linear slope and intercept
parameters.
Dspn = Median spawning day
(Julian Days) as a function of
elevation (m),
a & b = linear slope and intercept
parameters.
Spspn = post spawning survival of
resident spawners.
For tributary reaches,
a = 8.6, b = -1292.
a = 18.93, b = -6449.
Fecundity
(anadromous
spawners)
Fas = a∙Ls+b
Spawn timing
(resident and
anadromous)6
Dspn = a∙Elev+b+1
Post-spawning
survival (resident
spawners)
Emergence timing
Spspn
Demg = function of steam temperature.
Demg = days since spawning
required to accumulate at least 885
cumulative thermal units (°C)
Egg-to-fry survival
(resident and
anadromous)
Mating selectivity
(i.e., spawner
fidelity)
Segg
Segg = egg-to-fry survival
Segg = 20%
PMc = PMbasec ∙ Wc
c = Resident Female
and Resident Male
(RFRM), RFAM,
AFAM, and AFRM;
PMbasec:
RFRM = 0.95
RFAM = 0.05
AFAM = 0.76
AFRM = 0.24
Weighting factor for
mating cross
probability
Examples:
WAFRM = NRM /(NRM + NAM)
WRFAM = NAM /(NRM + NAM)
PMc = Probability of obtaining
mating cross c is a function of the
average baseline mating cross
probability PMbasec (i.e. PMc at
equal abundance of resident and
anadromous male spawners)
weighted by the relative abundance
of male spawners from each ecotype
Wc.
Wc = weighting factor based on
relative abundance of male spawners
from each ecotype.
From Yakima Steelhead
Recovery Plan, Conley et al.
(2008).
a = 0.103, b = 34.8
Pearsons et al. (2007).
Spspn = 55%
Wydoski and Whitney (2003)
Estimated from redd capping
data from the Upper Yakima
River, 1994-1996 (WDFW,
Gabriel Temple)
Snow Creek, Bley and
Morning (1989).
Values for AFAM and
AFRM from McMillan et al.
(2007) and McMillan pers.
comm. (12/18/08). Values for
RFRM and RFAM from
Snow Creek, WA spawner
data (Seamons et al. 2004).
42
1
Freshwater capacity is averaged over a period of 30 days based on the assumption that changes in capacity resulting from differences in flow, temperature, or
territory size would not occur instantaneously, but instead would occur gradually over an extended period (e.g. 30 days). Model users can modify this value to
test different time duration assumptions.
2
We apply a lag period of 14 days to density-dependent survival, such that, when abundance exceeds capacity, the mortality associated with this capacity
limitation is expressed over an extended period of time (e.g. 14 days) instead of occurring instantaneously.
3
Initial abundance values for anadromous origin fish were the same as resident starting values, except abundance for age 4 and 5 fish was set to 0 based on the
assumption that all anadromous smolts would migrate to the ocean by age 3.
4
Winter dispersal calculations only apply to anadromous offspring rearing in tributaries.
5
Age at smoltification is determined via the growth model and an assumed threshold size for smoltification. For a given age, the proportion of the fish on
emigration date
6
Because no significant difference was observed between spawn timing of resident and anadromous populations and to simplify model calculations, we used a
linear regression model developed from resident fish to predict spawn timing for both resident and anadromous populations.
43
Appendix B. Habitat suitability criteria for resident O. mykiss used in 2-D modeling to produce
discharge habitat relationships. Gray cells represent unsuitable ranges of depth and velocity and
white cells represent suitable values.
Fry (Age
0)
0.00
0.06
0.09
0.12
0.15
0.18
0.21
0.24
0.27
0.30
0.34
0.49
0.61
0.70
0.76
0.79
1.01
1.10
1.13
1.25
1.31
1.40
1.49
1.74
2.01
2.74
2.99
3.87
4.51
4.88
5.00
Suitable depth (meters)
Juvenile (Age
Adult (Age
1)
2-5)
0.00
0.00
0.06
0.06
0.09
0.09
0.12
0.12
0.15
0.15
0.18
0.18
0.21
0.21
0.24
0.24
0.27
0.27
0.30
0.30
0.34
0.34
0.49
0.49
0.61
0.61
0.70
0.70
0.76
0.76
0.79
0.79
1.01
1.01
1.10
1.10
1.13
1.13
1.25
1.25
1.31
1.31
1.40
1.40
1.49
1.49
1.74
1.74
2.01
2.01
2.74
2.74
2.99
2.99
3.87
3.87
4.51
4.51
4.88
4.88
5.00
5.00
Suitable velocity (meters per second)
Fry (Age
Juvenile (Age
Adult (Age
0)
1)
2-5)
0.00
0.00
0.00
0.06
0.06
0.06
0.09
0.09
0.09
0.12
0.12
0.12
0.15
0.15
0.15
0.18
0.18
0.18
0.21
0.21
0.21
0.24
0.24
0.24
0.27
0.27
0.27
0.30
0.30
0.30
0.34
0.34
0.34
0.40
0.40
0.40
0.49
0.49
0.49
0.64
0.64
0.64
0.67
0.67
0.67
0.70
0.70
0.70
0.73
0.73
0.73
0.76
0.76
0.76
0.79
0.79
0.79
0.82
0.82
0.82
0.88
0.88
0.88
0.91
0.91
0.91
0.94
0.94
0.94
1.01
1.01
1.01
1.19
1.19
1.19
1.25
1.25
1.25
1.37
1.37
1.37
1.58
1.58
1.58
2.44
2.44
2.44
2.56
2.56
2.56
2.71
2.71
2.71
44
Appendix C. Description of parameter values used in sensitivity analyses.
Model parameter
Default value
Smolt-to-adult survival (lower basin, upper basin)
1.25%, 2.88%
Winter dispersal (age-0, age-1)
50%, 30%
Cross-ecotype production (AF X RM)
61%
Anadromous spawner fidelity
76%
Low value
5th percentile = 0.45%, 1.0%
Default * 0.5 = 25%, 15%
Default * 0.5 = 30.5%
Default - 25% = 50%
High value
95th percentile = 4.0%, 9.3%
Default * 1.5 = 75%, 45%
Default * 1.5 = 91.5%
Default + 25% = 100%
Growth (food availability)
Age-at-maturity (ages 1, …, 5)
Length threshold for smoltification
Maximum survival Mar-Oct (ages 1-5)
Default - 16% = 0.84
Default - 10% = 0%, …, 80%
Default - 30 = 120mm
40%
Default + 16% = 1.16
Default + 10% = 18%, ..., 100%
Default + 30 = 180mm
90%
qe = 1
8%, 22%, 47%, 73%, 90%
150 mm
80%
45
Appendix D. Summary of model results for each of eight different sensitivity scenarios for the
Kittitas, Taneum, and Toppenish Mid reaches. Reach names and corresponding estimates of relative
reproductive success under default model settings are shown in italics.
Relative reproductive success
Change from default
Kittitas
Default =
0.23
Model parameter
Low
High
Low - Default High - Default
Smolt-to-adult survival
0.06
1.33
-0.17
1.10
Winter dispersal
NA
NA
NA
NA
Cross-ecotype production (AF X RM)
0.15
0.41
-0.08
0.18
Anadromous spawner fidelity
0.23
0.40
-0.01
0.17
Growth (food availability)
Age-at-maturity
Length threshold for smoltification
Maximum survival Mar-Oct
0.11
0.45
0.09
0.25
0.25
0.15
0.26
0.25
-0.12
0.22
-0.14
0.02
0.01
-0.08
0.03
0.02
Taneum
Model parameter
Smolt-to-adult survival
Winter dispersal
Cross-ecotype production (AF X RM)
Anadromous spawner fidelity
Growth (food availability)
Age-at-maturity
Length threshold for smoltification
Maximum survival Mar-Oct
Default =
Low
0.12
0.70
0.30
0.61
0.29
1.03
0.67
0.56
0.65
High
3.67
0.35
1.59
1.49
1.73
0.42
0.41
0.68
Low - Default
-0.53
0.05
-0.35
-0.04
-0.36
0.38
0.02
-0.09
High - Default
3.02
-0.30
0.94
0.84
1.08
-0.23
-0.24
0.03
Toppenish Mid
Model parameter
Smolt-to-adult survival
Winter dispersal
Cross-ecotype production (AF X RM)
Anadromous spawner fidelity
Growth (food availability)
Age-at-maturity
Length threshold for smoltification
Maximum survival Mar-Oct
Default =
Low
0.38
2.12
0.76
1.97
0.22
4.08
1.65
2.51
2.04
High
10.17
1.86
7.70
6.98
3.31
1.20
0.66
2.01
Low - Default
-1.66
0.08
-1.28
-0.07
-1.82
2.04
-0.39
0.46
High - Default
8.13
-0.19
5.66
4.93
1.27
-0.84
-1.38
-0.03
46