United States
Environmental Protection
Agency
EPA-910-D-01-002
May 2001
SEPA Issue Paper 2
Salmonid Distributions
and Temperature
Prepared as Part of EPA Region 10
Temperature Water Quality Criteria
Guidance Development Project
Jason Dunham, U.S. Forest Service
Jeff Lockwood, National Marine Fisheries Service
Chris Mebane, Idaho Department of Environmental Quality

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Contents
Abstract 	1
Introduction 	1
What is a "distribution"? 	2
Ontogenetic variation	2
Life history variation 	3
Historical vs. contemporary vs. potential distribution	3
Examples 	4
What are the direct effects of temperature?	12
Examples 	12
What are indirect effects of temperature on fish distributions?	13
Biotic interactions 	13
Habitat size and isolation	13
What is meant by "scale" and "level?" At what "level" should we be concerned with
temperature criteria to protect fish distributions? 	13
Information on thermal relationships of salmonids comes from a variety of laboratory
and field studies—how do we integrate work conducted at different scales or
levels of organization (e.g., population vs. individuals)? 	14
EPA Fish and Temperature Database Matching System (FTDMS) 	15
"I saw fish in hot water."	15
Is unoccupied habitat relevant to temperature requirements of salmonids? 	15
Conclusion	17
Literature Cited 	18
Salmonid Distributions and Temperature

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Issue Paper 2
Salmonid Distributions and Temperature
Prepared as Part of Region 10 Temperature Water Quality Criteria
Guidance Development Project
Jason Dunham, Jeff Lockwood, and Chris Mebane
Abstract
Distributions of native salmonid fish in the Pacific Northwest are strongly tied to
temperature conditions in their habitat. Salmonid populations have declined in conjunction with
thermal changes and the loss and fragmentation of large and interconnected cold-water habitats.
Temperature affects the health of not only individual fish but also entire populations and groups
of species. Temperature changes have obvious direct effects, and also interact with other factors
to indirectly affect salmonids.
The best way to protect existing populations and restore depleted populations is to create
temperature criteria that explicitly consider salmonids' temperature requirements at different
times and places. Natural temperature conditions must be preserved whenever possible.
Because current fish distributions and populations are significantly reduced from their historical
numbers, protection and restoration of their thermal environment must often extend beyond the
boundaries of their existing or suitable habitat.
Attempts to set temperature criteria must balance what is known and not known about the
habitat and biological requirements of salmonids. Full consideration of current and potential fish
distribution and habitat, including thorough documentation of assumptions and knowledge gaps,
is needed in establishing and implementing temperature criteria to support healthy (viable,
productive, and fishable) salmonid populations.
Introduction
Under natural conditions, freshwater salmonid habitat is defined by physical and
chemical characteristics of the environment, including water quality, flow, geological and
topographic features of the stream and its valley, and cover (National Research Council 1996).
Common factors influencing fish distribution include size and accessibility of suitable habitat,
connectivity between areas of suitable habitat, biological interactions, and "historical" factors
(e.g., postglacial dispersal and geographic barriers) (Matthews 1998). Many of these factors act
directly or indirectly with temperature to determine the distribution of a species. This is
especially true for cold-water fishes such as salmonids.
This paper is not intended to be an exhaustive review of the status or declines in
salmonid populations or distributions. These are widely documented elsewhere. We briefly
review some examples of declines in salmonid populations and habitats to provide some context
Salmonid Distributions and Temperature
1

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for these issues, but our focus is not on declines per se. Furthermore, this issue paper is not
intended to be an exhaustive review of the effects of temperature on salmonid distributions in the
Pacific Northwest (see McCullough 1999). Rather, it is intended to describe a basic framework
for thinking about salmonid distributions and appropriate biological criteria to protect salmonid
populations from adverse effects of altered factors affecting thermal regimes.
This paper describes in a question-and-answer format five main issues related to
salmonid distributions and temperature criteria:
1.	Definition of a "distribution"
2.	Direct effects of temperature
3.	Indirect effects of temperature
4.	Relevance of scale
5.	Importance of unoccupied habitat
What is a "distribution "?
Often, the word "distribution" is used without reference to what is specifically meant.
Like any other organism, salmonid fishes (and temperatures) are not distributed equally across
landscapes. Within stream basins, limits to fish distributions may be obvious, but even within
continuous areas of suitable habitat, discontinuities in distributions may arise (Angermeier et al.
in press, Dunham et al. in press).
A common example of "distribution" for animals can be found in popular bird
identification and field guides. Distribution maps for birds often cover broad areas. In some
cases, ranges of different "races" or recognized subspecies are distinguished. Within these areas,
it is obvious that birds do not occur everywhere. For example, a wading bird may only be found
in wetland areas, though it is broadly distributed across the continent (because wetlands are
broadly distributed). Furthermore, this bird may only be found in particular kinds of wetlands
(those with sufficient cover and food to support reproduction). This bird may be found in
different areas, depending on the season. Birds may appear in "unusual" habitats while
migrating, or may shift habitat use from year to year, depending on climate (wet vs. dry years).
Similar analogies apply to salmonid fishes. There are several things to consider when using the
term "distribution" for salmonids: ontogenetic variation; life history variation; and historical,
contemporary, and potential distribution.
Ontogenetic variation. "Ontogenetic variation" refers to changes in habitat use during
the life cycle of an individual. Here, the term "life cycle" refers to the sequence of events (egg -
alevin - parr - smolt - juvenile - adult) that must occur within an individual's life for successful
reproduction. Ideally, temperature criteria established for salmonids should address spatial and
temporal distribution of thermal habitats that protect all life stages.
Habitat requirements vary considerably as salmonids begin their lives as eggs in (or on)
the substrate and progress through developmental stages to reproduction as an adult. Different
life stages may have different thermal requirements (Magnuson et al. 1979; Physiology issue
paper). However, thermal requirements may also overlap considerably among life stages.
Salmonid Distributions and Temperature
2

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Furthermore, some life stages are relatively insensitive to temperature whereas others (such as
egg incubation) are extremely sensitive (see Physiology issue paper).
Life stage requirements may be tied to specific spatial or temporal frames. Many
salmonids' life stages may use certain habitats only on a seasonal or intermittent basis. For
example, the timing of migration and spawning for most species is strongly tied to temperature
(Bjornn and Reiser 1991).
Often, assessments for salmonids focus on the distribution of areas used for spawning
and early rearing (Dunham et al. 2001). Even though the importance of spawning and rearing
habitat is obvious, other components of the life cycle may be key to viability or productivity,
particularly for species with obligate life histories. Such habitats can include migratory
corridors, feeding areas, and seasonal refuges (Northcote 1997). In many species, loss or severe
degradation of these habitats can cause extinction even if spawning and rearing habitats are in
good condition. An obvious example is extinction of migratory salmonid populations that used
spawning habitats now blocked by dams. As of 1991, at least 106 major populations of salmon
and steelhead on the West Coast of the United States had become extinct, with inadequate fish
passage at dams a primary cause (Nehlsen et al. 1991).
Life history variation. Life history refers to how an individual completes the life cycle.
Salmonids may adopt a "resident" or "migratory" life history. Resident fish remain very close to
their natal habitats throughout their life cycle, whereas migratory fish use a much broader range
of habitat. Each of these broad categories has its own variations. For example, spawning
migrations vary by time and location (e.g., summer vs. winter steelhead; fall vs. winter chinook).
The length of juvenile residence in natal areas may also be important (e.g., "stream" vs. "ocean"
type chinook).
Some species have relatively fixed life cycles and life history patterns (e.g., pink salmon,
Groot and Margolis 1991); others exhibit considerable variation or polymorphism (e.g., cutthroat
trout). Most Pacific salmon die after spawning, whereas most species of trout and char do not
(iteroparous). Some species, subspecies, races, or populations have flexible life histories
(referred to as "facultative"); others have fixed life history patterns (referred to as "obligatory")
(Rieman and Dunham 1999). Species in the latter category may be less resistant to
environmental change.
Historical vs. contemporary vs. potential distribution. Both fish distributions and
stream temperatures can be considered in terms of "historical," "contemporary," or "potential"
distribution. Historical refers to the distribution of native salmonids before European settlement.
Contemporary refers to the present distribution of native salmonids. Potential refers to the
distribution of native salmonids we would expect if natural habitat conditions were restored to
the fullest extent possible, given the current natural capacity (Ebersole et al. 1997) of the system.
In other words, potential distribution allows for the possibility that physical systems have been
altered such that historical distributions are no longer attainable. Widespread declines of
salmonids observed in most areas (Nehlsen et al. 1991, Lee et al. 1997, Thurow et al. 1997)
suggest that many streams are not currently at their full natural potential or capacity.
Salmonid Distributions and Temperature
3

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A primary concern of managers is protecting or restoring fish distributions that maximize
population viability (most recently reviewed by McElhany et al. 2000). Many efforts are under
way to define thermal habitat potential using predictive physical models (reviewed by Bartholow
2000). Prediction of physical responses is complex, but is much simpler than predicting
biological responses.
Restoration of the physical system (temperature, thermal regime) should be considered
together with biological requirements (viability, productivity) of a species. The physical
potential of a system constrains what can be achieved biologically. There are four possible
scenarios in which physical system potential and biological requirements or potential are
considered:
1.	System potential attained, biological goal attained. This is the best of all worlds, where
protection to maintain existing conditions would be a prudent management option.
2.	System potential attained, biological goal not attained. This is a situation where nothing
can be done to enhance the potential of the natural system to attain a biological goal.
3.	System potential not attained, biological goal not attained. This is a situation where
enhancement of system potential could result in a biological benefit.
4.	System potential not attained, biological goal attained. This is a situation where
enhancement of system potential could result in a biological benefit, but the current state
of the biological system is satisfactory from a regulatory viewpoint.
It may be difficult to balance the attainment of biological goals versus physical system
potential, but the answer is essential to long-term viability and productivity of salmonid
populations. In reality, these four scenarios represent extremes along a continuum of biological
requirements and physical system potential. In practice, it is much easier to define physical
system potential than to define "how much is enough?" from a biological perspective. Thus, it
may be difficult to discern different scenarios based on biological requirements. In practice,
most management to date has focused on system potential.
Defining of system potential can be challenging. First, it is critical to realize that
perspectives on attainment of system potential may depend on scale. For example, a local reach
of stream may be at system potential, but part of a larger degraded system in need of restoration.
Second, it is difficult, if not impossible, to restore all aquatic habitats to their historic condition.
There usually are insufficient data to definitively document "historic" conditions, but even
limited information on historic habitat conditions and fish populations can provide a useful
perspective. Such determination involves finding what is "irreversible" (e.g., removal of major
dams and urban centers) and what can likely be accomplished through basin management.
Examples. The historical and contemporary distributions of resident and anadromous
fish have been documented in the Columbia River Basin (CRB) by the Interior Columbia Basin
Ecosystem Management Project (Figures 1 to 7). About 12,452 km of the 16,935 km of streams
that originally were accessible are now blocked (Quigley and Arbelbide 1997), including some
large subbasins and many smaller watersheds. Other factors contributing to the decline of
Salmonid Distributions and Temperature
4

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MONTANA
WASHINGTON
'REGON
IDAHO
DATA SOURCES:
LEGEND:
| | HISTORIC RANGE
18831 CURRENT RANGE
	 1:2M STREAMS
Historic Range:
Interior Columbia Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Maiagement Project
Data for analysis and display were compiled
from the best availiable sources. No liability
is assumed for the accuracy or completeness
of the information contained herein.
COLUMBIA BASIN FALL CHINOOK DISTRIBUTION
(EAST OF CASCADES ONLY)
CANADjA,
Siieosvwes
UTAH
WYOMING
NEVADA
Figure 1. Columbia Basin fall chinook distribution.
Salmonid Distributions and Temperature
5

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WASHINGTON
iREGON
IDAHO
DATA SOURCES:
Historic Range:
Interior Columbi Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Management Project
DISCLAIMER:
Data for analysis and display were compiled
from the best availiab!e sources. No liability
is assumed for the accuracy or completeness
of the information contained herein.
LEGEND:
| | HISTORIC RANGE
CURRENT RAN3E
	 1:2M STREAMS
COLUMBIA BASIN BULLTROUT DISTRIBUTION
(EAST OF CASCADES ONLY)
CANADA
OnneosTMES
NEVADA
UTAH
WYOMING
Figure 2. Columbia Basin bulltrout distribution.
Salmonid Distributions and Temperature
6

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MONTANA
'ASHINGTON
'REGON
IDAHO
DATA SOURCES:
Historic Range:
Interior Columbia Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Management Project
DISCLAIMER:
Data for analysis and display were compiled
from the best avalliabie sources. No liability
is assumed for the accuracy or completeness
of the information contained herein.
LEGEND:
I 1 HISTORIC RANGE
$$$& CURRENT RANGE
	 1:2M STREAMS
COLUMBIA BASIN SPRING CHINOOK DISTRIBUTION
(EAST OF CASCADES ONLY)
CAMADA
UNITED STATES
WYOMING
UTAH
NEVADA
Figure 3. Columbia Basin spring chinook distribution.
Salmonid Distributions and Temperature

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COLUMBIA BASIN REDBAND TROUT DISTRIBUTION
(EAST OF CASCADES ONLY)
LEGEND:
I | HISTORIC RANGE
CURRENT RANGE
1:2M STREAMS
OS
CANADA	
United states
WYOMING
DATA SOURCES:
Historic Range:
Interior Columbia Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Management Project
DISCLAIMER:
Data for analysis and display were compiled
from the best availiaDle sources. No liability
is assumed for the accuracy or completeness
of the information contained herein.
NEVADA
UTAH
150 Miles
ISO Kilometers
Figure 4. Columbia Basin redband trout distribution.
Salmonid Distributions and Temperature
8

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WASHINGTON
I REGON
IDAHO
LEGEND:
I I HISTORIC RANGE
CURRENT RANGE
	 1:2M STREAMS
Historic Range:
interior Columbia Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Management Project
Data for analysis and display were compiled
from the besl availiable sources. No liability
is assumed for tho accuracy or complctonoss
of the information contained herein.
COLUMBIA BASIN WESTSLOPE CUTTHROAT TROUT
DISTRIBUTION
(EAST OF CASCADES ONLY)
NEVADA
UTAH
WYOMING
Figure 5. Columbia Basin westslope cutthroat trout distribution.
Salmonid Distributions and Temperature

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MONTANA
iSHINGTON
iREGON
IDAHO
DATA SOURCES:
LEGEND:
I | HISTORIC RANGE
CURRENT RANGE
	 1:2M STREAMS
Historic Range:
Interior Columbia Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Management Project
Data for analysis and display were compiled
from the best availiable sources. No liability
is assumed for the accuracy or completeness
of the information contained herein.
COLUMBIA BASIN STEELHEAD DISTRIBUTION
(EAST OF CASCADES ONLY)
c^naoa
unites stME
NEVADA
UTAH
WYOMING
Figure 6. Columbia Basin steelhead distribution.
Salmonid Distributions and Temperature
10

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MONT/
WASHINGTON
'REGON
IDAHO
LEGEND:
I | HISTORIC RANGE
CURRENT RANGE
1 ;2M STREAMS
Historic Range:
Interior Columbia Basin Ecosystem Management Project
Current Range:
Interior Columbia Basin Ecosystem Management Project
Data for analysis and display were compiled
from the best available sources. No liability
is assumed for the accuracy or completeness
of the information contained herein.
COLUMBIA BASIN YELLOWSTONE CUTTHROAT TROUT
DISTRIBUTION
(EAST OF CASCADES ONLY)
CANADA
Sited svmbs
WYOMING
NEVADA
UTAH
Figure 7. Columbia Basin Yellowstone cutthroat trout distribution.
Salmonid Distributions and Temperature
11

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salmonids in the CRB are habitat loss (including thermal degradation), harvest, and direct and
indirect effects of hatcheries (Lichatowich 1999). The current known and predicted distribution
of steelhead trout in the CRB encompasses 46% of the historical range. For chinook salmon, the
current known and predicted distribution encompasses 28% of the historical range for stream-
type chinook and 29% for ocean-type chinook (Quigley and Arbelbide 1997). For many of these
species and populations, it is likely that both system potential and biological goals are not
attained (scenario 3). In other words, widespread enhancement of system physical potential to
minimize adverse effects of altered temperature conditions is needed in the region.
What are the direct effects of temperature?
Temperature may constrain the distribution of fish through direct effects on physiological
function. If temperatures are too warm, metabolic rates may rise to the point at which energy
intake (e.g., food consumption) is insufficient to maintain basic physiological functions. Growth
ceases, and compounding effects of temperature may result in death. Cold temperatures may
also be important, particularly where growing seasons are short and fish must endure a long
season (e.g., winter) of scarce resources (Shuter and Post 1990). For salmonids in the Pacific
Northwest, the concern is unsuitably warm summer temperatures. Currently, there are no
criteria that directly address excessively cool temperatures.
Examples. Studies of thermal effects on regional salmonid distributions are numerous
(see McCullough 1999). These studies use a wide variety of indicators. At larger scales, it is
common to use climate indicators of thermal regimes, such as air temperature, elevation, or
geographic location (e.g., Meisner 1990, Flebbe 1994, Keleher and Rahel 1996, Dunham et al.
1999). Geographic variation in distribution of fish populations is typically studied with these
large-scale indicators. At finer scales, air temperature can be a poor indicator of fish
distributions. Within streams, variation in local climate is minimal, but variation in water
temperatures is often obvious. For example, geographic variation in the distribution of cutthroat
trout is strongly tied to climate gradients (Dunham et al. 1999). At a smaller scale within
streams, water temperature is the best indicator (in terms of water temperature) of suitable
conditions for fish (Dunham 1999). At even smaller scales (e.g., stream reach or unit) it may be
possible to distinguish habitat use patterns, if local variation in water temperature is large enough
to elicit a biologically significant response (e.g., Torgerson et al. 1999; Ebersole et al., in press).
Most attempts to relate salmonid distributions to temperature are based on air
temperatures, which are widely available. Air temperature and groundwater temperature are
known to be related (either directly or indirectly), which is believed to explain the association
between air temperatures and salmonid distributions on a regional scale (e.g., >104 m, or 6th field
hydrologic unit code; see Rieman et al. 1997). However, air temperature generally has only a
weak direct influence on surface water temperature (Poole and Berman in press).
More recent studies have focused on direct associations between fish distributions and
surface water temperatures. As digital data loggers and remote sensing (e.g., forward-looking
infrared ideography, Torgerson et al. 1999) become more accessible, reliance on indirect
measures of aquatic thermal regimes (e.g., regional climatic or air temperatures) will be less
necessary.
Salmonid Distributions and Temperature
12

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What are indirect effects of temperature on fish distributions?
The direct effects of temperature are obvious, but indirect effects can be important as
well. Multiple stressors (see Multiple Stressors issue paper) can modify the effect of
temperature on probability of survival under different thermal regimes. In colder seasons, fish
may be vulnerable to warm-blooded predators, including birds and mammals (Conduce et al.
1998). In warm seasons, thermal stress may similarly render fish susceptible to predators,
competitors, or disease. Patterns of habitat use may change. Abundance of prey may also
change (e.g., Li et al. 1994). Interactions of these factors with temperature may affect fish
distributions and responses to temperature, as in the following examples.
Biotic interactions. The response of a species to a given thermal environment can be
modified dramatically by biotic interactions (e.g., competition, disease, predation) within or
among species. In some cases, this influence may affect the distribution of a species.
Within a species, temperature may affect important life history attributes, such as size
and age at emigration and return times for spawning adults. Within cohorts, intraspecific
competition for limited resources (e.g., food, shelter, mates) may be affected, possibly leading to
variation in competitive ability and fitness of individuals and changing patterns of growth and
survival. The subtle influences of these factors on the distribution of fish within aquatic habitats
has not been documented in the published literature.
There is better evidence for the influence of temperature on distribution of fishes among
different species. In studies by Reeves et al. (1987), juvenile steelhead production was the same
at water temperatures of 53.6-59°F (12-15°C) whether red shiners were present or not. At
warmer temperatures (66.2-71.6°F [19-22°C]), steelhead production was lower when shiners
were present than when shiners were absent. Additional examples can be found in the Behavior
and Multiple Stressors issue papers.
Habitat size and isolation. Thermal gradients often result in erratic distribution of fish
populations (Dunham et al. 2001). Changes in the size and distribution of habitats result in
habitat fragmentation, which has been documented for several species (e.g., Rieman and
Dunham 2000). Generally, as habitat size decreases and isolation increases, the occurrence of
fish decreases. For bull trout (Salvelinus confluentus) and cutthroat trout (Oncorhynchus clarki)
limited evidence suggests these species are unlikely to be found in watersheds with surface areas
of less than roughly 10s ha (Dunham et al., in press). Available data are not sufficient to propose
minimum area requirements for any species (Rieman and Dunham 2000). Populations in smaller
habitats are assumed to be more vulnerable to chance extinction, or extinction caused by
deterministic factors such as replacement by competitors or land use impacts (see McElhany et
al. 2000).
What is meant by "scale" and "level?" At what "level" should we be concerned with
temperature criteria to protect fish distributions?
Temperature can affect fishes at several scales and levels. Scale refers to the space or
time dimensions of a problem, whereas level refers to the ways in which physical or biological
processes are organized (for details, see Allen 1998). A local population may be considered as a
Salmonid Distributions and Temperature
13

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level of biological organization, for example. Local populations for salmonids may correspond
to the distribution of spawning and rearing areas (Dunham et al. in press). In terms of scale,
local populations can occupy very small or very large watersheds. Thus, "scale" and "level" are
not exactly synonymous.
Research on salmonid habitat has addressed a wide variety of spatiotemporal scales and
levels. In the 1970s and 1980s, research often focused on fishery production. Numerous studies
addressed the relationship between standing crop of salmonids and site-specific habitat
characteristics (e.g., pools, cover, substrate). These models were often limited by their lack of
transferability in space or time and poor predictive ability (Fausch et al. 1988). Existing EPA
temperature criteria (U.S. EPA 1998) are site-specific and do not address larger scale issues of
landscape processes and fish distributions.
Recent models have addressed aquatic habitat at larger spatial scales (Johnson and Gage
1997; see Spatial-Temporal Issue Paper) and focused more on patterns of species diversity,
distribution, and occurrence than on standing crop (Dunham et al. in press, Angermeier et al.
2001). Larger scale approaches to salmonid habitat focus on both habitat characteristics and the
spatial context of a habitat in the landscape (Rieman and Dunham 1999). Part of the motivation
for a larger scale approach is the need for models that address habitat requirements at the
population level. Population-level concerns (e.g., occurrence, persistence, diversity) are
increasingly critical in this region as the list of threatened, endangered, and sensitive salmonids
grows.
Information on thermal relationships of salmonids comes from a variety of laboratory
and field studies—how do we integrate work conducted at different scales or levels of
organization (e.g.,population vs. individuals)?
One important issue related to scaling is the connection between laboratory studies of
thermal tolerance and thermal habitat use in the field. EPA criteria for temperature (Federal
Register 1998, Brungs and Jones 1977) are based on laboratory tests of individual fish responses
to temperature. These experiments provide a mechanistic basis for understanding the effects of
temperature on individual fish. In the laboratory, a rigorous experimental design can isolate the
effects of specific factors and test for interactions among factors.
It is difficult to extrapolate results obtained under laboratory conditions to the field,
where many uncontrolled factors interact simultaneously. Nonetheless, it is in the field that
temperature has a potentially important role. Although it is sometimes possible to conduct large-
scale field experiments, field studies more often involve analyses of correlation or association
between factors, such as fish distribution and temperature. Obviously, such correlations do not
necessarily constitute cause and effect. Development of temperature criteria must therefore
involve a combination of approaches, including laboratory experiments and field studies.
Integrating pattern and process across multiple scales or levels of biological organization is
essential for correct ecological inference (Werner 1998). Following are several examples.
Salmonid Distributions and Temperature
14

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EPA Fish and Temperature Database Matching System (FTDMS). A simple
example of integrating field and laboratory studies comes from results of the EPA's FTDMS,
Eaton et al. 1995). Eaton et al. (1995) found a close correspondence between laboratory-derived
thermal tolerance limits and maximum water temperatures in the field. For salmonids,
maximum water temperatures in the field were 33.8-39.2°F (1-4°C) cooler than limits indicated
by laboratory studies. The fact that fish distributions in the field corresponded to cooler
temperatures suggests that sublethal effects may be important. This may occur when
temperature directly or indirectly acts as one of multiple stressors (see Temperature Interaction
issue paper).
"I saw fish in hot water." Sometimes simple observations of fish in unusually warm
water are used to support (or reject) proposed temperature criteria or thresholds. Such
observations do not indicate anything about individual fitness or population health.
Furthermore, they ignore the essential chain of inference that should be made using both
laboratory and field observations (Werner 1998). Observations of fish in "unusual" (or any)
conditions should be interpreted in a probabilistic context. For example, given the observed
thermal regime, what is the probability that a given species will occur? Determining this
requires information (and evidence) on a fuller range of thermal conditions and is much more
informative. Salmonid fish may occasionally occur in "hot" water, but in general they are much
more likely to occur when temperatures are cooler. This is illustrated by the wide range of
temperatures where salmonids and other species are observed to occur (Figure 8).
A useful perspective can be found in humans' use of high-temperature environments. In
many cultures, it is common to engage in recreational or ritual use of steam baths, saunas, sweat
lodges, hot springs, or other extremely warm microclimates. Although limited use of these
environments is common, they are by no means suitable in the long term; the negative health
effects are obvious to humans. Fish, like humans, will occasionally be found in thermal habitats
that are unsuitable for long-term (and sometimes even short-term) health. In some cases, short
forays into physiologically stressful habitats may provide a net benefit. Many prey organisms
make use of predator-free space, which often exists at the extremes of physiological tolerance for
predators (e.g., Rahel et al. 1994).
Is unoccupied habitat relevant to temperature requirements of salmonids?
Thermal habitat can be utilized at a variety of spatial and temporal scales (see also
Spatial/Temporal issue paper). Spatial and temporal variation in the availability of thermal
habitat may be an important constraint. Temperature criteria should address all temperatures
likely to be used by fish, not just upper, lower, or "optimal" temperatures. It is the full range of
thermal variability that provides a context for continued evolution of species (Lichatowich
1999). Distribution of "habitat" can extend well beyond that which is currently occupied by a
species or population.
Because of natural variation in space and time, most fish occupy landscapes with a
considerable amount of suitable but unoccupied habitat. Unoccupied habitat is a natural
consequence of extinction and recolonization, natural habitat succession, and human influences
on fish populations and habitats (Reeves et al. 1995, Rieman and Dunham 2000). Therefore, the
distribution of habitat needed by fish may extend well beyond that which is currently occupied.
Salmonid Distributions and Temperature
15

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Coldwater adapted
(stenothermal) species
Coolwater adapted
(mesothermal) species
A
T
*

*
*

*
b
cd
&
><
bed
cd
bed
de
*
*
*
Bull SSC CT SLSC
89 164 346 35
BK RBT BR
245 332 31
MW MSC PSC LND SD LSS BSU
39 67 89 73 130 14 64
RS
76
Figure 8. Distribution of summertime point temperatures at which selected fish species occurred in Idaho. Boxes
indicate the median and upper and lower quartiles (the central 50% of the values), the whiskers extend up to 1.5X the
interquartile value, and asterisks show outlying values. Plots marked with the same letter indicate that their means are
not significantly different at P<0.05 using Tukey's multiple comparison procedure. Data from the Idaho Department of
Environmental Quality beneficial use reconnaissance program.
Code
Common Name
Scientific Name
Code
Common Name
Scientific Name
Bull
Bull trout
Salvelinus confluentus
MW
Mountain whitefish
Prosopium williamsoni
SSc
Shorthead sculpin
Cottus confusus
MSC
Mottled sculpin
Cottus bairdi
Cutt
Cutthroat trout
Oncorhynchus clarki
PSC
Paiute sculpin
Cottus beldingi
SLSC
Slimy sculpin
Cottus cognatus
LND
Longnose dace
Rhinichthys cataractae
BK
Brook trout
Salvelinus fontinalis
SD
Speckled dace
Rhinichthys osculus
RBT
Rainbow trout
Oncorhynchus mykiss
LSS
Largescale sucker
Catostomus macrocheilus
BR
Brown trout
Salmo trutta
BSU
Bridgelip sucker
Catostomus columbianus



RS
Redside shiner
Richardsonius balteatus
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This is especially true for most threatened and endangered species because current distributions
are reduced or declining.
Unoccupied habitat is a potentially controversial issue, particularly because it can be
difficult to identify areas needing protection. When fish are present, the choice of habitat to
protect or restore can be relatively obvious. When fish are not present, the choice must be
guided by information on historical and potential distributions of fish and suitable habitat, and
potential sources of natural recolonization.
Conclusion
Salmonids in the Pacific Northwest evolved in habitats with large amounts of cold, clean
water. Their life histories and ecology are strongly tied to natural thermal regimes. Region-wide
declines in salmonids have paralleled the loss and fragmentation of formerly large and
interconnected cold water habitats, and changes in thermal regimes (see Spatial/Temporal issue
paper). Temperature is widely appreciated as an important factor affecting not only the health of
individual fish, but also entire populations and species assemblages. Direct effects of
temperature may be obvious, or temperature may interact with other important variables to
indirectly affect salmonids.
Temperature criteria that explicitly consider the thermal requirements of salmonids at
multiple spatial and temporal scales (see also Spatial/Temporal issue paper), and the connection
between salmonids and natural thermal regimes, will be most protective of existing populations
and offer a means for restoring depressed populations. In many cases, protection and restoration
of thermal habitat must extend beyond the current boundaries of existing occupied and/or
suitable habitat, because current fish distributions are significantly reduced from their historical
extent.
Attempts to set temperature criteria must balance what is known and not known about the
physical system potential and biological requirements of salmonids. Consideration of unknown
factors is essential in determining precautionary measures to avoid adverse effects related to
temperature criteria. General guidelines for assessing salmonid populations (e.g., McElhany et
al. 2000) provide a means for determining the "knowns" and "unknowns." Full consideration of
the weight of evidence about current and potential fish distribution and physical system
potential, including thorough documentation of assumptions and knowledge gaps, is needed in
establishing and implementing temperature criteria to support healthy (viable, productive, and
fishable) salmonid populations.
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