Biological Services Program

L;~ !"ary

Nc'.Lional Wetlands Resea rch Center

U. S. Fi3h and Wildlife Service

Cajundome Boulevard

Lafayette, La. 70506

FWS/OBS·82/10.5

FEBRUARY 1982

HABITAT SUITABILITY INDEX MODELS:

CUTTHROAT TROUT

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_. >hand Wildlife Service

Department of the Interior

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The Biological Services Program was established within the U.S. Fis h

and Wild life Service to supply scientific information and methodologies on

key environmental issues that impact fish and wi ldli fe reso urces and their

supporting ecosystems. The mission of the program is as follows:

• To strengthen the Fish and Wi ldl i fe Service i n its role as

a primary source of information on national fish and wildlife resources, particularly in respect to environmental

impact assessment.

• To gather, analyze, and present info rmation that wi ll aid

decisionmakers in the identification and re sol'ution of

problems associated with major changes in land and water

use.

• To provide bet ter ecological informat ion and eval uat ion

for Department of the Interior development programs , such

as those relating to energy developme nt.

Informat i on developed by the Biol ogi cal Ser vices Program is i nt ended

for use in the planning and decisionma ki ng process to prevent or mi nimize

the impact of development on fi sh and wi ldli fe. Rese arch activ ities and

tec hnical assistance services are based on an analysis of t he issues , a

determina tion of t he decisionmake rs involved and the i r i nformati on needs,

and an evaluation of the state of t he art to iden ti fy i nformation Haps

and to determine priorit ies. This is a st ra t egy t hat wi l l ensure t hat

the products produced and disseminated are t ime ly and useful.

Proj ect s -have been initiated i n t he following areas: coal extract ion

and conversion ; power plants; geot hermal , mi neral and oil shal e developmen t; wat er reso urce analysis , i ncl udi ng stream alterations and west ern

water allocation ; coastal ecosystems and Outer Con tinent al Shelf devel opment ; and systems invento ry , inc l uding Na ti onal Wetl and Inventory,

habitat classi f i cation and analysis, and informat i on transfer .

The Biological Servi ces Program consists of the Office of Bi ol ogi cal

Ser vi ces in Washingt on. D.C ., wh i ch is res po nsi ble for overal l planning and

ma nagement; Nat.io.nal Teams, whi ch provide the Program's cent ral scien tifi c I

and tec hnical expertise and arrange for contract i ng bi ol o9ica l services

studies with states, universities, consulting firms , and othe rs; Regiona l

Staffs, who provide a link to problems at the operating leve l ; and staffs at

certain Fish and Wil dli fe Ser vice resear ch fa cili t i es , wh o conduct i n-house

research studi es.
FWS/OBS-82/10.5

February 1982

HABITAT SUITABILITY INDEX MODELS: CUTTHROAT TROUT

by

Terry Hickman a

and

Robert F. Raleigh

Habitat Evaluation Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

Drake Creekside Building One

2625 Redwing Road

Fort Collins, Colorado 80526

Western Energy and Land Use Team

Office of Biological Services

Fish and Wildlife Service

U.S. Department of the Interior

Washington, D.C. 20240

aCurrent address: U.S. Fish and Wildlife Service, Endangered Species Office,

1311 Federal BUilding, 125 S. State Street, Salt Lake City, Utah 84138.
This report should be cited as:

Hickman, T., and R. F. Raleigh. 1982. Habitat suitability index models:

Cutthroat trout. U.S.D.!. Fish and Wildlife Service. FWS/OBS-82/10.5.

38 pp.

i i
PREFACE

The habitat use information and Habitat Suitability Index (HSI) models

presented in this document are an aid for impact assessment and habitat management activities. Literature concerning a species ' habitat requirements and

preferences is reviewed and then synthesized into HSI models, which are scaled

to produce an index hetween 0 (unsuitable habitat) and 1 (optimal habitat).

Assumptions used to transform habitat use information into these mathematical

models are noted, and guidelines for model application are described. Any

models found in the literature which may also be used to calculate an HSI are

cited, and simplified HSI models, based on what the authors believe to be the

most important habitat characteristics for this species, are presented.

Use of the models presented in this publication for impact assessment

requires the setting of clear study objectives and may require modification of

the models to meet those objectives. Methods for reducing model complexity

and recommended measurement techniques for model variables are presented in

Appendix A.

The HSI models presented herein are complex hypotheses of species-habitat

relationships, not statements of proven cause and effect relationships.

Results of mode~erformance tests, when available, are referenced; however,

models that have demonstrated reliability in specific situations may prove

unreliable in others. For this reason, the FWS encourages model users to

convey comments and suggestions that may help us increase the utility and

effectiveness of this habitat-based approach to fish and wildlife planning.

Please send comments to:

Habitat Evaluation Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

2625 Redwing Road

Ft. Co 11 ins, CO 80526

iii

CONTENTS

PREFACE iii

ACKNOWLEDGEMENTS vi

HABITAT USE INFORMATION 1

Genera 1 1

Age, Growth, and Food 1

Reproduct ion 2

Anadromy 2

Specifi c Habitat Requi rements 3

HABITAT SUITABILITY INDEX (HSI) MODELS................................ 7

Model Applicability............................ 7

Model Description - Riverine 9

Suitability Index (SI) Graphs for

Model Variables............................................... 10

Riverine Model.................................................. 22

Lacustrine Model 26

Interpreting Model Outputs...................................... 27

ADDITIONAL HABITAT MODELS 31

Modell......................................................... 31

Model 2 31

Model 3 31

Model 4 32

REFERENCES CITED 32

v
ACKNOWLEDGMENTS

We are indebted to Ron Anderson and Dave Meyer for their aid with early

literature reviews and development work of sUitability index graphs and narrative sections of the cutthroat trout summary. The manuscript was reviewed by

Robert Behnke, Norman Benson, Jim Mullen, and James Johnston. Their review

contributions are gratefully acknowledged, but the authors accept full

responsibility for the contents of the summary. Word processing was provided

by Dora Ibarra and Carolyn Gulzow. The cover of this document was illustrated

by Jennifer Shoemaker.

vi
CUTTHROAT TROUT (Salmo clarki)

HABITAT USE INFORMATION

General

Cutthroat trout, Salmo clarki, are a polytypic species consisting of

several geographically distinct forms with a broad distribution and a great

amount of genetic diversity (Hickman 1978; Behnke 1979). Behnke (1979)

recognized 13 extant subspecies: Coastal cutthroat (S. c. clarki) in coastal

streams from Prince William Sound, Alaska to the Eel Rlver in California;

mountain cutthroat (~. ~. alpestris) in upper Columbia and F~Dser River

drainages of British Columbia; west slope cutthroat (S. c. lewisi) in the

upper Columbia, Salmon, Clearwater, South Saskatchewan and upper Missouri

drainages of Montana and Idaho; an undescribed subspecies in the Alvord basin,

Oregon; Lahonton cutthroat (S. c ..henshawi), Pauite cutthroat (S. c.

seleniris), and an undescribed- subspecies in the Humboldt River drafnage of

the Lahontan basin of Nevada and California; Yellowstone cutthroat (S. c.

bouvieri) in the Yellowstone drainage of Wyoming and Montana and the Snake

River drainage of Wyoming, Idaho, and Nevada; an undescribed subspecies (fine

spotted) in the upper Snake River, Wyoming; Bonneville cutthroat (S. c. utah)

in the Bonneville basin in Utah, Nevada, Idaho, and Wyoming; Colorado River

cutthroat (~. ~. pleuriticus) in the Colorado River drainage in Wyoming, Utah,

New Mexico, and Colorado; greenback cutthroat (S. c. stomias) in the South

Platte and Arkansas River systems; and Rio Grande cutthroat (~. ~. virginalis)

in the Rio Grande River drainage of Colorado and New Mexico. Many of these 13

subspecies are included on Federal or State endangered or threatened species

lists.

Temperature and chemical preferences, migration, and other ecological and

life history attributes vary among cutthroat subspecies (Behnke 1979). Differences in growth rate (Carlander 1969; Scott and Crossman 1973; Behnke 1979)

and food preferences have also been reported (Trojnar and Behnke 1974) between

some subspecies.

Age, Growth, and Food

Most male cutthroat trout mature at ages II to III, whereas females

usually mature a year later (Irving 1954; Drummond and McKinney 1965; Johnston

and Mercer 1977). In Washington streams that contain anadromous populations

of cutthroat, which predomi nant ly smo 1t at age I I, 1ess than 15% of the

cutthroat returning to the river for the first time are sexually mature females

(Mercer and Johnston 1978). The maximum life expectancy for coastal cutthroat

is about 10 years (Johnston and Mercer 1976), whereas the maximum reported age

for interior cutthroat is 7 years (Behnke 1979). Size at maturity will vary

depending on environmental conditions. Cutthroat mature at a smaller size in

small headwater streams (Behnke and Zarn 1976).

Trout are opportunistic feeders (Behnke and Zarn 1976), but their diet

consists mainly of aquatic insects (Allen 1969; Carlander 1969; Baxter and

Simon 1970; Scott and Crossman 1973; Griffith 1974). Other foods, such as

zooplankton (McAfee 1966; Carlander 1969; Trojnar and Behnke 1974), terrestrial
insects (Carlander 1969; Trojnar and Behnke 1974; Hickman 1977), and fish

(Carlander 1969) are locally or seasonally important. Cutthroat trout usually

become more piscivorous as they increase in size (McAfee 1966; Carlander 1969;

Baxter and Simon 1970).

Reproduction

Cutthroat trout are stream spawners. The fertilized ova are deposited in

redds constructed primarily by the female in the stream gravels (Smith 1941,

1947). Spawning begins in spring, as early as February (Behnke 1979), but can

occur as late as August in colder areas (Juday 1907; Fleener 1951). The time

of spawning depends on water temperature, runoff (Lea 1968), ice melt (Calhoun

1944), elevation and latitude (Behnke and Zarn 1976).

Anadromy

Coastal cutthroat are the most abundant of the thirteen recognized

cutthroat subspecies and consist of both resident and anadromous populations.

Both populations are usually found in the same watershed. The resident populations are frequently, but not always, segregated from the anadromous stock by

some barrier to anadromy. Although resident and anadromous populations have

been reported to occur in sympatry in streams and lakes, more information is

needed to determine if gene flow between populations is absent (Johnston

1979).

Anadromous coastal cutthroat spend less time in saltwater than steelhead

trout or salmon. Although some cutthroat overwinter in saltwater, most return

to freshwater after 3 to 8 months (Johnston 1979). During this period in

saltwater, the cutthroat stay close to shore and are rarely found at depths

greater than 3 m. Preferred habitats in saltwater are gravel beaches vegetated

above the high tide mark and gravel spits created by tidal currents. Cutthroat

are rarely found in saltwater in areas with silt, mud, or solid rock substrate.

Anadromous cutthroat utilize cover during upstream migration from saltwater.

Coastal cutthroat initially smolt at age II, III, or IV. Some smolt at

age I, whereas others may not migrate to salt water until age VI (Jones 1974,

1975, 1976). In Washington, the smallest cutthroat entering salt water weigh

from 25 to 45 gms and are 120 to 170 mm long. Physiological adaptation to

salt water appears to be related to size rather than age (Johnston and Mercer

1976) .

In Washington and Oregon, smolt movement to salt water begins in March,

peaks in mid-May, and is completed by mid-June (Johnston and Mercer 1976). In

Alaska, migration begins in April (Armstrong 1971; Jones 1974, 1975, 1976),

peaks at the end of May, but may continue into August. Most seaward migrations

occur at night. Re-entry into fresh water in Washington and Oregon begins in

July, peaks in September and October, and 1asts until the end of October

(Giger 1972; Johnston and Mercer 1976). In smaller coastal streams, re-entry

begins in October, peaks in December and January, and continues into March.

Migrations into small stream-lake systems in Alaska begin as early as mid-May,

peak in September, and last until October (Armstrong 1971; Jones 1974, 1975,

1976).

2
Specific Habitat Requirements

Optimal cutthroat trout riverine habitat is characterized by clear, cold

water; a silt free rocky substrate in riffle-run areas; an approximately 1:1

pool-riffle ratio with areas of slow, deep water; well vegetated stream banks;

abundant instream cover; and relatively stable water flow, temperature regimes,

and stream banks (Ra1ei gh and Duff 1981). Cutthroat trout tend to occupy

headwater stream areas, especially when other trout species are present in the

same river system (Glova and Mason 1976).

Optimal lacustrine habitat is characterized by clear, cold, deep lakes

that are typically oligotrophic, but

particularly in reservoir habitats.

require tributary streams with gravel

tion to occur.

may vary in size and chemical quality,

Cutthroat trout are stream spawners and

substrate in riffle areas for reproducTrout literature does not clearly distinguish between feeding stations,

escape cover, and winter cover requirements. Prime requisites for optimal

feeding stations appear to be low water velocity and access to a plentiful

food supply, e.g., energy accretion at a low energy cost. Water depth is not

clearly defined as a selection factor, and overhead cover is preferred but not

essential. Escape cover, however, must be nearby (Raleigh and Duff 1981).

The feeding stations of dominant adult trout will include overhead cover when

available. The feeding stations of subdominant adults and juveniles, however,

may not always include overhead cover.

Cover is recognized as one of the essential components of trout streams.

Boussu (1954) was able to increase the number and weight of trout in stream

sections by adding artificial brush cover and to decrease numbers and weight

by removing brush cover and undercut banks. Lewis (1969) reported that the

amount of cover was important in determining the number of trout in sections

of a Montana stream. Cover for adult trout consists of areas of obscure

stream bottom in areas of water 15 cm deep with a low velocity of

s 15 cm/sec (Wesche 1980). Wesche (1980) reported that, in larger streams,

the abundance of trout 15 cm in length increased with depth; most were at

depths of at least 15 to 45 cm. Cover is provided by overhanging vegetation;

submerged vegetation; undercut banks; instream objects, such as debris piles,

logs, large rocks; and pool depth or surface turbulence (Giger 1973). A cover

area of 25% of the total stream area will provide adequate cover for adult

trout; a cover area of 15% is adequate for juveniles. The main use of

summer cover is probably for predator avoidance and resting. In winter,

salmonids occupy different habitat areas than in the summer (Hartman 1965;

Everest 1969; Bustard and Narver 1975a).

In some streams, the major factor limiting salmonid densities may be the

amount of adequate overwintering habitat rather than summer rearing habitat

(Bustard and Narver 1975a). Winter hiding behavior in salmonids is triggered

by low temperatures (Chapman and Bjornn 1969; Everest 1969; Bustard and Narver

1975a,b). Cutthroat trout were found under boulders, log jams, upturned

roots, and debris when temperatures neared 4 to 8° C, depending on velocity

(Bustard and Narver 1975a). Everest (1969) found juvenile rainbows 15 to

30 cm deep in the substrate, which was often covered by 5 to 10 cm of anchor

3
ice. Lewis (1969) reported that, during winter, adult rainbow trout tended to

move into deeper wa t er (class 1 pools). Bjornn (1971) indicated that downstream movement during or preceding winter did not occur if sufficient winter

cover was locally available. Trout move to winter cover to avoid physical

damage from ice scouring (Hartman 1965; Chapman and Bjornn 1969) and to

conserve energy (Chapman and Bjornn 1969; Everest 1969).

Headwater trout streams are relatively unproductive. Most energy inputs

to the stream are in the form of allochthonous materials, such as terrestrial

vegetation and terrestrial insects (Idyll 1942; Chapman 1966; Hunt 1975).

Aquatic invertebrates are most abundant and diverse in riffle areas with

rubble substrate and on submerged aquatic vegetation (Hynes 1970). However,

optimal substrate for maintenance of a diverse invertebrate population consists

of a mosaic of gravel, rubble, and boulders with rubble being dominant. The

invertebrate fauna is much more abundant and diverse in riffles than in pools

(Hynes 1970), but a ratio of about 1:1 of pool to riffle area (about 40 to 60%

pool area) appears to provide an optimal mix of trout food producing and

rearing areas. In riffle areas, the presence of fines (> 10%) reduces the

production of invertebrate fauna (based on Cordone and Kelly 1961; Crouse et

a1. 1981).

Canopy cover is important in maintaining shade for stream temperature

control and in providing allochthonous materials to the stream. Too much

shade, however, can restrict primary productivity in a stream. Stream temperatures can be increased or decreased by controlling the amount of shade.

About 50 to 75% mid-day shade appears optimal for most small trout streams

(Anonymous 1979). Shading becomes less important as stream gradient and size

increases. In addition, a well vegetated riparian area helps to control

watershed erosion. In most cases, a buffer strip about 30 mdeep, 80% of which

is either well vegetated or has stable rocky stream banks, will provide adequate erosion control and maintain undercut stream banks characteristic of

good trout habitat. The presence of fines in riffle-run areas can adversely

affect embryo survival, food production and cover for juveniles.

There is a definite relationship between the annual flow regime and the

quality of trout habitat. The most critical period is typically the base flow

(lowest flows of late summer to winter). A base flow 50% of the average

annual daily flow is considered excellent, a base flow of 25 to 50% is considered fair, and a base flow of < 25% is considered poor for maintaining quality

trout habitat (adapted from Binns and Eiserman 1979; Wesche 1980).

Of 66 streams sampled in British Columbia, those containing cutthroat

trout had a pH of 6.0 to 8.8 (Hartman .and Gill 1968). Thirteen streams in

Wyoming containing populations of Colorado River cutthroat trout had pH levels

of 7.1 to 8.3 (Binns 1977). Sekulich (1974) reported that the pH in three

reservoirs containing cutthroat trout ranged from 7.8 to 8.5. Platts (1974)

analyzed three streams in Idaho with cutthroat trout where the pH ranged from

7.3 to 7.9 and total dissolved solids ranged from 41 to 63 mg/l. Some isolated

populations of cutthroat trout in the Great Basin area have developed a unique

tolerance for high pH, alkalinity, total dissolved solids, and temperature

conditions. The Lahontan basin cutthroat trout persist in Pyramid and Walker

Lakes, Nevada, where total dissolved solids exceed 5,000 mg/l and 10,000 mg/l,

respectively (Behnke and Zarn 1976). The largest cutthroat trout ever recorded

came from Pyramid Lake, which has a pH range between 9.0 and 9.5. The Lahontan

4
basin cutthroat also lives in alkaline waters, such as Walker Lake, Nevada

(alkalinity of 2,900 mg/l). These conditions are probably lethal to other

cutthroat trouts (Behnke and Zarn 1976). Precise pH tolerance and optimal

ranges for cutthroat trout are not well documented. Most cutthroat populations

can probably tolerate a pH range of 5 to 9.5, with an optimal range of 6.5 to

8. The Lahontan basin cutthroat appear to have developed a tolerance to

higher pH conditions, with regional pH tolerance and optimal ranges of 5 to 10

and 6.5 to 8.5, respectively.

Bachmann (1958) reported that, at turbidities above 35 ppm, cutthroat

trout stopped feeding and moved to cover. Turbidities of less than 25 JTU and

total dissolved solids from 38 to 544 rng/l characterized 13 Wyoming streams

containing cutthroat trout (Binns 1977).

Adult. Dissolved oxygen requirements vary with species, age, prior

acclimation temperature, water velocity, activity level, and concentration of

substances in the water (McKee and Wolf 1963). As temperature increases, the

dissolved oxygen saturation level in the water decreases while the dissolved

oxygen requirements for the fish increases. As a result, an increase in

temperature resulting in a decrease in dissolved oxygen can be detrimental to

the fish. Optimal oxygen levels for cutthroat trout are not well documented,

but appear to be > 7 mg/l at temperatures 15° C and 9 mg/l at temperatures

> 15° C. Doudoroff and Shumway (1970) demonstrated that swimming speed and

growth rates for salmonids declined with decreasing dissolved oxygen levels.

At temperatures 15° C, cutthroat trout generally avoid water with dissolved

oxygen levels of less than 5 mg/l (Trojnar 1972; Sekulich 1974).

Cutthroat trout usually do not persist in waters where maximum temperatures consistently exceed 22° C, although they may be able to withstand brief

periods of water temperature as high as 26° C if considerable nightime cooling

takes place (Behnke and Zarn 1976). The Humboldt River cutthroat trout in the

Lahontan basin, however, occupy waters where temperatures may reach a summer

maximum of 25° C (Behnke 1979). Needham and Jones (1959) reported cutthroat

trout actively feeding at 0° C. Bell (1973) reported a preferred temperature

range of 9 to 12° C for cutthroat trout. Dwyer and Kramer (1975) reported the

greatest scope for activity in cutthroat trout occurred at 15° C when tested

at 5° C increments. We assume that scope for activity is a better measure of

optimal temperature than temperature preference tests and have selected

12-15° C as an optimal temperature range for cutthroat trout.

Focal point velocities for adult cutthroat trout on territorial stations

in Idaho streams were primarily between 10 and 14 em/sec, with a maximum of

22 em/sec (Griffith 1972).

Embryo. Incubation time varies inversely with temperature. Eggs usually

hatch within 28 to 40 days (Cope 1957), but may take as long as 49 days (Scott

and Cr-o ssmau 1973). Bell (1973) reported that cutthroat trout spawning temperatures ranged from 6 to 17° C. The optimal temperature for embryo incubation

is approximately 10° C (Snyder and Tanner 1960). Calhoun (1966) reported

increased mortalities of rainbow embryos at temperatures < 7° C and normal

development at temperatures s 12° C. Hooper (1973) and Thompson (1972)

reported spawning velocities ranging from 31 to 92 em/sec, while Hunter (1973)

reported the range as 11 to 40 em/sec. Average water column velocities for

embryo development apparently range from 11 to 92 em/sec. We assume that

5
optimal velocities range from 30 to 60 em/sec. The combined effects of

temperature, dissolved oxygen levels, water velocity, and gravel permeability

are important for successful incubation (Coble 1961). In a 30% sand and 70%

gravel mixture, only 28% of implanted steel head embryos hatched; of the 28%

that hatched, only 74% emerged (Bjornn 1969; Phillips et al. 1975). We assume

that these same results would be true for the closely related cutthroat trout.

We further assume that optimal spawning gravel conditions include 5% fines,

and that 30% fines will cause low survival of embryos and emerging yolk-sac

fry. Suitable incubation substrate is gravel 0.3 to 8 em in diameter (Duff

1980). Optimal substrate size will depend on size of spawners, but we assume

it will average 1.5 to 6.0 em in diameter. Doudoroff and Shumway (1970)

reported that salmonids incubated at low dissolved oxygen levels were weak and

small with slower development and more abnormalities. Dissolved oxygen

requirements for cutthroat trout embryos are probably similar to the requirements for adults.

Fry. Cutthroat trout remain in the gravel for about two weeks after

hatching (Scott and Crossman 1973) and emerge 45 to 75 days after egg fertilization, depending on water temperature (Calhoun 1944; Lea 1968). When moving

from natal gravels to rearing areas, cutthroat trout fry exhibit three

distinctly different genetically controlled patterns: 1) downstream to a

larger river or lake; 2) upstream from an outlet river to a lake; or 3) local

dispersion within a common spawning and rearing area to areas of low velocity

and cover (Raleigh and Chapman 1971). Fry of lake resident fish may either

move into the 1ake from nata 1 streams duri ng the fi rst growi ng season or

overwinter in the spawning stream and move into the lake during subsequent

growing seasons (Raleigh 1971; Raleigh and Chapman 1971). Salmo clarki lewisi

average two growing seasons, but may spend 1 to 4 years in ~stream before

migrating to the lake (Roscoe 1974).

Fry residing in streams prefer shallower water and slower velocities than

other life stages (Miller 1957; Horner and Bjornn 1976). Fry utilize velocities of less than 30 em/sec, but less than 8.0 em/sec are preferred (Griffith

1972; Horner and Bjornn 1976). Fry survival decreases with increased velocity

after optimal velocity has been reached (Bulkley and Benson 1962; Drummond and

McKinney 1965). A pool area of 40% to 60% of the total stream area is assumed

to provide optimal fry habitat. Cover in the form of aquatic vegetation,

debris piles, and the interstitial spaces between rocks is critical. Griffith

(1972) states that younger trout live in shallower water and stay closer to

escape cover than do older trout. Few fry are found more than 1 m from cover.

As the young cutthroat grow, they move to deeper, faster water. Everest

(1969) suggested that one reason for this movement was the need for cover,

which is provided by increased water depth, surface turbulence, and larger

substrate.

Trout fry usually overwinter in shallow areas of low velocity near the

stream margin, with rubble being the principle cover (Bustard and Narver

1975a). Optimal size of substrate used as winter cover by steelhead fry and

small juveniles ranges from 10 to 40 em in diameter (Hartman 1965; Everest

1969). An area of substrate of this size class (10-40 em) of 10% of the

total habitat will probably provide adequate cover for cutthroat fry and small

juveniles. The use of smaller diameter rocks (gravel) for winter cover may

result in increased mortality due to greater shifting of the substrate (Bustard

6
and Narver 1975a). The presence of fi nes (~ 10%) in the ri ffl a-r-un areas

impairs the value of the area as cover for fry and small juveniles.

Juvenile. Juvenile cutthroat trout in streams are most often found in

water depths of 45 to 75 em and velocities of 25 to 50 em/sec (Nickelson

unpublished data). Griffith (1972) reported focal point velocities for

juvenile cutthroat in Idaho of between 10 and 12 em/sec, with a maximum

velocity of 22 em/sec. Metabolic rates are highest between 11 and 21° C with

an apparent optimal temperature of 15° C (Dwyer and Kramer 1975) .

Bustard and Narver (1975b) demonstrated that juvenile cutthroat trout

used rubble and overhanging banks as cover. The juveniles also showed a

preference for clean, as opposed to silted, rubble for cover. Common types of

cover for juvenile trout are upturned roots, logs, debris piles, overhanging

banks, and small boulders (Bustard and Narver 1975a). Young salmonids occupy

different habitats in winter than in summer, with log jams and rubble important

as winter cover. Wesche (1980) observed that larger cutthroat trout (> 15 em

long) tended to use streamside cover (overhanging banks and vegetation) more

often than instream substrate objects, while juveniles (::=; 15 em) preferred

instream substrate cover.

HABITAT SUITABILITY INDEX (HSI) MODELS

Figure 1 depicts the theoretical relationships among model variables,

components, and HSI for the cutthroat trout model.

Model Applicability

GeographiC area. The following model is applicable over the entire range

of cutthroat trout distribution. Where differences in habitat requirements

have been identified for different races of cutthroat trout, suitability index

graphs have been constructed to reflect these differences. For this reason,

care must be excercised in use of the individual graphs and equations.

Season. The model rates the freshwater habitat of cutthroat trout for

all seasons of the year.

Cover types. The model is applicable to freshwater riverine or lacustrine

habitats.

Minimum habitat area. Minimum habitat area is the mlnlmum area of continuous habitat that is required for a species to live and reproduce. Since

cutthroat can move considerable distances to spawn or locate suitable summer

or winter rearing habitat, no attempt has been made to define a minimum habitat

size for the species.

Verification level. An acceptable level of performance for this cutthroat

trout model is for it to produce an index between 0 and 1 that the authors and

other biologists familiar with cutthroat trout ecology believe is positively

correlated with the carrying capacity of the habitat. Model verification

consisted of testing the model outputs from sample data sets developed by the

authors to simulate high, medium, and low quality cutthroat trout habitat.

7
Habitat variables

Average thalweg depth (V~)

Model components

~~ adult cover (V 6A) -----~-_=::::::::=~ Adult

~~ pools (VIa)

Pool class (VIS)

~6 juveni 1e cover

~~ p00 1s (VI a) ----------------=::~J uvenil e

Pool class (VIS)

~~ substrate size (V8)~FrY

% pools (VIa) --~ ------------~HSI

% riffle fines (V16B)

Ave. max. temp. (V z )

Ave. min. O. (V3 )

Water velocity (Vs ) ----

Ave. gravel SiZ~

~~ fines (VI6A)

Max. temperature (VI)

Base flow (VI4)

Dominate substrate (V9 )

% vegetation (VII)

% vegetation (erosion) (VIZ)

% riffle fines (VI6B)---~

._-===::::::~ Em bry 0

*Variables that affect all life stages.

Figure 1. Diagram illustrating the relationships among model

variab1es, components, and HSI.

8
Model Description - Riverine

The riverine HSI model consists of five components, adult (CA), Juvenile

(CJ), Fry (CF), Embryo (CE), and Other (CO), Each 1ife stage component contains variables specifically related to that component. The component Co

contains variables related to water quality and food supply that affect all

life stages of cutthroat trout.

The model utilizes a modified limiting factor procedure. This procedure

assumes that model variables and components with suitability indices in the

average to good range, > 0.4 to < 1.0, can be compensated for by higher suitability indices of other related model variables and components. However,

variables and components with suitabilities 0.4 cannot be compensated for,

and thus become limiting factors on habitat suitability.

Adult component. Variable V6, percentage of instream cover, is included

because standing crops of adult trout have been shown to be correlated with

the amount of cover available. Percentage of pools (VI O) is included because

pool s provide cover and resting areas for adult trout. Variable VlD al so

quantifies the amount of pool habitat that is needed. Variable VIS' pool

class, is included because pools differ in the amount and quality of escape

cover, winter cover, and resting areas that they provi de. Average thalweg

depth (V4 ) is included because average water depth affects the amount and

quality of pools and instream cover available to adult trout and migratory

access to spawning and rearing areas.

Juvenile component. Variables V6, percentage of instream cover; VIO,

percentage of pools; and VIS, pool class are included in the juvenile component

for the same reasons listed above for the adult component. Juvenile cutthroat

trout use these essential stream features for escape cover, winter cover, and

resting areas.

Fry component. Variable Va, substrate size class, is included because

trout fry utilize substrate as escape cover and winter cover. Variable VIO,

percent pools, is included because fry use the shallow, slow water areas of

pools and backwaters as resting and feeding stations. Variable VI6, % fines,

is included because the percent fines affects the ability of the fry to utilize

the rubble substrate for cover.

Embryo component. It is assumed that habitat suitability for trout

embryos depends primarily on water temperature, V2 ; dissolved oxygen content,

V3 ; water velocity, Vs; spawning gravel size, V7 ; and percent fines, VI 6.

Water velocity, Vs ; gravel size, V7 ; and percent fines, VI 6, are interrelated

factors that have been shown to effect the transport of dissolved oxygen to

the embryo and the removal of the waste products of metabolism from the embryo.

9
These functions have been shown to be vital to the survival of trout embryos.

In addition, the presence of too many fines in the redds will block movement

of the fry from the incubating gravels to the stream.

Other component. This component contains model variables for two subcomponents, water quality and food supply, that affect all life stages. The

subcomponent water quality contains four variables, maximum temperature (Vi)'

minimum dissolved oxygen (VJ ) , pH (V1 J ) , and base flow (V1 4 ) . All four variables have been demonstrated to affect the growth and survival of all 1ife

stages except embryo, whose water quality requirements are included with the

embryo component. The subcomponent food supply contains three variables,

substrate size (V9 ) , percent vegetation (Vii), and percent fines (V1 6 ) .

Dominant substrate type (V9 ) is included because the abundance of aquatic

insects, an important food item for cutthroat trout, is correlated with substrate type. Variable V1 6 , percent fines in riffle-run and spawning areas, is

included because the presence of excessive fines in riffle-run areas will

reduce the production of aquatic insects. Variable Vii is included because

allochthonous materials are an important source of nutrients to cold, unproductive trout streams. The waterflow of all streams fluctuate on an annual

seasonal cycle. It has been demonstrated that a correlation exists between

the average annual daily streamflow and the annual low base flow period in

maintaining desirable stream habitat features for all life stages. Variable

V1 4 is included to quantify the relationship between annual water flow fluctuations and trout habitat suitability.

Variables ViZ and Vi? are optional variables to be used only when needed

and appropriate. Percentage of streamside vegetation, ViZ' is an important

means of controlling soil erosion, a major source of fines in streams. Variable Vi?' percentage of mid-day shade, is included because studies have shown

that the amount of shade can affect water temperature and photosynthesis in

streams. Variables ViZ and Vi? are used primarily for streams:$; 50 m wide

with temperature, photosynthesis, or erosion problems or when changes in the

riparian vegetation are part of a potential project plan.

Suitability Index (SI) Graphs for Model Variables

This section contains suitability index graphs for 17 model variables.

Equations and instructions for combining groups of variable SI scores into

component scores and component scores into cutthroat trout HS1 scores are

included.

The graphs were constructed by quantifying information on the effect of

each habitat variable on the growth, survival, or biomass of cutthroat trout.

The curves were built on the assumption that increments of growth, survival,

or biomass originally plotted on the y-axis of the graph could be directly

converted into an index of suitability from 0 to 1.0 for the species; 0 indicates unsuitable conditions and 1.0 indicates optimal conditions. Graph trend

lines represent the author's best estimate of suitability for the various

10
levels of each variable presented. The graphs have been reviewed by biologists

familiar with the ecology of the species, but obviously some degree of SI

variability exists. The user is encouraged to vary the shape of the graphs

when existing regional information indicates that the variable suitability

relationship is different.

The habitat measurements and SI graph construction are based on the

premise that it is the extreme, rather than the average, values of a variable

that most often limit the carrying capacity of a habitat. Thus, measurement

of extreme conditions, e.g., maximum temperatures and minimum dissolved oxygen

levels, are often the data used with the graphs to derive the SI values for

the model. The letters Rand L in the habitat column identify variables used

to evaluate riverine (R) or lacustrine (L) habitats.

Habitat

R,L

Variable

Average maximum water

temperature (OC) during

the warmest period of

the year (adult,

juvenile, and fry).

For lacustrine habitats,

use temperature strata

nearest ootimal in

dissolved oxygen zones

of > 3 mg/l.

A = General

B = Lahontan Basin

Suitability Graph

1. 0 -+----~_"""IIt_...........~-...........+ ><

Q)

"'0

oS 0.8

>,

+->

;:: 0.6 .,....

..0

ro

.~ 0.4 ::::l

(/)

0.2

R Average maximum water

temperature (OC) during

embryo development.

11

><

Q)

-g 0.8 .......

>,

+->

.,.... 0.6 ..-

.,-

::::l

<n 0.2

10

10

20 30

20
R,L Average mlnlmum

dissolved oxygen

(mg/l) during the

late growing season

low water period and

during embryo development (adult, juvenile,

fry, and embryo).

For lacustrine habitats,

use the dissolved oxygen

readings in temperature

zones nearest to optimal

where dissolved oxygen

is>3 mg/l.

A= $: 15° C

B

=> 15° C

1.0

x

OJ 0.8 -0

s::

.......

~0.6

-r-:

:0 0.4 n:::l

.; 0.2 U1

3

6

mg/l

9

R Average thalweg depth

(em) during the late

growing season low

water period. A = $: 5 m stream width B = > 5 m stream width

1.0

0.8 -0

s::

.......

>, 0.6 .r0.4 n:::l

0.2

15 30

em

45 60

R Average ve1oe ity

(em/sec) over spawning

areas during embryo

development.

1.0

x

0.8 s::

.......

0.6 .r-

:0 0.4 n:::l

.r0.2

12

25 50

em/sec

75 100
R (V6 ) Percent cover 1.0

during the late

growing season x

low water period OJ 0.8 -0

at depths 15 em c:

and velocities 0.6

< 15 em/sec.

J = Juveniles 0.4

A = Adults ..0

l1:l

+>

:J 0.2 V1

10 20 30 40

%

R (V7) Average size of sub- 1.0 strate between 0.3-

8 em diameter in x

spawning areas, OJ

-0 0.8

preferably during the c:

spawning period. 0.6

To derive an average

value for use with graph ..0 0.4 l1:l

V7, include areas con- +>

''--

:J 0.2 taining the best spawning V1

substrate sampled until

all potential spawning

sites are included or 5 10

unt i 1 the sample contains

an area equal to 5% of the em

total cutthroat habitat

being evaluated.

R (Va) Percent substrate size 1.0

class (10-40 em) used

for winter and escape x

OJ 0.8 cover by fry and small -0

c: juveniles. >,0.6

+-'

..0 0.4 l1:l

+>

:J

V1 0.2

5 10 15 20

%

13
- f-

-

-

-

R (V 9 ) Dominant (~ 50%) 1.0

substrate type in

riffle-run areas for x

food production. 0.8 s::

.......

A) Rubble or small 0.6

boulders or aquatic ''--

vegetation in spri ng ''--

0.4

areas dominant with

limited amounts of ''--

:=l

gravel, 1arge (/) 0.2

boulders, or bedrock.

B) Rubble, gravel,

boulders, and fines

occur in approximately

equal amounts or gravel

is domi nant. Aquatic

vegetation mayor may

not be present.

C) F,ines, bedrock, or

large boulders are

dominant. Rubble

and gravel are

insignificant (s 25%).

A B C

R Percent pools during

the late growing

season low water

period.

1. 0 +-o-...............&.~_ .......-.....:~ ................+

x

0.8 s::

.......

0.6 ''--

.---

:0 0.4 res

0.2

14

25 50

%

75 100
R (V 11) Average percent vege- 1.0

tation (trees, shrubs,

and grasses-forbs) x

<lJ 0.8 along the streambank "'0

s:::

during the summer for .......

allochthonous input. b 0.6

Vegetation Index = 2 (% shrubs) + 1.5 ',...

.0 0.4 (% grasses) + (% trees) ro

+-'

+ 0 (% bareground), ,,...

::::::l

Vl 0.2

(For streams 50 m wide)

100 200 300

%

R (V 1 2 ) Average percent rooted 1.0

(Optional) vegetation and stable

rocky ground cover along x the streambank during the <lJ 0.8 "'0

summer (erosion control). s:::

.......

>, 0.6

+-'

',...

.0 0.4 ro

+-'

-r-'

::::::l

Vl 0.2

25 50 75 100

%

R,L (V 1 3 ) Annual maximal or 1.0 minimal pH. Use the

measurement with the

lowest 51 value. x

<lJ 0.8 "'0

s:::

For lacustrine habitats, .......

>, 0.6

measure pH in the zone +-'

of the best combination ',...

of dissolved oxygen and .0 0.4 ro temperature, +-' ',...

::::::l 0.2

A General Vl

= B = Lahontan Basin

5 6 7 8 9 10

pH

15
R Average annual base

flow regime during the

late summer or winter

low flow period as a

percentage of the

.average annual daily

flow.

1.0

::<

0.8 s::

.......

~0.6

.... 0.4

-j-)

::::l

Vl 0.2

R Pool class rating during

the late growing season

low flow period. The

rating is based on the

% of the area containing

pools of 3 classes as

described below.

A) 30% of the area

is comprised of

1st-class pools.

B) 10%-< 30% 1stclass pools or

50% 2nd-class

pools.

C) < 10% 1st-class

pools and < 50%

2nd-class pools.

(See pool class descriptions below)

1.0

::<

0.8 s::

.......

>,0.6

-j-)

:0 0.4 ro

-j-)

0.2

A

25 50

%

B

75

C

100

A) First-class pool: Large and deep. Pool depth and size are sufficient to provide a low velocity resting area for several adult

trout. More than 30% of the pool bottom is obscure due to depth,

surface turbulence, or the presence of structures, e.g., logs,

debris piles, boulders, or overhanging banks and vegetation. Or,

the greatest pool depth is 1.5 m in streams s 5 m wide or z 2 m

deep in streams> 5 mwide.

16
B) Second-class pool: Moderate size and depth. Pool depth and size

are sufficient to provide a low velocity resting area for a few

adult trout. From 5 to 30% of the bottom is obscure due to surface

turbul ence, depth, or the presence of structures. Typi ca 1 second

class pools are large eddies behind boulders and low velocity,

moderately deep areas beneath overhanging banks and vegetation.

C) Third-class pool: Small or shallow or both. Pool depth and size

are sufficient to provide a low velocity resting area for one to

very few adult trout. Cover, if present, is in the form of shade,

surface turbulence, or very limited structure. Typical third-class

pools are wide, shallow pool areas of streams or small eddies behind

boulders. Virtually the entire bottom area is discernable.

R (V 1 6 ) Percent fin es « 3 mm) 1.0

in ri ffl e-run and in

spawning areas during x

OJ 0.8

summer flows. -0average s::

A Spawning >, = +-' 0.6

B Riffle-run ''--

=

.0

ro 0.4

+-'

:::::l

(/) 0.2

15 30

%

45 60

R (V1 7 ) Percent of stream area 1.0

(Optional) shaded between 1000 and x

1400 hrs (for streams OJ

-0 0.8

s 50 m wide). Do not s::

use on cold «18°C) >,

unproductive streams. +-' 0.6 ''--

''--

.0

ro 0.4

+-'

''--

:::::l

(/) 0.2

25 50

%

75 100

References to sources of data and the assumptions used to construct the

above suitability index graphs for cutthroat trout HSI models are presented in

Table 1.

17
Table 1. Data sources for cutthroat trout suitability indices.

Variable and source

Needham and Jones 1959

Bell 1973

Behnke and Zarn 1976

Behnke 1979

Dwyer and Kramer 1975

Snyder and Tanner 1960

Bell 1973

Calhoun 1966

Doudoroff and Shumway 1970

Trojnar 1972

Sekulich 1974

Delisle and Eliason 1961

Estimated by authors

Thompson 1972

Hooper 1973

Hunter 1973

Assumption

Average maximal daily water temperatures have a greater effect on trout

growth and survival than minimal

temperatures. The maximal temperature related with the greatest scope

for activity is optimum.

The average maximal daily water temperature during the embryo development period related to the highest

survival and normal development of

the embryo is optimum. Those

temperatures that reduce survival

are suboptimum.

The average minimal daily dissolved

oxygen level during embryo development

and the late growing season that is

related to the greatest growth and

survival of cutthroat trout and trout

embryos is optimal. Those that reduce

survival and growth are suboptimum.

The average thalweg depths that

provide the best combination of

pools, instream cover, and instream

movement of adult trout is optimum.

The average velocities over spawning

areas affect the suitability with

which dissolved oxygen and waste

products are carried to and from

the developing embryos. Average

velocities which result in the

highest survival of embryos are

optimum. Those that result in

reduced survival are suboptimum.

18
Variable and source

Boussu 1954

Elser 1968

Lewis 1969

Bjornn 1969

Phillips et al. 1975

Duff 1980

Table 1 (continued)

Assumption

Trout standing crops are correlated

with the amount of usable cover

present. Usable cover is associated

with water 15 em deep and velocities

15 em/sec. These conditions are

associated more with pool than riffle

conditions. The best ratio of habitat

conditions is about 50% pool to 50%

riffle areas. Not all of a pool IS area

provides usable cover. Thus, it is

assumed that optimal cover conditions

for trout streams can be reached at

< 50% of the total area.

The average size of spawning gravel

that is correlated with the best water

exchange rates, proper redd construction, and highest fry survival is

assumed to be optimum for average sized

cutthroat trout. The percentage of

total spawning area needed to support a

good trout population was calculated

from the following assumptions:

1. Excellent riverine trout habitat

will support about 500 kg/hectare.

2. Spawners comprise about 80% of

the weight of the population.

500 kg x 80% = 400 kg of

spawners.

3. Cutthroat adults average about

0.2 kg each

400 kg _ 0.2 kg - 2,000 adult spawners

4. There are two adults per redd

2,000 = 1 000 pairs 2 '

5. Each redd covers 0.5 m2

1,000 x 0.5 = 500 m2 per hectare

6. There are 10,000 m2 per hectare

105~~0 = 5% of total area ,

19
Variable and source

Table 1 (continued)

Assumption

Hartman 1965

Everest 1969

Bustard and Narver 1975a, b

Pennak and Van Gerpen 1947

Hynes 1970

Binns and Eiserman 1979

Needham 1940

Elser 1968

Hunt 1971

Horner and Bjornn 1976

Idyll 1942

Delisle and Eliason 1961

Chapman 1966

Hunt 1975

Anonymous 1979

Raleigh and Duff 1981

Hartman and Gi 11 1968

Platts 1974

Sekulich 1974

Behnke and Zarn 1976

Binns 1977

Binns 1979

Adapted from Duff and

Cooper 1976

The substrate size range selected

for escape and winter cover by cutthroat fry and small juveniles is

assumed to be optimum.

The dominant substrate type containing

the greatest numbers of aquatic insects

is assumed to be optimum for insect

production.

The percent pools during late summer

low flows that is associated with the

greatest trout abundance is optimum.

The average percent vegetation along

the streambank is related to the

amount of allochthanous materials

deposited annually in the stream.

Shrubs are the best source of

allochthanous materials, followed by

grasses and forbs, and then trees.

The vegetational index is a reasonable

approximation of optimal and suboptimal

conditions for most trout stream

habitats.

The average percent rooted vegetation

and rocky ground cover that provides

adequate erosion control to the stream

is optimum.

The average annual maximal or minimal

pH levels related to high survival of

trout are optimum.

Flow variations affect the amount and

quality of pools, instream cover, and

water quality. Average annual base

flows associated with the highest

standing crops are optimum.

20
Variable and source

V1 5 Lewis 1969

Raleigh (in press)

V1 6 Bjornn 1969

Cordone and Kelly

Pl atts 1974

McCuddi n 1977

Crouse et al. 1981

V17 Sabean 1976, 1977

Anonymous 1979

1961

Table 1 (concluded)

Assumption

Pool classes associated with the

highest standing crops of trout are

optimum.

The percent fines associated with the

highest standing crops of food organisms,

embryos, and fry in each designated area

is optimum.

The percent of stream area shaded that

is associated with optimal water temperatures and photosynthesis rates is

optimum.

The above references include data from studies on related salmonid species.

This information has been selectively used to supplement, verify, or complete

data gaps on the habitat requirements of cutthroat trout.

21
Riverine Model

This model uses a life stage approach with five components: adult,

juvenile, fry, embryo, and other.

Case 1:

Case 2: 1/2 where V6 is (V lD x ViS)

If V4 or (V i Dx Vis) 1/ 2 is 0.4 in either equation, then C

A

= the lowest

factor score.

Juvenile (CJ).

or, if any variable is 0.4, then C

J = the lowest variable score.

or, if Vi Dor (V s x V1 6 ) 1/ 2 is 0.4, then CF = the lowest factor score.

22
Steps:

A. A potential spawning site is an 0.5 m2 area of gravel 0.3-8.0 cm in

size covered by flowing water 15 cm deep. At each spawning site

sampled, record:

1. The average water velocity over the site;

2. The average size of all gravel 0.3-8.0 cm;

3. The percentage of fines < 0.3 cm in the gravel; and

4. The total area in m2 of each site.

B. Derive a spawning site suitability index (V ) for each site by combining

Vs , V7 , and V1 6 values for each site. s

C. Derive a weighted average (V s) for all sites included in the sample.

Select the best V scores until all sites are included, or until

s

a total spawning area equal to, but not exceeding, 5% of the total

cutthroat trout habitat has been included, whichever comes first.

n

Vs I A. = i=l 1

total

where:

V .

Sl

habitat area /0.05 (output cannot> 1.0)

Ai = the area of each spawning site in m2 , but I A. cannot

exceed 5% of the total cutthroat habitat. 1

V .

Sl

= the individual SI scores from the best spawning areas

until all spawning sites have been included or until

Sl ls from an area equal to 5% of the total cutthroat

habitat being evaluated has been included, whichever

occurs first.

D. Derive CE

CE

= the lowest score of V2 , V3 , or V

s

23
Other (CO),

(V g x VI6) 1/ 2 + VII

2

1/2

where: N = the number of variables within the parentheses. Note

that variables VI 2 and VI 7 are optional and, therefore,

may be omitted (see page 18).

HSI determination. HSI scores may be derived for a single life stage, a

combination of two or more life stages, or all life stages combined. In all

cases, except for the embryo component (CE), an HSI is obtained by combining

one or more life stage component scores with the other component (CO) score.

1. Equal Component Value Method. The equal component value method assumes

that each component exerts equal influence in determining HSI. This

method should be used to determine HSI unless information exists that

individual components should be weighted differently. Components:

CA, CJ , CF, CE, and Co

or, if any component is 0.4, then HSI = the lowest component value,

or if C

A is < the equation value, then HSI = CA'

where: N = the number of components in the equation.

Solve the equation for the number of components to be included in the

evaluation. There will be a minimum of two, one or more life stage

components and the component (CO), unless only the embryo life stage

(CE) is being evaluated; then HSI = CEo

2. Unequal Component Value Method. This method also uses a life stage

approach with five components: adult (CA), juvenile (CJ ) , fry (CF),

embryo (CE), and other (CO), However, the Co component is divided into

two subcomponents, food (COF) and water quality (COQ)' It is assumed that

the C

OF subcomponent can either increase or decrease the suitability of

24
the habitat by its effect on growth at each life stage except embryo. The

C

OQ subcomponent is assumed to exert an influence equal to the combined

influence of all other model components in determining habitat suitability.

The method also assumes that water quality is excellent, C

OQ

= 1. When

C

OQ is < 1, HSI is decreased. In addition, when a basis for weighting

exists, model component and subcomponent weights can be increased by

multiplying each index value by multipliers> 1. Model weighting

procedures must be documented.

Components and subcomponents: C

A, C

J , CF, CE, C

OF' and COQ

Steps:

A. Calculate the subcomponents (COF and COQ) of Co

(V V )1/ 2 V

9 x 16 + 11

COF = 2

or, if any variable is 0.4, then C

OQ = the value of the lowest

variable.

B. Calculate HSI by either the noncompensatory or the compensatory option.

Noncompensatory option. This option assumes that degraded water

quality conditions cannot be compensated for by good physical habitat

conditions. This assumption is most likely true for small streams

(~ 5 m wide) and for persistent degraded water quality conditions.

or, if any component is 0.4, then HSI = the lowest component

value x COQ'

where: N = the number of components and subcomponents inside the

parentheses or, if the model components or subcomponents

have unequal weights, then N = I of weights selected.

25
If only the embryo component is being evaluated, then H5I = CE x COQ'

Compensatory option. This method assumes that moderately degraded

water quality conditions can be partially compensated for by good

physical habitat conditions. This assumption is useful for large

rivers (~ 50 m wide) and for temporary, or short term, poor water

quality conditions.

1) H5I ' = (CA x CJ x CF x CE x COF)1/N

or, if C

A is 0.4, then H5I ' = CA

where: N = the number of components and subcomponents in the

equation, or if the model components or subcomponents have unequal weights, then N = I of weights

selected.

2) If C

OQ is < H5I ' ,then H5I = H5I ' x [1- (H5I ' - COQ)]; if not,

H5I = H5I '.

3) If only the embryo component is being evaluated, follow the

procedure in step 2, substituting CE for H5I '.

Lacustrine Model

The following model is available to evaluate cutthroat trout lacustrine

habitat. The lacustrine model consists of two components: water quality and

reproduction.

Water Quality (CWQ)'

or, if the 51 scores for VI or V3 are 0.4, then C

WQ = the lowest 51

score for VI or V3 •

Note: Lacustrine cutthroat require a tributary stream for spawning and

embryo development. If the embryo life stage habitat is to be included in

the evaluation, use the embryo component steps and equations in the riverine

model above, except that the area of spawning gravel needed is only about 1%

of the total surface area of the lacustrine habitat.

26
n

V

s

L A. V . = i=l 1 Sl

total habitat area /0.01 (output cannot> 1.0)

HSI determination.

HSI

If only the lacustrine habitat is evaluated, then HSI = CWO'

Interpreting Model Outputs

Model HSI scores for individual life stages, composite life stages, or for

the species are a relative indicator of habitat suitability for the evaluation

element. The HSI models, in their present form, are not intended to consistently predict standing crops of fishes throughout the United States. Standing

crop limiting factors, such as interspecific competition, predation, disease,

water nutrient levels, and length of growing season, are not included in the

aquatic HSI models. The models contain physical habitat variables important

in maintaining viable populations of cutthroat trout. If the model is

correctly structured, a high HSI score for a habitat would indicate near

optimal regional conditions for cutthroat trout for those factors included in

the model, intermediate HSI scores would indicate average habitat conditions,

and low HSI scores would indicate poor habitat conditions. An HSI of 0 does

not always mean that the species is not present. An HSI of 0 means that the

habitat is very poor and the species will be scarce or absent.

Cutthroat trout tend to occupy riverine habitats with very few other fish

species present. They are usually competitively excluded by other trout

species. Thus, factors of disease, interspecific competition, and predation

usually will have little effect on the model. When the cutthroat trout model

is applied to cutthroat trout streams with similar water quality and length of

growing season, it should be possible to calibrate the model output to reflect

size of standing crops within some reasonable confidence limits. This possibility, however, has not been tested with the present model.

Sample data sets selected by the authors to represent high, intermediate,

and low habitat suitabilities are given in Table 2, along with the 5I Is and

H5I Is generated by the cutthroat trout riverine model. The model outputs

calculated from the sample data sets (Tables 3 and 4) reflect what the authors

believe carrying capacity trends would be in riverine habitats with the listed

characteristics; thus, the model meets the specified acceptance level.

27
Table 2. Sample data sets using the riverine cutthroat trout HS1 model.

Data set 1 Data set 2 Data set 3

Variable Data SI Data SI Data SI

Max. temperature

(OC) V1 14 1.0 15 1.0 16 1.0

Max. temperature

( 0C) V2 12 1.0 15 0.66 17 0.4

Min. dissolved O2

(mg/l) V3 9 1.0 7 0.73 6 0.42

Ave. depth (cm) V4 25 0.9 18 0.6 18 0.6

Ave. velocity

(cm/s) Vs 30 1.0 25 0.7 20 0.57

% cover V6 20 A 0.95 10 A 0.65 10 A 0.65

J 1. 0 J 0.92 J 0.92

Ave. gravel size

(cm) V7 4 1.0 3 1.0 2.5 1.0

Dam. substrate

size (cm) Vs 15 1.0 8 0.7 8 0.7

Dam. substrate

class V9 A 1.0 B 0.6 B 0.6

% pools V10 55 1.0 15 0.65 10 0.46

%Alloch.

vegetation Vll 225 1.0 175 1.0 200 1.0

% bank vegetation V12 95 1.0 50 0.6 40 0.5

Max. pH V13 7.1 1.0 7.2 1.0 7.2 1.0

% Ann. base flow V14 37 0.8 30 0.6 25 0.5

Pool class V15 A 1.0 B 0.6 C 0.3

% fines (A) V16 5 1.0 20 0.5 20 0.5

0/

/0 fines (B) V16 15 0.9 30 0.6 30 0.6

0/

/0 shade V17 60 1.0 60 1.0 60 1.0

28
Table 3. Average value method.

Data set 1 Data set 2 Data set 3

Variable Data SI Data SI Data SI

Component

C

A 0.95 0.62 0.37

C

J 1. 00 0.77 0.37

CF 0.97 0.65 0.55

CE 1. 00 0.66 0.40

Co 0.96 0.78 0.40

Species HSI 0.98 0.70 0.37

29
Table 4. Average value, probability method.

Data set 1 Data set 2 Data set 3

Variable Data 51 Data 51 Data SI

Component

C

A 0.95 0.62 0.37

CJ 1.0 0.77 0.37

CF 0.97 0.65 0.55

C

E 1. 00 0.66 0.40

COF 0.97 0.80 0.80

C

OQ 0.96 0.80 0.67

Species HS1

Noncompensatory 0.94 0.56 0.25

Compensatory 0.96 0.70 0.37

30
ADDITIONAL HABITAT MODELS

Mode 1 1

Optimal riverine cutthroat trout habitat is characterized by:

1. Clear cold water with an average maximum summer temperature of < 22° C;

2. An approximate 1:1 pool-riffle ratio;

3. Well vegetated, stable stream banks;

4. 25% of stream area providing cover;

5. Relatively stable water flow regime, < 50% annual fluctuation from

average annual daily flow;

6. Relatively stable summer temperature regime, averaging about

13° C ± 4° C; and

7. A relatively silt free rocky substrate in riffle-run areas.

Model 2

HSI

= number of attributes present

7

A riverine trout habitat model by Binns and Eiserman (1979). Transpose

the model output of pounds per acre to an index of 0-1.

HSI = model output of pounds per acre

regional optimal pounds per acre

Model 3

Optimal lacustrine cutthroat habitat is characterized by:

1. Clear, cold water with an average summer midepilimnion temperature

of < 22° C;

2. A midepilimnion pH of 6.5 to 8.5;

3. Dissolved oxygen content of epilimnion of 8 mg/l; and

31
4. Access to riverine spawning tributaries.

Model 4

HSI = number of attributes present

4

A low effort system for predicting habitat suitability of planned cool

water and cold water reservoirs as habitat for individual fish species by

McConnell et al. (1982).

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32
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33
Cope, O. B. 1957. The choice of spawning sites by cutthroat trout. Proc.

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Utah.

1977 .

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habitat zones

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34
Griffith, J. S. 1972. Comparative behavior and habitat utilization of brook

trout (Salvelinus fontinalis) and cutthroat trout (Salmo clarki) in small

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1974. Utilization of invertebrate drift by brook trout

(Salvelinus fontinalis) and cutthroat trout (Salmo clarki) in small

streams in Idaho. Trans. Am. Fish. Soc. 103(3):440-447.

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ecology.

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Contract 56, Prog. Rep. 1975. 38 pp.

Hunt, R. L. 1971. Responses of a brook trout population to habitat development in Lawrence Creek. Dept. Nat. Res. Tech. Bull. 48, Madison, Wise.

35 pp.

1975. Food relations and behavior of salmonid fishes.

137-151 in A. D. Hasler (ed.). Coupling of land and water systems.

Studies,Vol. 10. Springer Verlag.

Pages

Eco1.

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Trans. Am. Fish. Soc. 84(2):275-296.

35
Johnston, J. M. 1979. Coastal cutthroat (Salmo clarki clarki). Washington

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a trout

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36
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37
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Resources Ser. 78. 46 pp.

38
5027'2 -101

REPORT ~~MENTAT'ON l.l'-FWSioBS·-82/10.5

3. Recipient's Accession No.

4. Titte and Subtitle

_ Habitat Suitability Index Models: Cutthroat trout

Februarv 1982

7. Author(s)

Terry J. Hickman, and Robert F. Raleigh

a. Performing O,..anizatlon RePt- No.

9. Performing Oraanization Name and Address Ha bi ta t Eva 1uati on Procedures Group

Western Energy and Land Use Team

U.S. Fish and Wildlife Service

Drake Creekside Building One

2625 Redwing Road

Fort Collins, Colorado 80526

10. Project/Task/Worlc Unit No.

11. Contract(C) or Grant(Gl No.

(C)

«n

12. Sponsoring Organization Name and Address Western Energy and Land Use Team

Office of Biological Services

Fish and Widlife Service

U.S. Department of Interior

Washinaton D.C. 20240

13. Type of Report & Period Covered

14.

15. Supplementary Notes

·18. Abstract (Limit 200 words)

Cutthroat trout (Salmo clarki) characteristics are described, including distribution,

age, growth, food, reproduction and anadromy. A detailed literature review of specific

habitat requirements is also included. 17 habitat variables are then evaluated and

quantified in suitability index graphs. These data are subsequently synthesized into

Habitat Suitability Index (HSI) models for the cutthroat trout.

This is one in a series of publications developed to provide information on the habitat

requirements of selected fish and wildlife species. HSI models are designed to provide

information for use in impatt assessment and habitat management activities. The HSI

technique is a corollary to the U.S. Fish and Wildlife Service's Habitat Evaluation

Procedures.

17. Oocument Analysis a. Oescriptors

Habi tabil ity

Fishes

b. Identlfiers/Ol)en-Ended Terms

Index

Habitat

Habitat Suitability

Cutthroat trout

Salmo clarki t c. COSATI Field/Group

18. Availability Statement

Unlimited

! 19. Security Class (This Report)

! Unclassified

I 20. Security Class (This Pagel

. Unclassified

1

21_ No. of Pag"

I 38

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REGION 2

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u.s. FISH ..WILDLIFE f>ERVICE

DEPARTMENT OF THE INTERIOR hi?]

u.s.FISH ANDWILDLIFESERVICE "'-r rw T " "

As the Nation's pri ncipal conservation agency, the Department of the Interior has responsibility for most of our ,nationally owned public lands and natural resources . This includes

fostering the wisest use of our land and water resources, protecting our fish and wildlife,

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