· BIOLOGICAL REPORT 82(10.133)

FEBRUARY 1987

NAfiuNAL iiliETLANDS

RESEARCH CENTER LIBRARY

700 Cajundome Blvd.

Lafayette, LA 70506-3152 "" - =en ,! el l:: ll

Lc foyette, Lou isiana 050

HABITAT SUITABILITY INDEX MODELS:

JUVENILE ENGLISH SOLE

SK

361

.U54

no.82-

10.133

:lnd Wildlife Service

Department of the Interior
MODEL EVALUATION FORM

Habitat models are designed for a wide variety of planning applications

where habitat information is an important consideration in the decision

process. It is impossible, however, to develop a model that performs equally

well in all situations. Each model is published individually to facilitate

updating and reprinting as new information becomes available. Assistance from

users and researchers is an important part of the model improvement process.

Please complete this form following application or review of the model. Feel

free to include additional information that may be of use to either a model

developer or model user. We also would appreciate information on model

testing, modification, and application, as well as copies of modified models or

test results. Please return this form to:

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Thank you for your assistance.

Species----------

Geographic

Location------------------

Management Action Analysis _

Habi tat or Cover Type(s)------------------------

Type of Application: Impact Analysis _ Baseline Other---------------------

Variables Measured or Evaluated

._--------------------

Was the species information useful and accurate? Yes No

If not, what corrections or improvements are needed?------------
Were the variables and curves clearly defined and useful? Yes No

If not. how were or could they be improved? __

Were the techniques suggested for collection of field data:

Appropriate? Yes No

Clearly defined? Yes No

Easily applied? Yes No

If not, what other data collection techniques are needed?

We.re the model equations logical? Yes No

Appropriate? Yes No

How were or could they be improved?

Other suggestions for modification or improvement (attach curves,

equations, graphs, or other appropriate information) _

Additional references or information that should be included in the model:

Model Evaluator or Reviewer Date ----------------------- ------------ Agency _

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Biological Report 82(10.133)

February 1987

HABITAT SUITABILITY INDEX MODELS: JUVENILE ENGLISH SOLE

by

Christopher L. Toole

University of California Cooperative Extension

Sea Grant Marine Advisory Program

Foot of Commercial St.

Eureka, CA 95501

and

Roger A. Barnhart

U.S. Fish and Wildlife Service

California Cooperative Fishery Research Unit

Humboldt State University

Arcata, CA 95521

and

Christopher P. Onuf

U.S. Fish and Wildlife Service

National Wetlands Research Center

1010 Gause Boulevard

Slidell, LA 70458

Performed for

National Wetlands Research Center

Research and Development

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
This report may be cited as:

Toole, C.L., R.A. Barnhart, and C.P. Onuf.

models: juvenile English sole. U.S.

82(10.133). 27 pp.

1987. Habitat suitability index

Fish Wild1. Servo Bio1. Rep.
PREFACE

The juvenile English sole habitat suitability index (HSI) model is

intended for use in the habitat evaluation procedures (HEP) developed by the

U.S. Fish and Wildlife Service (1980) for impact assessment and habitat

management. The model was developed from a review and synthesis of existing

information and is scaled to produce an index of habitat suitability between 0

(unsuitable habitat) and 1 (optimally suitable habitat). Assumptions involved

in developing the HSI model and guidelines for model applications, including

methods for measuring model variables, are described.

This model is a hypothesis of species-habitat relations, not a statement

of proven cause and effect, and has not been field tested. For this reason

the U.S. Fish and Wildlife Service encourages model users to convey comments

and suggestions that may help increase the utility and effectiveness of this

habitat-based approach to fish and wildlife management. Please send any

comments or suggestions to the following address:

Information Transfer Specialist

National Wetlands Research Center

U.S. Fish and Wildlife Service

1010 Gause Bl vd , Slidell, LA 70458.

iii

CONTENTS

PREFACE .......................................... ACKNOWLEDGMENTS .................................. iii

vi

INTRODUCTION ••..•••••.•

Distribution •.....•••

Life History Overview .....•

SPECIFIC HABITAT REQUIREMENTS

Cover and Substrate

Food •.•••..

Temperature

Salinity

Dissolved Oxygen Special Considerations ••....

HABITAT SUITABILITY INDEX (HSI)

Model Applicability

Model Description Suitability Index (SI) Graphs

Component Index Equations and

Field Use of the Model

Interpreting Model Outputs

REFERENCES

MODEL

for Model Variables

HSI Determination

v

1

1

2

4

4

5

6

7

7

8

8

8

9

11

15

15

16

19
ACKNOWLEDGMENTS

The authors wish to thank Gregor Cailliet, Moss Landing Marine

Laboratories, Moss Landing, California; Perry Herrgesell, California

Department of Fish and Game, Stockton; and William Pearcy, Oregon State

University, Corvallis, for valuable comments on an earlier draft of this

report. Other reviewers of the model were U.S. Fish and Wildlife Service

personnel from the Ecological Services Field Offices of Sacramento,

California, and Portland, Oregon. Christopher Dewees, Elizabeth Strange, and

Debbie Marshall of the University of California Cooperative Extension assisted

with the initial literature search and literature compilation. Word

processing was provided by Mrs. Delores Neher of the California Cooperative Fishery Research Unit. A portion of this work was performed under grant

number NA80AA-D-00120 through the Department of Commerce National Sea Grant

College Program and through the California State Resources Agency, project number A/EA-l. Other funding was provided by U.S. Fish and Wildlife Service.

Patrick J. Lynch illustrated the cover.

vi
ENGLISH SOLE (Parophrys vetulus)

INTRODUCTION

English sole (Parophrys vetulus) is one of the major commercial

groundfish species caught along the Pacific coast. Landings in the United

States and Canada averaged 4,947 t/yr between 1975 and 1984, placing it third

in importance among flatfish caught by Pacific coast trawlers (Pacific Marine

Fisheries Commission 1985). Juvenile English sole are also among the most

abundant fishes in many bays and estuaries along the Pacific (Westrheim 1955;

Sopher 1974; Ambrose 1976; Rogers 1985). The English sole is not an important

recreational species.

Distribution

English sole are found from San Cristobal Bay, Baja California, to Unimak

Island, Alaska (Miller and Lea 1972; Hart 1973; Knaggs et al. 1976). Adults

have been caught at depths of 550 m (Hart 1973), but are most common at depths

between 40 and 150 m (Forrester 1969a; Demory et al. 1976; Pearcy 1978).

Juveniles (0- and I-age fish) are common in intertidal and subtidal waters of

bays and estuaries from Elkhorn Slough, California, to at least Grays Harbor,

Washington (Villadolid 1927; Westrheim 1955; Smith and Nitsos 1969; Olson and

Pratt 1973; Samuelson 1973; Pearcy and Myers 1974; Sopher 1974; Ambrose 1976;

Misitano 1976, 1977; Collins 1978; Hulberg and Oliver 1979; Toole 1980; Bayer

1981; Barry 1982; Rosenberg 1982; Yoklavich 1982; Rogers 1985; Krygier and

Pearcy 1986). They are apparently not common in embayments south of Monterey

(Fierstine et al. 1973; Horn 1980). In northern Washington and British

Columbia juvenile English sole are common in shallow protected inland coastal

areas (Ketchen 1956; Allen et al. 1959; Kendall 1966; English 1967; Alderdice

and Forrester 1968; Forrester 1969a; Van Cleve and El-Sayed 1969; Moulton et

al. 1974; Simenstad et al. 1977; Thornburgh 1918; Westrheim et al. 1981;

Stocker et al. 1981; Fargo et al. 1982, 1983). Juveniles are also distributed

in shallow waters in open coastal areas of Oregon and Washington (Laroche and

Holton 1979; Rosenberg 1982; Rogers 1985; Krygier and Pearcy 1986).

Tagging and meristic studies indicate that at least eight separate stocks

of English sole exist along the Pacific coast (Ketchen 1950; Ketchen and

Forrester 1955; Van Cleve and Pruter 1956; Ketchen 1956; Forrester 1969b;

Holland 1969; Jow 1969; Pattie 1969; Barss 1976; Day 1976). Stocks are

currently separated for management purposes according to the International

North Pacific Fisheries Commission geographical regions which, in some cases,

may encompass two or more localized stocks (Pacific Fishery Management Council

1982).

1
Life History Overview

Measurements are of total length throughout this report unless indicated

otherwise. Female English sole are usually mature at 3 years of age and 26-35

cm long. Median length at maturity has been reported as 29.5-31 cm. Males

may mature when 2 years of age and 21-29 cm long. Median length at maturity

is about 22-23 cm (Ketchen 1947; Harry 1959; Allen and Forrester 1966;

Forrester 1969a).

Fecundity ranges from 150,000 eggs for 30 cm fish, 3 years of age, to

1.95 million eggs for 43.7 cm fish, 10 years of age. Fish of average size in

the Strait of Georgia commercial fishery (33-38 cm, ages 4-6) produce between

0.5 and 1.0 million eggs (Ketchen 1947).

The spawning period in English sole is highly protracted and variable.

Some spawning may occur in all months, but the peak season is September

through April (Budd 1940; Ketchen 1956; Harry 1959; Jow 1969; Laroche and

Richardson 1979). Examination of ovaries does not indicate serial spawning,

so the protracted spawning season is probably due to the spawning of different

individuals at different times (G. Hewitt, pers. comm., cited in Kruse and

Tyler 1983). Kruse and Tyler (1983) showed that the spawning season is

strongly correlated with bottom temperature and upwelling patterns, but the

exact method of control was not determined. Hayman et al. (1980) showed a

strong correlation between fall upwelling patterns and resulting cohort

strength. They hypothesized that when upwelling persists into the fall,

nearshore bottom temperatures are reduced, egg maturation is slowed, spawning

is delayed, and larvae are ready to feed closer to the time of spring

upwelling when feeding conditions are presumably more favorable.

Eggs are pelagic and hatch in 3.75 days at 13 °C to 11.8 days at 4 °C

(Budd 1940; Ketchen 1956; Alderdice and Forrester 1968; Orsi 1968). Hatching

rate is also dependent on salinity and dissolved oxygen levels (Alderdice and

Forrester 1968). Embryology and larval development have been described by

Budd (1940), Orsi (1968), and Ahlstrom and Moser (1975).

Duration of the pelagic larval phase was estimated at 6-10 weeks by

Ketchen (1956) based on the time between peak spawning of adults and the

appearance of transforming larvae in shallow waters. Laroche and Richardson

(1979) estimated that this phase lasted about 18-22 weeks based on analysis of

larval length frequency modes followed through time, but Laroche et al. (1982)

corrected this to 8-10 weeks based on a later analysis of otolith-estimated

age. During this time, larvae grow from approximately 2.8 mm standard length

(SL) at hatching to 18-22 mm SL, the size at which metamorphosis to the

benthic form occurs (Ahlstrom and Moser 1975; Misitano 1976; Laroche and

Richardson 1979). Growth rate and feeding during the larval phase have been

described by Laroche et al. (1982) and Gadomski and Boehlert (1984),

respectively. Growth rate during metamorphosis has been described by

Rosenberg and Laroche (1982).

Early stage larvae are rarely found in shallow water nursery areas,

although some have been found throughout the brackish water areas of San

Francisco Bay (P. Herrgesell, California Deoartment of Fish and Game,

2
Stockton; pers. comm.) Transforming larvae (stages IV and V of Shelbourne

1957) and recently settled juveniles are common (Westrheim 1955; Ketchen 1956;

Kendall 1966; Smith and Nitsos 1969; Van Cleve and El-Sayed 1969; Eldridge and

Bryan 1972; Olson and Pratt 1973; Pearcy and Myers 1974; Simenstad et ale

1977; Misitano 1976, 1977; Thornburgh 1978; Laroche and Holton 1979; Toole

1980; Bayer 1981; Rosenberg 1982; Rosenberg and Laroche 1982; Rogers 1985).

Transport of larvae into nursery grounds is poorly understood, although

winter-spring oce~nic onshore transport processes, such as those described by

Wyatt et al. (1972) and Parrish et al. (1981), may bring them into nearshore

areas, and "selective tidal transport," defined as the movement of young fish

off the bottom during flood tides, may effect their movement into embayments

(Pearcy and Myers 1974; Rogers 1985; Krygier and Pearcy 1986; Boehlert and

Mundy, unpubl. MS.).

Growth rates of juveniles in both estuarine and nearshore nursery areas

have been described by Westrheim (1955), Kendall (1966), Smith and Nitsos

(1969), Van Cleve and El-Sayed (1969), Rosenberg (1982), Yoklavich (1982),

Rogers (1985), and Krygier and Pearcy (1986). Growth rates of juveniles rai~ed under laboratory conditions have been described by Williams and

Caldwell (1978), Reported growth rates vary considerably, depending on

location of the study and analytical methods. In those studies where the same

methods were used to compare two areas, Rosenberg (1982) and Krygier and

Pearcy (1986) found no significant differences between growth rates of

juvenile English sole from Yaquina Bay and those from a nearshore open coastal

area, while Rogers (1985) found that fish from an offshore nursery area in

Washington had a higher growth rate than those from Grays Harbor. He noted,

however, that his estimates were probably biased by emigration of the largest

juveniles in the estuary to offshore areas.

Embayments appear to be important rearing areas for juvenile English

sole. Using a Peterson mark-recapture estimate, Misitano (1970) determined

that more than 100,000 juveniles were in the channels of Humboldt Bay in

September, 1969. Sole out-migration was already in progress at that time.

Using catch-per-effort data, Rogers (1985) calculated total juvenile English

sole populations of 4.8 million in 1983 and 10.9 million in 1984 in the

8,500 ha of Grays Harbor. In the adjacent offshore study area, which spanned

about 70 km of the coastline and was 18 times the Grays Harbor area, the total

juvenile English sole population was 3.0 million in 1983 and 16.6 million in

1984. In a survey of the shallow waters off the Oregon coast between

Tillamook Bay and the Umpqua River estuary, Krygier and Pearcy (1986) reported

a total population of 64.3 million O-age English sole in May-early June 1978,

which declined by almost 90% to 7 million by the end of June. The total

number of juvenile sole for five estuaries along the same 448 km stretch of

coast was 14 million in May. The estuaries were not sampled again at the end

of June; however, the results of other studies indicate that abundances either

decline much less rapidly in estuaries at this time of year (approximately 50%

in Grays Harbor [Rogers 1985]), change little (Yaquina Bay [Olsen and Pratt

1973]), or even increase (Y~quina Bay [Westrheim 1955]). Assuming a 50%

reduction in abundance within the estuaries, the same number of O-age sole

were in estuaries as offshore by the end of June 1978 in Krygier and Pearcy's

(1986) study area. This analysis suggests that estuaries and embayments may

provide rearing habitat for half the juvenile English sole population, at

3
least between the Umpqua estuary, Oregon, and Grays Harbor, Washington, while

occupying only about 5% of the area in which juveniles are found offshore.

Observations offshore of Yaquina Bay on the incidence of infection by

parasites that can only develop during estuarine residence led Olson and Pratt

(1973) to conclude that virtually all English sole surviving their first year

had reared in the estuary.

No data are available to make offshore-estuary comparisons for the

remaining four-fifths of the Pacific coast where English sole are common. Some

documented estuarine rearing areas such as Elkhorn Slough, San Francisco Bay,

and Humboldt Bay are present along this part of the coast. However,

relatively long stretches are devoid of suitable estuarine rearing areas,

possiblY increasing the proportionate contribution of offshore rearing.

Most English sole leave shallow water nursery areas and emigrate to deeper waters during the fall of their first year at an average size of 110-

150 mm, although some I-age fish remain in the nursery areas through the winter (Westrheim 1955; Ketchen 1956; Olson and Pratt 1973; Samuelson 1973;

Sopher 1974; Ambrose 1976; Simenstad et al. 1977; Rogers 1985). When English

sole reach deeper water their movements become fairly localized, although

there is evidence that adults make seasonal inshore-offshore migrations and

that a small proportion of the population makes extensive coastal movements

(Ketchen 1950; Ketchen and Forrester 1955; Van Cleve and Pruter 1956; Ketchen

1956; Alverson 1960; Forrester 1969b; Jow 1969; Pattie 1969; Barss 1976; Day 1976; Hewitt 1980).

Growth rates of adult English sole have been described by Manzer and

Taylor (1947), Holland (1969), Van Cleve and El-Sayed (1969), Smith and Nitsos (1969~ and Kreuz et al. (1982). Kreuz et al. (1982) demonstrated both

seasonal and annual variations in growth rates within a single population.

Variations were most strongly correlated with temperature. The maximum

recorded age of English sole is 22 years (Leaman and Beamish 1984). The

largest male recorded was 57 cm and the largest female 49 cm (Forrester

1969a).

Adult English sole are opportunistic feeders and prey on benthic

mollusks, ophiuroids, polychaetes, crustaceans (particularly amphipods),

squid, and small fish (Villadolid 1927; Forrester 1969a; Hilasky 1972; Pearcy

and Vanderploeg 1973; Kravitz et al. 1976; Hulberg and Oliver 1979).

English sole are capable of hybridizing with starry flounder (Platichthys

stellatus); the resulting offspring are known as forkline sole or (hybrid)

sole (Inopsetta ischyra) (Aron 1958; Hart 1973).

SPECIFIC HABITAT REQUIREMENTS

Cover and Substrate

Rosenberg (1982) compared growth of English sole in estuarine and coastal

areas and suggested that the primary value of estuaries for English sole is

not abundant food and enhanced growth, but escape from predation. Kendall

4
(1966), Toole (1980), Bayer (1981), and Borton (1982) have described the use

of intertidal habitats by recently metamorphosed English sole. Relatively few

piscivorous fish have been collected intertidally with English sole and, in

spite of possible increased predation by birds (Talent 1984), Toole (1980)

hypothesized that intertidal areas give recently metamorphosed English sole

protection from predation as well as decreased competition for food from

larger flatfish in channels.

Misitano (1970) found that, among intertidal habitats, areas with mud and

sparse eelgrass (Zostera marina) had the densest concentrations of English

sole in Humboldt Bay. Phillips (1984), in reviewing the use of eelgrass beds

by fish, noted that English sole are generally included among the eelgrass

fauna and pointed out the advantages of this cover to juvenile fish. Bayer

(1981), however, did not find English sole to be more abundant in eelgrass

beds than on mud flats in Yaquina Bay. Borton (1982) also found no

siqnificant difference between catch per haul of O-age English sole in

eelqrass and sand habitats of Alki Point in Puqet Sound. Toole (1980) found

that juvenile English sole were common over an-intertidal sand flat, but made

no comparison with other habitats. Kendall (1966) also found English sole to

be common in sandy intertidal areas.

Barry and Cailliet (1981) and Barry (1982) reported juvenile English sole

from shallow marshes and tidal creeks in Elkhorn Slough, and noted that cover

from plants and turbidity of the water helped to make this habitat ideal for

juvenile fish rearin9. The numbers of English sole reported from these

habitats are very low, however, possibly because of the extremes of

temperature and salinity. Chamberlain (in prep.) collected no juvenile

English sole from salt marshes in Humboldt Bay.

In subtidal channels, larger piscivorous fish and marine mammals (Brown

and Mate 19~3) contribute to increased predation, but by virtue of the strong

predominance of small, younq fish in shallow embayments, the rate is probably

lower than in offshore habitats. Rogers (1985) collected juvenile English

sole over sand, silty sand, and mud bottoms, either bare or with varying

types of epibenthic materials (e.g., shell hash, eelgrass). He found no

obvious differences in abundance attributable to substrate or bottom cover.

Similarly, surveys by Sopher (1974) and Ambrose (1976) indicate that trends in

abundance at stations in Humboldt Bay and Elkhorn Slough are controlled by factors other than substrate.

Food

Recently metamorphosed English sole occur in intertidal and shallow

subtidal sand, sand/eelgrass, and mud/eelgrass habitats, where they feed primarily on small epibenthic crustaceans such as calanoid and harpacticoid

copepods and cumaceans (Simenstad et al. 1977; Toole 1980). At lengths of

approximately 50-65 mm English sole begin to feed primarily on polychaetes and

other infaunal organisms. English sole exceeding 85 mm rarely feed in

intertidal areas (Toole 1980). In the subtidal channels of Elkhorn Slough and

Humboldt Bay, larger English sole (generally between 70 and 150 mm) feed

primarily on infaunal polychaetes and bivalves, but also on amphipods and a

variety of other organisms (Ambrose 1976; Collins 1978). On the basis of

5
comparisons between available prey items and composition of prey organisms in

stomach contents, juvenile English sole in estuarine channels are considered

non-selective feeders (Collins 1978).

Analyses of stomach contents by Collins (1978) showed that English sole

collected from stations near the entrance to Humboldt Bay consumed fewer

species, smaller numbers, and lower volumes of prey than elsewhere in the bay.

The percentage of English sole stomachs containing food times the percentage fullness of those stomachs averaged 38.4% for collections from the entrance

channel, and densities of sole were low. Values for the other areas of the

bay ranged from 61.6% to 77.6%, and averaged 69.7%. Also, densities of

English sole were 1.5 to 5 times higher in these other areas of the bay. The

substrate at stations near the entrance channel was shifting gr~vel and coarse

sand. The reduced suitability of the entrance channel as a feeding environment was attributed to sediment instability rather than to the

coarseness of the sediment, because at one sampling area with an even coarser

but stable substrate, densities and stomach fullness were the highest sampled

anywhere in the bay. Benthic samples collected by Boyd et al , (1975) and

analyzed by principal components analysis by Collins (1978) demonstrated that

the entrance channel of Humboldt Bay, with its coarse shifting sediments and

high current velocities, has sparse populations of invertebrate species

consumed by juvenile English sole. Gravel and coarser materials alone can

reduce suitability when they are the dominant substrate types, however, as

reported by Rogers (1985) for offshore sampling.

Temperature

High water temperatures are important in limiting the growth and

production of English sole. Sopher's (1974) lowest abundance stations for

English sole in Humboldt Bay had summer bottom temperatures between 16.7 and

20.3 °c, while bottom temperatures at his highest abundance stations were

13.9-17.3 °C. Yoklavich (1982), using Ambrose's (1976) distributional analysis

of English sole in Elkhorn Slough and records of temperatures during the same

period (Nybakken et al. 1977), noted that the fish tended to avoid waters

which warmed to 17-20 °C. Barry (1982) also found relatively few juvenile English sole in marshes and tidal channels of Elkhorn Slough, which often have

summer temperatures exceeding 18°C (G. Cailliet, Moss Landing Marine

Laboratories, Moss Landing, California; pers. comm. 1985).

Yoklavich's (1982) laboratory experiments showed that growth rates at

17°C were reduced 30% below growth rates at 13 °C. Similarly, Williams and

Caldwell (1978) found in laboratory experiments that growth of juvenile

English sole slowed 28%-47% at 18°C compared to growth at 15 °C, and stopped

completely at 21°C. Stober et al. (1971) and Ames et al. (1978) found the

mean upper lethal temperatures to be between 25.7 and 27.0 °C when juvenile

and adult English sole were subjected to a gradual rise in temperature.

High water temperatures also may be implicated in increased mortality

caused by parasitism. Olson (1976, 1981) has documented temperature

dependence in the infection of English sole by the microsporidean protozoan

Glugea stephani. The upper estuary is the only part of Yaquina Bay where

6
water temperature appreciably exceeds that of the ocean, and this is

juveniles become infected. Approximately 17% of infected fish die

1976).

where

(Olson

Low temperatures encountered in estuaries are probably not limiting. In

Humboldt Bay, most English sole emigrate to offshore waters in the fall' when

bottom temperatures range from 12 to 16°C; however, some English sole remain

in the bay all winter and are found at stations where bottom temperatures are

as low as 9.4 °c (Sopher 1974). Ambrose (1976) was not able to correlate the

emigration of English sole from Elkhorn Slough with marked chanqes in

temperature. Although he found no English sole at any stations between

November and January, the lowest temperature at any station during that time

was 11.5 °c (Nybakken et al. 1977). Rogers (1985) found juvenile English

sole at offshore stations with bottom temperatures as low as 7.6 °c. Mean

bottom temperatures in the offshore nursery area ranged from 9.2 to 11.3 °c.

Salinity

Olson and Pratt (1973) considered salinity to be the major factor

explaining the low density of English sole in the upper estuary of Yaquina Bay

versus the high density in the lower estuary. The lower estuary had bottom

salinities ranging from 25 to 35 ppt while the upper estuary had extremely

variable salinities ranginq from 0 to 34 ppt. Densities of English sole in

the lower estuary averaged 9-120 times greater than upper estuary stations

during April-October and 3-10 times greater during November-March.

Rogers (1985) collected English sole in Grays Harbor at salinities

ranging from 15 to 34 ppt. Over the whole April to October samplinq period in

1983, den~ity was not different among the three sampling areas where mean

salinity exceeded 23 ppt, but density was 63% lower in the one sampling area

where mean salinity was <23 ppt.

In Humboldt Bay, bottom salinities at Sopher's (1974) stations with the

lowest numbers of English sole ranged from 21.8 to 29.3 pot during the rainy

season of October-March and from 31.7 to 32.9 ppt during April-September. In

contrast, stations with highest sole abundance ranged from 27.4 to 31.9 ppt

during the rainy season and from 31.8 to 33.8 ppt during the remainder of the

year. Catch per effort was approximately three ~imes greater at high sole

abundance stations than at the low sole abundance sbations.

Ambrose (1976) never encountered salinities below 25.9 ppt at any of his

stations in Elkhorn Slough (Nybakken et al. 1977), and no correlations between

salinity and catch of English sole are apparent. Analysis of the numbers of

English sole versus salinity at stations in Netarts Bay, Oregon, also showed

no obvious correlations between salinity and catch (W. Pearcy, College of

Oceanography, Oregon State University, Corvallis; pers. comm. 1986).

Dissolved Oxygen

Levings (1980) demonstrated the response of adult English sole to changes

in dissolved oxygen under natural conditions. A normal density was maintained

7
until dissolved oxygen dropped below about 3 mg/l. English sole density

continued to decline as dissolved oxygen decreased, until all English sole had

either left the area or died by the time dissolved oxygen reached 1.0 mg/l. It

was unknown whether the English sole were physically impaired by low dissolved

oxygen levels or left the areas as their infaunal prey items died. Stations

containing English sole outside of the oxygen-depleted area had dissolved

oxygen levels up to 6.0 mg/l.

Special Considerations

In discussing declines in the commercial fishery for English sole in the

Strait of Georgia, Ketchen et al. (1983) proposed that pollution of the

intertidal and shallow subtidal habitats occupied by 0- and 1-year age group

fish may have increased the mortality of these juveniles and reduced

recruitment to the fishable stocks. Cancerous tumors were noted in English

sole over 50 years ago (Pacis 1932), and in recent years a large body of

literature has developed regarding the possible causes, mechanisms of

transmittal, and degree of mortality associated with the condition (e.g.,

Angell et al. 1975; Stitch et al. 1976; Varanasi and Gmur 1981; Krahn et al.

1984; Malins et al. 1984). Carcinogenic agents most closely examined in

recent studies were hydrocarbon derivatives.

Stitch et al. (1977) demonstrated that the incidence of skin tumors on

juvenile English sole in the Strait of Georgia is inversely related to

distance from major urban centers, the proposed sources of carcinogenic

contamination. However, other studies such as those of Cooper and Keller

(1969) and Kelly (1971) in San Francisco Bay have not shown this relationship.

HABITAT SUITABILITY INDEX (HSI) MODEL

Model Apolicability

Geographic area and cover types. This model is designed to apply to

juvenile English sole in estuaries and coastal lagoons year-round. The use of

estuarine areas by juvenile English sole begins with the immigration of

metamorphosing postlarvae and recently metamorphosed juveniles in late winter

and spring and ends with the emigration of most juveniles in the fall. Small

numbers overwinter in estuaries and emigrate the following spring. The major

area of application is along the Pacific coast from central California to

northern Washington. Marsh and marsh creek habitat are used to such a minor

extent (Barry and Cailliet 1981; Barry 1982; Chamberlain, in prep.) that they

are excluded from consideration in this model.

Minimum habitat area. This is defined as the mlnlmum area of contiguous

suitable habitat required for a species to successfully live and reproduce. No

attempt has been made to establish a minimum habitat area for English sole

because they must migrate between inshore nursery areas and offshore spawning

grounds. This model only applies to estuarine nursery areas.

Verification level. This model has not been verified in the field. It is

based upon existing information documented in the literature; however, none of

8
the studies examined were designed to compare different habitats within an

estuary in the comprehensive manner addressed in this model. Adjustments to

the model will undoubtedly be required if adequate field tests are conducted.

The followinq biologists have reviewed this model for accuracy and

applicability to various areas of the Pacific coast: Greqor CaillJet, Moss

Landing Marine Laboratories; William Pearcy, Oregon State University College

of Oceanography; and Perry Herrgesell, California Department of Fish and Game,

Stockton. Their comments have been incorporated when feasible, but the

authors are responsible for the final version of the model.

Model Description

Overview. The model is based upon the assumption that any environmental

variable that has an impact on the growth, survival, distribution, or

abundance of juvenile English sole can be expected to have an impact on the

carrying capacity of its habitat. Juvenile English sole habitat is assumed to

consist of two major life requisite components: food and water quality.

Variables contributing to each of these components are described below and the

relationships between habitat variables, habitat components, and habitat

suitability are described in Figure 1. Cover was not included as a component

in the model because the importance of cover in reducing predation and

influencing the abundance of Enqlish sole has not been demonstrated in the

existinq literature.

Food Component. Enqlish sole are generalized benthic feeders. By the

criteria of stomach fullness and density, the suitability of a site for

English sole is positively correlated with prey abundance (Collins 1978);

however, measurement of prey density is usually beYond the capabilities of a

HEP assessment team and is only possible for baseline assessments. Where

predictions of future conditions are required, the state of the food resource

would have to be estimated in terms of responses to alterations in the

Habitat variables

VI Hydrodynamic regime .

V3 Bottom water temperature

Bottom mean salinity .........

Vs Dissolved oxygen

concentration

Component

Food

Water quality

HSI

Figure 1. The relation of habitat variables, life requisite components, and

the habitat suitability index (HSI) for juvenile English sole in estuaries and

coastal lagoons.

9
physical environment. Water velocity and substrate type probably are the two

most important factors influencing the food resource for English sole. The

suitable ranges are broad. High water velocities can limit the food resource

by making the substrate so unstable as to prevent the development of a dense

benthic community (Collins 1978). Low velocities become limiting by not

providing sufficient exchange of materials to support a dense benthic

assemblage. Weak currents may also limit the accessibility of the interior

parts of large systems such as San Francisco Bay. There, only in years when

high freshwater inflow induces a countercurrent of marine water at the

bottom, do substantial numbers of English sole get beyond the Central Bay (Armor and Herrgesell 1985). However, the actual velocities at which

suitability diminishes have not been determined. Hiqh densities of food

orqanisms and English sole have been found associated with substrates

containing as much as 20% gravel (>2 mm diameter particles) (Collins 1978);

but English sole densities were low where gravel or rocks predominated (Rogers

1985), presumably because prey either were not abundant or were not

accessible. There is no indication of a decrease in suitability directly as a

result of very fine sediments.

For the purposes of this model, it is sufficient to characterize the

hydrodynamic regime in terms of the gross morphology and surficial

characteristics of the estuary (VI). The shifting deposits of the tidal

deltas at the mouths of estuaries are low in suitability, as are any other

areas where rapid erosion or deposition is occurring. At the other extreme,

suitability is low in peripheral areas where tidal exchange is impeded or the

turnover rate of the water is low, as in a deep cul-de-sac. In San Francisco

Bay, it will also be necessary to take account of distance from the mouth.

Results of the Bay-Delta study will allow quantification of this factor in the

near future (P. Herrgesell, pers. comm.). The substrate variable (V2)

indicates the reduced suitability of areas with substantial amounts of gravel

or rocks.

Water Quality Component. Water quality variables used for the English

sole HSI model are bottom water temperature (V3), bottom mean salinity (V4),

and dissolved oxygen concentration (Vs), all of which affect growth, survival,

and abundance. Because of the high mobility of English sole and their ability

to temporarily avoid suboptimal conditions, the duration of stressful high

temperatures and low oxygen concentrations was regarded as more important than how stressful those conditions may be. Therefore, the temperature variable

(V3) was constructed as the number of months in which bottom mean temperature

is high enough to reduce suitability and presumably induce avoidance, as

inferred by Yoklavich (1982). The threshold for suboptimal temperature was

selected as 15°C, approximately midway between the highest temperature

without and the lowest temperature with detectable effects on growth in the

experiments of Williams and Caldwell (1978) and Yoklavich (1982).

The oxygen variable (Vs) was constructed as the number of months in which

the mean concentration was below the threshold level of 3 mg/l (Levings 1980).

Note that this measure will overestimate suitability where anaerobic

conditions cause a die-off of food organisms. In this case, unsuitable

conditions will persist until prey populations recolonize, and an adjustment

should be made.

10
Classes 1 and 2 of the salinity variable (V4) are based on data for

Elkhorn Slough, California (Ambrose 1976; Nybakken et ale 1977), and Grays

Harbor, Washington (Rogers 1985). We are aware of no reports of English sole

from sites where mean salinity from April to October was <15 ppt and have

defined Class 3 accordingly.

Distance from sources of environmental contaminants was not included as a

variable because the clearest evidence of this relationship is based on a

relatively large section of coastline (Stitch et al. 1977). Within a given

estuary, this relationship has not been demonstrated (e.g., Cooper and Keller

1969; Kelly 1971; discussion in Angell et al. 1975).

Suitability Index (51) Graphs for Model Variables

This section provides graphic representations of the relations previously

described between the habitat variables (VI-VS) and estuarine (E) habitat

suitability for juvenile English sole. An SI value of 1.0 indicates optimal

conditions and a value of 0 indicates unsuitable conditions. Data sources and ass~mptions associated with documentation of the SI graphs are listed in

Table 1.

-

IHabitat Variable Suitability Graph

E VI Hydrodynamic regime 1.0

1) High energy (tidal delta, )( O~8

areas of rapid erosion or CD

'0 deposition) c

- 0.6 2) Intermediate energy (main >- body of estuary with

stable substrates) :E 0.4

3) Low energy (peripheral CD

::

areas with impacted tidal 0.2 exchange or deep cul-de- UJ

sacs).

0.0

11

1 2

Class

3
Habitat Vari abl e Suitabil ity Graph

1.0 E VI* Percent reduction in tidal

range (alternative to VI for

peripheral areas with )( 0.8

impacted tidal exchange). Q)

'tJ

C

>- 0.6

:=

:s 0.4 as

.:t:

:::s

en 0.2

0.0

0 20 40 60 80 100

"

1.0

E Vz Substrate composition -

percentage by weight of a )( 0.8 sample >2 mm in diameter. Q)

'tJ

C

- 0.6

>-

.:t:

:sas 0.4

.:t:

:::s

en 0.2

0.0

0 20 40 60 80 100

"

1.0

E V3 Water temperature - number of months of sub- )( 0.8 optimal bottom temperature Q)

'tJ from April to October c

(mean temperature >15 °C). - 0.6

>- :

-

Q 0.4 as

:

:::s

en 0.2

0.0

0 7

Months

12
Habitat Variable Suitability Graph

-

1.0

0.2

0.0

0.4

0.6

1) <15 ppt

2) 15-23 ppt

3) >23 ppt

V4 Bottom mean salinity

April to October

E

1 2 3

Class

1.

E Vs Dissolved oxyqen concentration - number of months 0.8 of suboptirna 1 dissolved

oxyqen (mean dissolved '0

oxygen <3 mg/l). .5 0.6

>-

:5

:s 0.4

CIS

..

i 0

o.

0 7

Months

Table 1. Data sources and assumptions for habitat variables included in the

juvenile English sole HSI model.

Variable and source Assumptions

Boyd et al . (1975)

Collins (1978)

G. Cailliet (pers. comm.)

W. Pearcy (pers. comm.)

High energy areas are low in suitability

because the substrate is not stable enough

for a dense benthic assemblage to develop. Peripheral areas with impeded tidal

exchange or low rates of turnover of water

are low in suitability because the exchange

(continued)

13
Variable and source

VI (continued)

V2 Boyd et al. (1975)

Coll ins (1978)

Rogers (1985)

Table 1. (Concluded)

Assumptions

of materials is too slow to support a

productive benthic community. Also, access

by larval or weak-swimming young juvenile

English sole may be impaired.

Sediments with as much as 20% gravel were

suitable by the criteria of stomach

fullness and density of English sole.

Densities were low where gravel and rocks

were the dominant substrate type; however,

even 100% gravel or coarser substrate is

assumed to provide some food for English sole.

Stober et al . (1971)

Sopher (1974)

Ambrose (1976)

Olson (1976)

Nybakken et al. (1977)

Ames et al. (1978)

Williams and Caldwell (1978)

Olson (1981)

Barry (1982)

Yoklavich (1982)

V4 Nybakken et al . (1977)

Rogers (1985)

Vs Levings (1980)

Infection by a parasite that only develops

at water temperatures >15 °c can cause 17%

mortality of English sole. Growth rates are

30%-50% less at temperatures >17-18 °c as

compared to lower temperatures, and English

sole avoid this range. Death occurs between

25.7 and 27.0 °C. Duration of suboptimal conditions is assumed to be more important

than the severity of suboptimal conditions

on the grounds that English sole are highly

mobile, will avoid even moderately stressful temperatures, and can quickly

return when temperatures cease to be

stressful.

On average, locations in Grays Harbor with

mean salinities <23 ppt from April to

October supported only 40% of the densities

of English sol~ that higher salinity areas

supported. In Elkhorn Slough, salinities

<25 ppt were not observed, and densities at

different sampling locations were not

correlated with salinity.

Suitability is optimal with a dissolved

oxygen concentration above 3 mg/l. Habitat

is unsuitable at 1 mg/l. Duration of

suboptimal conditions is assumed to be more

important than the severity of suboptimal conditions for reasons described for'V 3 •

14
Component Index Equations and HSI Determination

The SI values for the habitat variables are combined so component scores

for juvenile English sole can be obtained. The suggested equations for

obtaining food or water quality values for juvenile English sole follow.

Component

Food (F)

Equation

VI or V2 , whichever is lower

or VI* or V2 , whichever is lower

Water quality (WQ) V3 , V4 , or Vs, whichever is lowest

HSI = F or WQ, whichever is lower

The equation for determining HSI is based on the limiting factor

concept. No cumulative or compensatory relationships between variables are

assymed.

Sample data sets from which HSI values have been generated are given in

Table 2. Data set 1 represents a site on the tidal delta where the

instability of the substrate is the limiting factor; data set 2, a site in the

main body of the estuary where all conditions are satisfactory; and data set

3, a site in a shallow tributary embayment connected to the main stem of the

estuary through a culvert, where the impairment of tidal exchange probably is

the limiting factor, which in turn causes suboptimal conditions of salinity

and temperature. Although the data sets are not actual field measurements,

they do represent the kinds of values expected for estuarine habitats used by

English sole. The HSI's calculated from these hypothetical data reflect the

relative abundance of English sole in habitats with the characteristics listed

in Table 2.

Field Use of the Model

Small «85 mm) juvenile English sole are found in both intertidal and

subtidal areas within bays and estuaries. The importance of intertidal areas

for feeding and possibly refuge for small English sole has been described by Toole (1980). Large (85-140 mm) juvenile Englis~ sole are rarely found

intertidally, consume different prey items, and exhibit different foraging

strategies from small juveniles collected intertidally. Because intertidal

and subtidal habitats play different but equally important roles in the

ecology of juvenile English sole, they should be treated separately when

applying this model. Losses of shallow water habitat (to 1 m below Mean Lower

Low Water) can only be compensated by replacement or improvement of shallow

water habitat. Similarly, losses in deep water habitat can only be compensated

by replacement or improvement of subtidal areas.

The HSI model for English sole is simple. Nevertheless, it demands

considerable field sampling. An application using all five variables requires

at least monthly, and preferably more frequent, sampling of the water quality

variables from April to October. Rarely, a university study or a major

15
monitoring program may provide the necessary information, but only if the

timing and location of samples are appropriate. Extrapolation from outside

the project area should not be considered except in consultation with experts.

Alternatively, the model can be applied using primarily aerial

photography, maps, and existing data compilations. In this case, only

Variables 1 and 2 would be evaluated. Field observations would be used to

verify and update interpretations from these secondary sources. Applications

scaled down in this fashion will sacrifice some resolution but will still be

valid in our opinion, because the hydrodynamic regime is the driving force

determining water quality in most instances. Thus, where water quality limits

habitat suitability, some impediment to water exchange usually is responsible

for the impairment of water quality. Close monitoring of water quality will

yield a more precise determination of habitat suitability than hydrodynamic

regime and substrate can provide by themselves, but this greater sensitivity

may not always be necessary. The abbreviated version of the model is most

likely to be in error at the southern end of English sole's geographic range,

where limiting ~emperatures may not be adequately predicted by the

hydrodynamic regime, and in areas receiving major alloqenic discharges.

Suggested methods for measuring the variables are presented in Table 3.

Interpreting Model Outputs

The proper interpretation of the English sole HSI is one of

comparison. This model can be used to compare different habitats or the same

habitat at different times. The habitat with the higher HSI should be the

area that could potentially support more juvenile English sole.

Table 2. Calculations for food (F) and water quality (WQ) suitability indices

(SI) and the habitat suitability index (HSI) for three sample data sets, using

the juvenile English sole variables (V) and model equations.

Component

Data set 1

Data SI

Data set 2

Data SI

Data set 3

Data SI

VI 1 0.2 2 1.0

VI* 70% 0.3

V2 30% 0.73 10% 1.0 0% 1.0

V3 o mo 1.0 1 mo 0.86 3 mo 0.57

V4 32 ppt 1.0 25 ppt 1.0 20 ppt 0.4

Vs o mo 1.0 o mo 1.0 2 mo 0.72

F 0.2 1.0 0.3

WQ 1.0 0.86 0.4

HSI 0.2 0.86 0.3

16
Table 3. Suggested techniques for field measurements of variables used in the

English sole HSI model.

Variable Techni ques

The shifting deposits of tidal deltas at the mouths of estuaries are

evident in aerial photographs. Other areas subject to rapid erosion

and depositions can be identified from time series of aerial

photographs and by boaters familiar with the estuary. Areas with

impeded tidal exchange or a low turnover rate of the water usually

are the result of human activities. Look for linear features at the

periphery of the estuary - causeways partially separating peripheral

areas from the main body of the estuary, box-ended deep areas

abutting the shore. For peripheral areas with impeded tidal

exchange, an alternative, VI* - percent reduction in the tidal

range, provides a more precise criterion for evaluating suitability.

It requires measurement of high and low water levels on a spring

tide sequence inside and immediately outside the area of suspected

impeded tidal exchange. The tidal range inside is expressed as a %

of the tidal range outside.

Substrate samples can be obtained with an Ekman or a Ponar

Substrate composition can be determined from the proportion of

weight of substrate retained by a screen with 2 mm openings.

graded series of sieves is used, an analysis for skewness

indicate areas of rapid erosion or deposition (Folk 1974). See

grab.

known

If a

wi 11

VI •

Bottom water temperatures can be obtained by using either a

thermistor or a thermometer and a water sample collected with a

water bottle (American Public Health Association et al. 1976).

Salinity can be measured by titration or by a refractometer,

hydrometer, or conductivity meter (Strickland and Parsons 1968).

Dissolved oxygen can be measured by Winkler titration or with an

oxygen meter (Strickland and Parsons 196~; American Public Health

Association 1976).

17

Ahlstrom, E.H., and H.G. Moser.

the California current region: Fish. Invest. Atlas, Vol. 23.

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24

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27
50171-101

REPORT DOCUIiENTAnoN II. ItI[P'O," NO.

Report 82(10.133) 12.

s. !teeillie"". Acc..._ No.

PAGE Biological .. TItle end IuIltltle I. Report Dele

February 1987

Habitat Su i t abi 1ity Index Models: Juvenile English Sole L

7. Author<.) I. ~rfo""l... O..enlut*o It..... No.

C.L. Toole R.A. Barnhart and C.P. Onuf ,_ ~rfonnl.. O..enlletlon Neme end Add ..... 10. I"rolectlTnk/Wortl Unit No.

U.S. Fish and Wildlife Service

California Cooperative Fishery Research Unit u. eomrect(C) or Orent(O) No.

Humboldt State University (C)

Arcata. CA 95521 (G) 12. SlIOfIsorl... O..enlletlon Neme .nd Addre••

13. Type of Report & ~riocl Covered

National Wetlands Research Center

Research and Development

Fish and Wildlife Service 14.

U.S. Department of the Interior

Washington, DC 20240

15. Suppl.m.nte., Not••

o 11. Abstrect (Umlt: 200 word.)

A review and synthesis of existing information were used to develop a model for

evaluating juvenile English sole habitat. The model is scaled to produce an index

between 0 (unsuitable habitat) and 1 (optimal habitat) along the Pacific coast of

North America from central California to southern British Columbia in estuaries

and lagoons. Habitat suitability models are designed for use with the Habitat

Evaluation Procedures previously developed by the U.S. Fish and Wildlife Service.

Guidelines for model application and techniques for measuring model variables are

provided.

17. Docu ....nt An.'ysi• •• O.scrlptors

Estuaries

Fishes

Habitat

Mathemat ica1 models

b. Id.ntlfi.rs/Open·End.d Term.

Habitat Suitability Index

Impact assessment

Eng 1ish sole

Parophrys vetulus

c. COSATI Fi.ld/Group

11. A".U.blllty St.t.m.nt 19. Security cr... (Thi. R.port) 21. No. of P••••

Unclassified 27

Unlimited -- 20. Security Cles. (Thl. P....) %2. Pric.

Unclassified

(S.. ANSI-Z39.1Bl

U.S. GOVERNMENT PRINTING OFFICE: 1987-761-070140006

OP'T'ONAL fOR.. 272 (4-77)

(Fo",...ly NTl5-35)

Oepertm.nt of Comm.n:.
Take Pride in America

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