Biological Report 82(10.92)

JUNE 1985

HABITAT SUITABILITY INDEX MODELS:

SOUTHERN AND GULF FLOUNDERS

SK

361

. U54

no . 82-

10 .92

I and Wildlife Service

Department of the Interior
This model is designed to be used by the Division of Ecological Services in

conjunction with the Habitat Evaluation Procedures.

This is one of the first reports to be published in the new "Biological

Report" series. This technical report series, published by the Research and

Development branch of the U.S. Fish and Wildlife Service, replaces the

"FWS/OBS" series published from 1976 to September 1984. The Biological Report

series is designed for the rapid publication of reports with an application

orientation, and it continues the focus of the FWS/OBS series on resource

management issues and fish and wildlife needs.

, - " •• I
MODEL EVALUATION FORM

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1010 Gause Boulevard

Slidell, LA 70458

Thank you for your assistance.

Species _ Geographic

Location---------------------------------- Habitat or Cover Type(s)----------------------------- Type of Application: Impact Analysis Management Action Analysis _

Baseline Other---------------------------------- Va ri ab1es Measu red or Eva1uated-------------------------

Was the species information useful and accurate? Yes No

If not, what corrections or improvements are needed?-----------
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Appropriate? Yes _

Clearly defined? Yes

Easily applied? Yes----

collection

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No

No----

of field data:

If not, what other data collection techniques are needed? _

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Yes----

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Biological Report 82(10.92)

June 1985

HABITAT SUITABILITY INDEX MODELS: SOUTHERN AND GULF FLOUNDERS

Kevin M. Enge

and

Rosemarie Mulholland

Florida Cooperative Fish and Wildlife Research Unit

School of Forest Resources and Conservation

117 Newins-Ziegler Hall

University of Florida

Gainesville, FL 32611

Project Officer

Carroll L. Cordes

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

Performed for

National Coastal Ecosystems Team

Division of Biological Services

Research and Development

Fish and Wildlife Service

U.S. Department of the Interior

Washington, DC 20240
DISCLAIMER

Mention of trade names in this report does not constitute endorsement or

recommendation for use by the Federal Government.

This report should be cited as:

Enge, K. M., and R. Mul hall and. 1985. Habitat suitabil ity index model s:

southern and gulf flounders. U.S. Fish Wildl. Servo Biol. Rep.

82(10.92). 25 pp.
PREFACE

The habitat suitab-ility index (HSI) models for the southern and gulf

flounders are 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 models were developed from a review and

synthesi s of existi ng i nformati on and are scaled to produce an index of

habitat suitability between 0 (unsuitable habitat) and 1 (optimally suitable

habitat). Assumptions involved in developing the HSI models and guidelines

for their application, including methods for measuring model variables, are

described .:

The models are hypotheses of species-habitat relationships, not

statements of proven cause and effect. They have not been field-tested. For

these reasons, the U.S. Fish and Wildlife Service encourages model users to

convey comments and suggestions that may help increase the utility and effect i veness of thi s habitat-based approach to fi sh and wil dl i fe management.

Please send any comments or suggestions about the HSI models to the following

address.

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

iii

CONTENTS

PREFACE . • • •

AC KNOWL EDGME NTS

INTRODUCTION •• • . • • • • • • .

Distribution •• . • . •.••...••••

Life History Overview. . • . . ••• SPECIFIC HABITAT REQUIREMENTS. • ••.

Adu 1t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Emb ryo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

La rva . . . • . . • . . • . • . . . . • . . . . .

Juvenile . . . . . . HABITAT SUITABILITY INDEX (HSI) MODELS •..•.•

Model Applicability •.....•....

Model Description •.•.•.••••.•...••

Suitability Index (SI) Graphs for Model Variables.

Component Index (CI) and HSI Equations. Field Use of Models ..••

Interpreting Model Outputs

LITERATURE CITED. . • . . . •

v

iii

vi

1

1

1

4

4

7

7

8

10

10

10

12

16

18

19

21
ACKNOWLEDGMENTS

The habitat suitability index model for the southern and gulf flounder

was reviewed and constructively criticized by George H. Burgess, Jr., Florida

State Museum, Gainesville; John H. Finucane, National Marine Fisheries

Service, Panama City, Florida; and Elmer J. Gutherz, National Marine

Fisheries Service, Pascagoula, Mississippi. Thorough evaluations of model

structure and functional relationships were provided by personnel of the U.S.

Fish and Wildlife Service's (FWS) National Coastal Ecosystems Team (NCET).

Supporting narrative and model reviews also were provided by Regional FWS

personnel. Funding for model development and publication was provided by the

FWS. The cover illustration was prepared by David S. Maehr, Florida Game and

Fresh Water Fish Commission.

vi
SOUTHERN FLOUNDER (Paralichthys lethosti~ma)

GULF FLOUNDER (Paralichthys albigutta

INTRODUCTION

Both the southern and gulf flounders (paralichthrS lethostigma, f. albi~utta) are important commercial and recreationa species. Catch

statlstics for flounder do not differentiate between species; however,

southern flounders are more common than gulf flounders except on the gulf

coast of Florida (Topp and Hoff 1972). The commercial fishery consists of

incidental catches by shrimp trawlers. A total of 853,162 kg (1,880,900 lb)

of unclassified flounder worth $572,142 was caught from the Gulf of Mexico in

1976 (U.S. Dep. Commerce 1980). The sport fishery consists of both

hook-and-line fishing and gigging.

Distribution

The southern flounder is found along the shores of bays, sounds, and

lagoons in comparatively shallow estuarine waters, and occasionally in

freshwater, from Albemarle Sound, North Carolina, to northern Mexico (Gutherz

1967). The gulf flounder occurs on the Continental Shelf and in large bays

and sounds from Cape Lookout, North Carolina, to the Laguna Madre of southern

Texas (Gutherz 1967). Southern flounder are most abundant in the western

Gulf of Mexico, whereas gulf flounder are most abundant in Florida (Topp and

Hoff 1972). Southern flounder apparently do not 1ive in the area from the

Loxahatchee River along the southern Florida Atlantic coast to the

Caloosahatchee River along the southern Florida gulf coast.

Life History Overview

Life stages. Developmental stages were defined by Balon (1975) as

embryonic, larval, juvenile, and adult. The buoyant, pelagic eggs of

Paralichthys spp. contain a single oil globule in the yolk (Norman 1934). In

the laboratory, eggs of southern flounder hatch in 61 to 76 h (Arnold et al.

1977). The embryo becomes a larva when it switches from exclusively

endogenous feeding to exogenous feeding, and the larva becomes a juvenile

after metamorphosis produces an adultlike form (Balon 1975). In the

laboratory, southern flounder larvae began to metamorphose at 40 to 46 days

(8 to 11 mm, 0.3 to 0.4 inch total length, TL) and completed metamorphosis at

50 to 51 pays; they then assumed a completely demersal existence ,(Arnold et

al. 1977). The onset of metamorphosis probably is-more strongly related to

IAll measurements of length in this report are for total length unless

otherwise indicated.

1
size than age; at higher temperatures one would expect metamorphosis at an

earlier age but at a similar size (Policansky 1982). Southern flounder are

considered juveniles at about 11 to 300 mn (0.4 to 11.8 inches), the size at

which females may first become gravid (Stokes 1977; Etzold and Christmas

1979; Nall 1979). Female gulf flounder may become gravid at 145 mm (5.7

inches) (Topp and Hoff 1972).

Growth. Hi1debrand and Cable ( 1930) descri bed the growth of

paralichth1s spp. collected at Beaufort, North Carolina. Recently hatched

flounders 2.5 mm, 0.1 inch) were symmetrical and planktonic. At 7 ITIm (0.3

inch) TL, larvae lost their synunetry, and their right eye began migrating

dorsally. At 11 mm (0.4 inch), the right eye was on the ridge of the head,

the body was increasingly compressed, and the left side was more pigmented

than the right. At 16 mm (0.6 inch), flounders had a form and shape resembling the adult: the neurocranium had rotated 90°, and both eyes were on the

left side of the body. At 26 mm (1 inch), the body's shape was virtually

identical to that of an adult, and its pigmentation was restricted almost

solely to the left side. By the time a flounder was 77 mm (3 inches) long,

its body was completely scaled and had the shape, form, and pigmentation of

the adult.

There is some evidence that southern flounder from different areas grow

at different rates (Stokes 1977; Etzold and Christmas 1979; Nall 1979),

partly due to differences in genetic stock, prey availability, temperature,

or salinity (Deubler 1960; Stickney and White 1973; Peters and Kjelson 1975;

Laurence 1977).

Nall (1979) noted that the growth of southern flounder from the Florida

Panhandle and Alabama was isometric (weight increases directly with length),

and the greatest absolute and relative growth occurred in their first 2 years

of life, whereas the gulf flounder's growth was allometric (weight increases

slower than length). Males of both species grew slower than females, but

individuals of equal sizes in both sexes had similar weights (Stokes 1977).

The largest southern flounder collected by Nall (1979) was 492 mm (19.4

inches) standard length (SL) and was 8 years old. Etzold and Christmas

(1979) collected 4-year-olds up to 480 mm (18.9 inches) long. Stokes (1977)

found 5-year-old females up to 620 mm (24.4 inches) and 3.9 k~ (8.6 lb) and

3-year-old male southern flounders up to 320 mm (12.5 inches) and 0.36 kg

(0.79 lb). The largest southern flounder on record was 762 mm or 30.0 inches

long (Ginsburg 1952).

Gulf flounder are on the average smaller than southern flounder; most

gulf flounder are "less than 254 mm (10.0 inches) l onq (Ginsburg 1952).

Stokes (1977) collected 3-year-old female gulf flounder up to 420 mm (16.5

inches) and 1.01 kg (2.2 lb) and 2-year-old males up to 290 nun (11.4 inches)

and 0.27 kg (0.6 lb). The largest gulf flounder on record was 710 mm or 28.0

inches long and weighed 5 kg or 11 lb (Vick 1964).

Reproduction. I n Alabama and northern Flori da , Na 11 (1979) found

developing eggs in all female southern flounder over 6 years old but in only

8%, 5%, and 18% of 4-, 5-, and 6-year-olds, respectively. The smallest

2
maturing southern flounder female Nall (1979) collected was 308 mm (12.1

inches). Southern and gu 1f flounders from Texas progressed from havi ng no

apparent gonads to a maturing stage in their first year to being gravid

for the first time at 2 years (Stokes 1977). In Mississippi, southern

flounder began to mature as 2-year-olds, and all appeared to be mature at

3 years (Etzold and Christmas 1979). Gonadal examinations of gulf flounder

indicated that females mature at about 145 mm (5.7 inches) (Topp and Hoff

1972).

Adult southern and gulf flounders "in Texas waters were gravid from

mid-September through December (Stokes 1977). Ginsburg (1952) concluded that

southern and gulf flounders spawned offshore in the Gulf of Mexico in late

fall and early winter and some spawning extended beyond winter. Appearance

dates of newly metamorphosed juveniles support the opinion that gulf flounder

spawn from November through February in the Gulf of Mexico (Reid 1954;

Springer and Woodburn 1960; Topp and Hoff 1972).

In a laboratory environment, male southern flounder began following

gravid females 3 weeks before spawning. At midday, spawning females swam to

the surface and released eggs that a single attending male immediately fertilized. Each female produced about 40,000 eggs (an average of 9231 per

spawn) of whi ch 30% to 50% were fertil i zed and 6% to 35% of those hatched

(Arnold et al. 1977).

Movements. Recently hatched paralichthfs spp. larvae (2.5 mm, 0.1 inch)

were captured only at sea off the coast 0 North Carolina (Hildebrand and

Cable 1930). Newly metamorphosed juveniles (8 to 16 mm, 0.3 to 0.6 inch)

were captured from December through April in estuarine nursery areas, where

they remained for 18 to 20 months before mov i ng into the ocean to spawn

(Powell and Schwartz 1977). Young Paralichthys spp. from 6 to 10 mm (0.2 to

0.4 inch) were collected in North Carolina waters from 24 km (15 mi) offshore

to 11 km (7 mi ) into estuaries (Hildebrand and Cable 1930). Juvenile

flounder from 15 to 100 rnm (0.6 to 3.9 inches) appeared to migrate from the

main bodies of estuaries into brackish and freshwater creeks and ditches

(Hildebrand and Cable 1930). Because southern and gulf flounders have an

extended spawning season of variable duration (Stokes 1977) and are serial

spawners (Arnold et al. 1977), sizes of juveniles appearing in estuaries may

vary among months within a year and between years at the same locality.

Peak appearance of juvenile flounder in estuaries along the Atlantic

coast usually occurs at a time when stratification and tidal exchange ratios

are at a yearly maximum. Juveniles entering estuaries effectively

enter freshwater by apparently "riding out" the ebb tide on the bottom below

the saltwater boundary and gradually migrating upstr-eam-on the flood tide.

Active migration into marshes may occur when juveniles move to the surface at

night and are then carried laterally by the flood tide into tidal creeks

(Weinstein et al. 1980).

Adult southern and gulf flounders migrate from estuarine to marine water

in fall or early winter (Hildebrand and Cable 1930; Simmons and Hoese 1959;

Stokes 1977). Adult southern flounder migrate from Texas bays and estuaries

into the Gu 1f of Mexi co from September through December; males generally

3
migrate gulfward earlier than females (Simmons and Hoese 1959; Stokes 1977).

Adults in Texas return to estuaries from February through June, although

older males may remain in the gulf (Simmons and Hoese 1959; Stokes 1977). In

anyone year, not all adults migrate from an estuary in fall and early winter; those that remain in winter usually move into deeper estuarine waters

in response to low water temperature (Ogren and Brusher 1977; Stokes 1977).

Stokes (1977) found that migrating adults were gravid, whereas adults

remaining in estuaries in fall were not yet gravid. The migration of adult

southern and gulf flounders was generally preceded by a drop in water

temperature of 4° to 5° C (7.2° to 9.0° F) (Stokes 1977). Juvenile southern

flounder (17 to 54 mm, 0.7 to 2.1 inches) were collected in Texas estuaries

from December throuqh May (Gunter 1945; Simmons and Hoese 1959; Arnol d et a1.

1960; Breuer 1962; Stokes 1977).

Tagged southern flounder showed no consistent pattern of activity

between and within Texas bays; distances between points of recapture varied

from 0 to 18.2 km (11. 3 mi) over a peri od of 3 to 212 days. The fastest

rate of movement was 9.3 km (5.75 mi) in 3 days. A year after being tagged in Aransas Bay, a fish was recovered more than 451 km (280 mi) away in the

gulf. Gulf flounder more than 2 or 3 years old probably live exclusively in

the gulf (Stokes 1977).

SPECIFIC HABITAT REQUIREMENTS

Adul t

Temperature. In gulf coast populations of southern and gulf flounders,

adults are offshore in the warmer gulf waters during cooler weather

(September through April) and inhabit the warmer estuarine waters in warmer

weather (May through August). In a Texas estuary, Gunter (1945) collected

southern flounder at temperatures of 9.9° to 30.5° C (49.8° to 86.9° F).

In Louisiana estuaries, Perret (1971) collected adult southern flounder at

temperatures of 5.0° to 34.9° C (41.0° to 94.8° F), and Barrett et al. (1978)

collected southern flounder at 10.4° to 29.8° C (50.7° to 85.6° F). In

Florida, Reid (1954) collected gulf flounder in the Cedar Key area at surface

temperatures of 8.3° to 30.6° C (46.9° to 87.1° F), and Springer and Woodburn

(1960) collected gulf flounder in the Tampa Bay area at 11.2° to 32.5° C

(52.2° to 90.5° F). In Aransas Bay, Texas, Stokes (1977) collected both

southern and gulf flounders at temperatures of 10.0° to 31.0° C (50.0° to

87.8° F), and adults left the bay for gulf waters during the period when

the mean water temperature dropped from 23.0° C (73.4° F) in October to

14.1° C (57.4° F) in December. Of seven peak periods of emigration from

Aransas Bay, four occurred when cold fronts reduced water temperatures by 4°

to 5° C (7.2° to 9.0° F) (Stokes 1977). Based on findings for other fish

species, the upper temperature limit for flounder is assumed to be about 40°

C (104° F).

Salinit~. Adult southern flounder are highly euryhaline, but adult gulf

flounder avold low salinities. Simmons (1957) collected adult southern and

gulf flounders at salinities as high as 60 parts per thousand (ppt ), In

estuaries, however, adult southern flounder seem to prefer salinities between

4
5 and 20 ppt (Gunter 1945; Williams and Deubler 1968; Perret and Caillouet

1974; Tarver and Savoie 1976; Stokes 1977; Barrett et ale 1978), while adult

gulf flounder seem to prefer salinities higher than 20 ppt (Gunter 1945;

Springer and Woodburn 1960; Topp and Hoff 1972). In Florida, Springer and

Woodburn (1960) collected gulf flounder from the Tampa Bay area at salinities

of 13.7 to 33.7 ppt , with very few taken at salinities less than 20 ppt ,

Rei d (1954) co11 ected gu 1f flounder from the Ceda r Key area at sa1in ities

from 17.5 to 31.5 ppt , In Aransas Bay, Texas, southern flounder were taken

only at salinities of 6 to 36 ppt and gulf flounder only at salinities

averaging over 16 ppt (Stokes 1977). Although 1 of 12 gulf flounder

collected by Gunter (1945) in Texas was taken at a salinity of 9.6 ppt , 10

were taken at salinities between 25 and 35 ppt , Gunter (1945) collected

various numbers of southern flounder at different salinity ranges: 32 at 0 to

4.9 ppt, 53 at 5.0 to 9.9 ppt, 49 at 10.0 to 14.9 ppt, 56 at 15.0 to 19.9

ppt , 35 at 20.0 to 24.9 ppt , 7 at 25.0 to 29.9 ppt , and 9 at 2. 30.0 ppt , The 801 southern flounder collected by Perret (1971) in Louisiana estuaries

were distributed equally over salinities of 0 to > 30 ppt. In the St. Johns

River on Florida's Atlantic coast, 13 gulf flounder (51 to 168 mn, 2 to 7

inches SL) were taken at salinities of 7.7 to 28.0 ppt, whereas 94 southern

flounder (22 to 311 ITIm, 0.9 to 12 inches SL) were taken at salinities of 0 to

30.2 ppt (Tagatz 1967). For southern flounder, seasonal changes in osmoregulatory processes correspond to spawning migrations between estuarine and

offshore waters (Hickman 1968).

Dissolved oxyaen and pH. In Louisiana estuaries, southern flounder were

caught at dissolve oxygen concentrations of 4.0 to 10.5 mg/l (Barrett et ale

1978). Although the lower lethal limits of dissolved oxygen for southern and

gulf flounders are unknown, an oxygen level of 3.0 mg/l is limiting to other

fish species (Hoss and Peters 1976). Limiting dissolved oxygen concentrations are most likely during brief periods in summer when biological and

chemical oxygen demands are high and thermal or salinity stratification

inhibits mixing of the water column. In Aransas Bay, Texas, southern

flounder were caught at pH values from 7.65 to 8.60, and there was no

apparent relationship between pH values and flounder distribution and

abundance (Stokes 1977).

Depth. Although southern flounder prefer comparatively shallow waters,

in the Gulf of Mexico they are commonly caught at depths of 44 m (144 f t )

(Hildebrand 1954) and have been caught as deep as 66 m (216 ft) (Stokes

1977). In Aransas Bay, Texas, adult southern flounder were collected mostly

along the shore in summer and near a pass leading to the Gulf of Mexico in

fall. In winter, nonspawning adults were collected in the deepest water in

the middle of the bay (Stokes 1977). Gulf flounder are most frequently

collected near the mouths of large estuaries or offshore. While on the

Continental Shelf, gulf flounder generally occupy water less than 91 m

(300 f t ) deep, but may be found as deep as 128 m (420 f t ) (Gutherz 1967;

Benson 1982).

Substrate. According to Ginsburg (1952), southern flounder prefer mud

bottoms and gulf flounder prefer hard or sandy bottoms. Nall (1979)

collected 152 southern flounder on mud bottoms, 25 on mud and sand bottoms,

and none on sand bottoms. The substrate preference of gulf flounder was not

5
as definitive as that for southern flounder, but Chi square analysis

indicated a significant (P < 0.05) relation between bottom type and species

(Nall 1979). Only 5 gulf flounder were collected on mud bottoms, whereas 16

were co11 ected on mud and sand bottoms and 12 on sand bottoms (Na11 1979).

At two gigging stations in Aransas Bay, Texas, most southern flounder were

taken at the nearshore stati on wi th the fi ner sediments, whereas most gul f

flounder were taken at the channel station, where sediments were coarser

(Stokes 1977).

Although most studi es report southern flounder to be most abundant on

soft bottoms composed of rich organic mud, clay, or silt, Tabb and Manning

(1961) collected southern flounder in southwestern Florida on shell and firm

marl bottoms. The identity of the specimens collected by Tabb and Manni ng

(1961), however, was questioned by Topp and Hoff (1972) on the basis of a

range map. Dahlberg and Odum (1970) collected southern flounder in Georgia

from bays with primarily sand bottoms.

Although gulf flounder are occasionally found on muddy bottoms, they

prefer firmer bottoms which may consist of sand, shell, or coral. Springer

and Woodburn (1960) collected gulf flounder from all habitats in the Tampa

Bay area of Florida except in freshwater or offshore near rocky reefs;

however, Moe and Martin (1965) collected a few gulf flounder near offshore

reefs.

Apparent differences in substrate preference between the two flounder

species may be more an effect of salinity selection than substrate selection

since the two species are morphologically similar and prey upon similar food

items. Substrate compositi on varies dependi ng upon vol ume of freshwater

inflow. A high inflow of freshwater into an estuary results in low salinities, and high sediment loads from rivers result in high turbidity and muddy

substrates. Estuaries wi th low freshwater i nfl ow and correspondi ngly hi gh

salinity are usually characterized by low turbidity and firmer substrates.

Vegetati on. In the northern gulf, southern flounder have usually been

collected both in highly turbid bays with little rooted vegetation and in

brackish or saltwater marshes and small tidal creeks dominated by cordgrass

(Spartina alterniflora), black rush (Juncus roemerianus), wiregrass (Spartina

patens), and threecorner grass (Scirpus olneyi) (Reid 1955; Darnell 1958; Fox

and White 1969; Perret 1971; Livingston 1976; Subrahmanyam and Coultas 1980).

In Aransas Bay, Texas, most southern flounder were gi gged at a site where

cordgrass lined the shore and extended out into the water; most gulf flounder

were gigged at a site with an unvegetated shoreline (Stokes 1977). Near

Cedar Key, Florida, most gulf flounder over 70 mm (2.8 inches) long were

collected during warm weather from sparsely vegetated channels or coves with

muddy sandy bottoms and during cold weather from shallow flats devoid of

thick plant growth (Reid 1954). Near Tampa Bay, Florida, gulf flounder

appeared to inhabit sandy areas in marine grass beds (Springer and Woodburn

1960), occurring at depths of less than 1.8 m (6 f't ) (Goodell and Gorsline

1961) .

Food. Southern and gulf flounders are highly predaceous, feeding on

both benthi c and pel agi c fi shes and crustaceans. The feedi ng behavi or of

6
southern flounder is probably similar to that of the summer flounder

(paralichth'lS dentatus), a primarily diurnal feeder that can capture prey equally we' on the bottom or 'in the water column (Olla et al. 1972; Powell

and Schwartz 1979). Although flounder may bury their bodies in the sediment,

they do not ambush passi nq prey. Instead they stalk prey on the bottom,

striking at speeds of 40 to 50 cm/s (16 to 20 inches/s) (Olla et ale 1972).

Larger southern and gulf flounders tend to prey proportionally more on

fish than on other types of prey (Reid 1954; Springer and Woodburn 1960; Topp

and Hoff 1972; Stokes 1977; Powell and Schwartz 1979), but often eat penaeid

shrimps and portunid crabs (Fox and White 1969; Topp and Hoff 1972;

Overstreet and Heard 1982). In southern and gulf flounders over 150 mm (6

inches), fish constituted 70% and 72% of the food items, respectively; penaeid shrimp were the most frequent invertebrates, followed by blue crabs

(Callinectes sapidus) (Stokes 1977).

Fishes commonly eaten by southern flounder in the Gulf of Mexico include

anchovy, (Anchoa sp.); mullet (MU9~1 sp.) menhaden (Brevoortia sp.), Atlantic

croaker, (Micropogonias undulatus , pinfish (La~odon rhornboides), and fat

sleeper (Dormitator maculatus) (Darnell 1958; ox and White 1969; Stokes

1977; Overstreet and Heard 1982). Relatively minor items in the diet of

southern flounder include mysids, gastropods, amphipods, stomatopods, schizopods, clams, and polychaetes (Fox and White 1969; Stokes 1977;

Overstreet and Heard 1982).

Fishes commonly eaten by gulf flounder include mullet, menhaden,

Atlantic croaker, pinfish, pigfish (Orthopristis chrysoptera), longnose

killifish (Fundulus similis), inshore lizard ish (S~nodus foetens), pipefish (Syngnathus sp.) and anchovy (Reid 1954; Springer an Woodburn 1960; Topp and

Hoff 1972; Stokes 1977). The frequency of occurrence of different fish

species in the diet of southern flounder, and presumably gulf flounder,

depends on seasonal prey abundance; there is no tendency for larger flounders

to eat larger prey (Darnell 1958; Fox and White 1969).

Embryo

Since southern and gulf flounders spawn offshore, thei r embryos are

adapted to developing in seawater at salinities of 30 to 35 ppt. Nutrition

is endogenous at this stage, so prey availability is not a limiting factor. With the possible exception of thermal shock, predation is probably

the only factor limiting the buoyant, planktonic embryos.

Larva

The larval period for the southern flounder, and presumably the gulf

flounder, lasts less than 2 months (Arnold et ale 1977) and is spent in

marine waters as the fish move with the currents toward inshore waters. The

planktonic larvae of marine fish, including flounders, feed on many kinds of

organisms, but copepod nauplii predominate in their diets (Houde and

Taniguchi 1979). Plankton density is an important determinant of larval

survival, and a "critical period" occurs at the onset of exogenous feeding

i niti ati on and immediately thereafter (Laurence 1977). Prey concentrati ons

7
affect the growth of larval fish, thereby influencing the length of the

larval period, during which fish are most vulnerable to predation (Houde and

Schekter 1980).

Juvenile

Temperature. Juveni 1e southern and gulf flounders began immi grati ng

into Texas estuaries from the Gulf of Mexico when the water temperature was

as low as 13.8° C (56.8° F), but peak immi9ration occurred when water

temperatures were between 16.0° and 16.2° C (60.8° and 61.2° F) (Stokes

1977). Juvenile southern flounder (17 to 40 mm, 0.7 to 1.6 inches) were

captured in Texas estuaries by Gunter (1945) at water temperatures of 14.5°

to 21.6° C (58.1° to 70.9° F). Wi 11 'jams and Deubler (1968) reported the

capture of juvenile southern and gulf flounders in Atlantic estuaries at

water temperatures as low as 2° to 4° C (35.6° to 39.2° F). The optimal

temperature for maximum feeding of juvenile southern flounder weighing 0.1 g

probably is above 30° C (86° F), the highest temperature tested in the

1aboratory, and probably decreases as the fi sh grow. The temperature for

maximum conversion efficiency in juvenile southern flounder increases as

sa 1i nity decreases (Peters and Kjel son 1975). Temperature may i ndi rectly

affect flounder survival by determining the length of time they are in a size

class that is vulnerable to predation.

Salinity. Although juvenile southern flounder are highly euryhaline and

can survi ve abrupt transfers from seawater (30 ppt) to freshwater (Deubler

1960), young juveniles « 0.1 g) may not be adapted to low salinities

(Stickney and White 1973). Gunter (1945) captured juvenile southern flounder

(17 to 40 mm, 0.7 to 1.6 inches) in Texas estuaries at salinities of 19.6 to

30.0 ppt. Stokes (1977) found no young juvenile southern flounder in Texas

bay areas with low salinities (10 to 12 ppt) until March; however, these

areas were farthest from the gulf. Williams and Deubler (1968) collected

juvenile southern flounder (lengths not indicated) in North Carolina

estuaries at salinities ranging from 0.2 to 35.0 ppt , but they did not

mention sizes. Juvenile gulf flounder were taken only near inlets where

salinities were of 22 to 35 ppt (Williams and Deubler 1968). In Pamlico

Sound, North Carolina, juvenile gulf flounder were collected at salinities

of 6 to 35 ppt but were most abundant near the estuary mouth where salinities

were highest (Powell and Schwartz 1977).

Small juvenile southern flounder «0.1 g) from North Carolina reared in

the laboratory at salinities of 0 to 30 ppt showed a positive linear

relationship between salinity and growth (Deubler 1960). Larger juveniles

(0.5 g) from North Carolina reared in the laboratory at salinities of 5, 15,

25, and 35 ppt grew at about the same rate, until the 10th week when growth

leveled off at 35 ppt but continued to increase at 5, 15, and 25 ppt

(Stickney and White 1973). This plateau in the growth rate may have occurred

because older juveniles are physiologically adapted to lower salinities,

since in nature they migrate from seawater to lower salinity estuarine waters

by the time they reach 0.5 g (Stickney and White 1973). However, in smaller

juveniles (0.15 g) from Georgia reared at salinities of 5,10,15,20,25,

8
and 30 ppt, growth rates were highest after 6 weeks at the lowest salinities

(5 and 10 ppt ) (Stickney and White 1973). In a laboratory study, juvenile southern flounder of the same size from North Carolina and Georgia were held

at a salinity of 15 ppt and at equal but increasing water temperatures (Stickney and White 1973). The North Carolina fish attained a mean weight of

0.5 g after 6 weeks, whereas the Georgia ones attained a mean weight of only

0.15 g after 8 weeks (Stickney and White 1973).

Dissolved oxygen. Juvenile southern flounder in a laboratory study

gradually withdrew to more highly oxygenated water when the dissolved oxygen

concentration fell below 3.7 mg/l (Deubler and Posner 1963). Although general activity increased as water temperatures increased, there was no

change in sensitivity to oxygen depletion at temperatures of 6.1°, 14.4°, and

25.3° C (43.0°, 57.9°, and 77.5° F) (Deubler and Posner 1963).

Ve~etation. In Texas, juvenile southern flounder were most abundant in

the sprlng at seine stations in estuaries characterized by dense patches (at

least 161 culms per 25 square inches) of shoal qras s (Halodule wrightii)

covering 30% to 60% of the total area (Stokes 1977). Juvenile gulf flounder

were taken from areas characterized by dense patches of shoal grass covering

30% to 60% of the area and from areas where light stands (less than 61 culms

per 25 square inches) of shoal grass occurred in patches covering less than

30% of the area. Juvenile gulf flounder were abundant from January through

May on shallow flats in the Cedar Key area of Florida (Reid 1954). No

j uvenil e southern or gulf fl ounders were captured in Texas bay areas where

turbidity exceeded 65 Jackson turbidity units (Stokes 1977).

Food. In the 1aboratory, newly metamorphosed juven"j 1e southern flounder

readi1"Yfed on rotifers (Branchionus rlicatilis) and brine shrimp nauplii

(Artemia sp.) (Deubler 1958; Lasswe 1 et al. 1977). Juvenile southern

flounder (10 to 150 mm, 0.4 to 6 inches) feed primarily on small crustaceans,

particularly mysids (Diener et al . 1974; Stokes 1977; Overstreet and Heard

1982). Numerically, 95% of food items in juvenile flounder from Texas were

invertebrates, 33% of which were mysids, followed by Acetes, Penaeus,

sergestids, and amphipods (Stokes 1977). Although small fish were minor

components of the diets of juvenile southern flounder in gulf estuaries,

juveniles (100 to 200 mm, 4 to 8 inches) in Atlantic estuaries selected fish

(65% of stomach contents by volume) over mysids (29%) despite the abundance

of mysids (Powell and Schwartz 1979). Juvenile gulf flounder feed primarily

on small crustaceans, such as penaeid shrimp and amphipods, but may also eat

small fishes and marine worms (Reid 1954; Springer and Woodburn 1960; Topp

and Hoff 1972). They become more piscivorous after reaching a length of

about 45 mm (2 inches). Numerically, 84% of food items in small gulf

flounder (10 to 150 mm, 0.4 to 6 inches) from Texas were invertebrates, 57%

of which were mysids (Stokes 1977).

9
HABITAT SUITABILITY INDEX (HSI) MODELS

Model Applicability

The habitat sUitability index (HSI) models in this report are developed

for appl ication in estuarine habitats of the northern and eastern Gulf of

Mexico from Florida to Texas. The models can be used to evaluate habitat for

juvenile southern and gulf flounders year-round. The models also evaluate

habitat quality for adults while they are in estuaries, primarily from May through August.

Minimum habitat area. Minimum habitat is defined as the contiguous suitable habitat required for a species to live and reproduce. No minimum

spati a1 requi rements have been reported for either the southern or gulf flounders, but they do require estuarine areas with open access to offshore

areas.

Verification level. The acceptable output of the HSI models is a value

between 0 and 1. This value has a positive relation to the carrying capacity for southern or gulf flounders in the estuarine habitat. Sample data sets,

which are presented later, were used to verify that HSI's determined with

these models were reasonable and acceptable. The southern and gulf flounder

HSI models were reviewed and evaluated by these biologists: G. H. Burgess,

Jr., Florida State Museum, Gainesville; J. H. Finucane, National Marine

Fisheries Service, Panama City, Florida; and E. J. Gutherz, National Marine

Fisheries Service, Pascagoula, Mississippi. Although their comments have

been incorporated, the authors are responsible for the final versions of the

models.

Model Description

Overview. The HSI models for southern and gulf flounders in estuarine

habitat are composed of two interacting life requisites, water quality and

cover. The larval stage is not considered in the models, because it is spent

in marine habitat. Given suitable water quality and cover, food is assumed

to be a nonlimiting life requisite for both juveniles and adults. The

relationship of habitat variables and life requisites to the HSI is

illustrated in Figure 1.

Water quality component. Water quality variables used in the HSI models

for both southern and gulf flounders are average annual salinity (V,), bottom

water temperature (V 2)' and average mi nimum di sso1ved oxygen concentration

(V3) •

Salinity (VI) affects water quality for southern and gulf flounders.

Within an estuary, salinity varies depending on the tidal cycle, season,

storm events, and long-term climatic trends; therefore, care must be taken in

measuring salinity values. Average annual salinity is the preferred measure,

since minor variations in salinity are of little consequence to the

euryha1i ne southern flounder. The norma 1 sal i nity range for juvenil e and

adult southern flounder in estuaries is a to 36 ppt, and the optimal salinity

10
Habitat variable LHe requisite Habitat

Estuarine HSI

Temperature Water quality

Dissolved oxygen

average daily minimum

Substrate Cover

composition

Salinity

V2

V

3

V

4

VI

......

......

Figure 1. Relationship of habitat variables and life requisites to the habitat suitability index

(HSI) for southern and gulf flounders in estuarine habitats.
is assumed to be 5 to 20 ppt , The normal salinity range for juvenile and

adult gulf flounders in estuaries is 17 to 36 ppt and the optimal salinity is

assumed to be 20 to 35 ppt.

The second variable that determines water quality for flounder is water

temperature (V2). Optimal temperatures for juvenile southern flounder< 0.1 g are probably over 30° C (86° F) and decrease as the fish grow older.- From

May to August, the optimal water temperatures for southern and gulf flounders

are assumed to be 20° to 35° C (77° to 95° F).

Dissolved oxygen concentration (V) also affects water quality for

southern and gulf flounders. The lower3lethal limit is probably about 3.0

mg/l and usually occurs only for brief periods in summer. Areas of low

dissolved oxygen levels can normally be avoided even by juvenile flounder and

do not permanently alter habitat suitabil ity. Juvenil e and adul t southern

flounders have been collected at oxygen levels of 4.0 mg/l.

Cover component. The only cover variable included in the HSl models is

substrate composition (V~). Adult southern flounder are collected most

frequently from muddy substrates, and adult gulf flounder from hard or sandy

substrates. It is assumed that southern and gulf flounders are responding both to sal i ni ty and substrate and that juveni 1e flounder have substrate

preferences similar to those of adults.

Vegetative cover is not included as a model variable because both

southern and gulf flounders are found in densely vegetated tidal marshes, in

vegetated and unvegetated bays, and off barren beaches. It is assumed that

the presence of flounder in different vegetative cover types is determined

primarily by salinity and possibly by substrate. The use of marine grass

flats and tidal marshes by juvenile flounder may be related to increased food

availability and/or decreased predation.

Water depth is assumed to be unimportant, since both southern and gulf

flounders have been collected near shore and in the deepest parts of bays.

The preference of juvenile and adult gulf flounders for mouths of estuaries

(Williams and Deubler 1968; Powell and Schwartz 1977; Stokes 1977) is assumed

to be due to the presence of higher salinities in these localities.

Suitability Index (SI) Graphs for Model Variables

This section presents graphic representations of the relationships

between various measurements of estuarine (E) habitat variables and the

habitat suitability for southern and gulf flounders. The SI values are read

directly off the graph (1.0 = optimum suitability, 0.0 = no suitability) for

any variable.

The SI graphs are based on the assumption that the sui tabil t ty of a

particular habitat variable can be represented by a two-dimensional response surface and is independent of other variables that contribute to habitat

suitabil ity. Data sources and assumptions associ ated with documentation of

the SI graphs are summarized in Table 1.

12
Habitat Variable Suitability Graph

1. E VI Average annual salinity

S 10 to 15 cm (3.9 to

5.9 inches) above the )( 0.8

bottom for southern Q)

flounder. "

.5 0.6

>- :=

:a OA

a:I

..

'3

en 0

O.

0 10 20 30 40 50 60

ppt

E V

1G Average annual salinity

10 to 15 cm (3.9 to

5.9 inches) above the

bottom for gulf flounder. )(

Q)

.5

"

>-

:5

:a

a:I

..

"3

en

o 10 20 30 40 50 60

ppt

20 30 40

°c

0.8

0.2

0.4

1.0..,....----.....,.111111111-..........

0.6

O.O~__r_~-__,..-..,....~_~

o

)(

Q)

"

.5

Average temperature 10

to 15 cm (3.9 to 5.9

inches) above the

bottom for southern

and gulf flounders, May

to August.

E

13
Habitat Variable Suitability Graph

1.0

Average minimum dissolved

oxygen concentration

>C 0.8 10 to 15 cm (3.9 to Q)

5.9 inches) above the '0

c bottom for southern and 0.6

gulf flounders May to >-

:= August. :0 0.4 ca

:!::

:;,

rn 0.2

0.0

1 2 3 4 5 6

mgJl

4

2 3 4

Class,

2 3

Class

1

1

-

-

r-

- ,..

1.0

>C 0.8

Q)

'0

C

- 0.6

:>.

E

.Q 0.4 ca

:!::

:;,

rn 0.2

0.0

1.0

0.0

>C 0.8

Q)

'0

C

.... 0.6

>- :=

0.4 ..oS

rn 0.2

E V4 Substrate composition

S for southern flounder

1) >66% mud,

rema inder si1 t

or sand

2) 34% to 66% mud or

si1 t, rema i nder

sand or shell

3) <34% soft

sediments,

remainder sand

or shell

4) Rock or coral

E V4 Substrate composition

G for gulf flounder

1) >66% mud,

remainder silt

or sand

2) 34% to 66% mud or

silt, remainder

sand or shell

3) <34% soft

sediments,

remainder sand

or shell

4) Rock or coral

14
Table 1. Variable sources and assumptions for habitat suitability indices of

southern and gulf flounders.

Variable and source

VI Gunter 1945; Simmons 1957;

S Williams and Deubler 1968;

Perret and Caillouet 1974;

Tarver and Savoie 1976;

Stokes 1977; Barrett

et al , 1978.

VI Gunter 1945; Reid 1954;

G Simmons 1957;

Springer and Woodburn 1960;

Tagatz 1967; Topp and Hoff 1972;

Stokes 1977.

V2 Gunter 1945; Reid 1954;

Springer and Woodburn 1960;

Perret 1971; Peters and

Kjelson 1975 Stokes 1977;

Barrett et al. 1978.

V3 Deubler and Posner 1963;

Hoss and Peters 1976;

Barrett et al. 1978.

V4 Ginsburg 1952;

S Stokes 1977;

Na 11 1979.

V Ginsburg 1952;

4G Springer and Woodburn 1960;

Topp and Hoff 1972; Stokes 1977;

Na 11 1979.

15

Assumptions

Optimal salinities for the growth

of southern flounder are those

salinities (5 to 20 ppt) at which

they are most frequently collected.

Optimal salinities for the growth of

gulf flounder are those salinities

(20 to 35 ppt) at which they are most

frequently collected.

The optimumal temperature for growth and feeding of southern flounder is

near 30° C (860 F), and the upper

temperature limit is 40° C (104° F).

Optimal temperatures for growth and

feeding rates of gulf flounder are

similar to those for southern

flounder since the two species follow similar movement patterns in

time and space.

Southern and gulf flounder are

similar to other fish species

in having a lower lethal limit

of dissolved oxygen near 3 mg/l.

Both juvenile and adult southern

flounders prefer soft substrates, and

the preferences are not totally a

product of salinity.

Both juvenile and adult gulf flounder

prefer firmer substrates, and the

preferences are not totally a

product of salinity.
Component Index (CI) and HSI Equations

To obtain an HSI for either southern or gulf flounder in the estuarine

habitat, combine SI values for each habitat variable or life requisite.

Because salinity (V ) appears to be the most important water quality variable for flounder, it~ SI value is squared in the water quality component

equation for southern and gulf flounders. The SI values for water

temperature (V ) and minimum dissolved oxygen (V ) are not raised to any

power in the wtter quality component equations, sinte only extreme values are

limiting and these are normally avoided and do not permanently alter habitat

suitability. The SI value for substrate in both species is equal to the

cover component index.

X SI X SI )1/4 V2 V3·

Equation

Equation

Gulf flounder

Southern flounder

Component

Water quality (CIWQ)

Cover (CI C)

((SI v1)2

SI G V

4G

The following steps must be taken to determine an HSI for any

application.

Component

Water quality (CIwQ)

Cover (CIc)

1. Review the section on model applicability to determine if the model

is valid for the intended application.

2. Obtain data for each variable used in the model. Obtain SI values

from the SI graphs and calculate component indices by using SI

graphs and component index equations.

3. Use the following equation to determine the HSI for either

southern or gulf flounder:

HSI = (CI X CI )1/2 WQ C

Three estuari ne data sets deri ved from pub1i shed sources were used in

Table 2 to calculate suitability indices, component indices, and habitat

suitability index values for southern and gulf flounders. The authors

believe that the HSI's calculated from these data sets reflect relative

carrying capacities of these areas for flounder.

16
Table 2. Suitability indices (SI), water quality component indices (C1wa),

cover component indices (C1r.)' and habitat suitability indices (HSI) for

southern and gulf flounders i~ three gulf estuaries.

Model

component

Old Tampa Bay, FL

Data sl

Barataria Bay, LA

Data S1

Upper Laguna Madre, TX

Data s1

Southern flounder

VI 26 ppt 0.85 20 ppt 1.00 50 ppt 0.25

S

V2 30°C 1.00 29°C 1.00 30°C 1.00

V3 6 mg/l 1.00 6 mg/l 1.00 6 mg/l 1.00

V4 Class 3 0.30 Class 1 1.00 Class 2 0.70

S

C1WQ 0.92 1.00 0.50

C1C 0.30 1.00 0.70

HS1 0.53 1.00 0.59

Gulf flounder

VI 26 ppt 1.00 20 ppt 1.00 50 ppt 0.40

G

V2 30°C 1.00 29°C 1.00 30°C 1.00

V3 6 mg/l 1.00 6 mg/l 1.00 6 mg/l 1.00

V4 Class 3 1. 00 Class 1 0.30 Class 2 0.70

G

C1WQ 1.00 1.00 0.63

C1 C 1.00 0.30 0.70

HS1 1.00 0.55 0.67

17
Field Use of Model

Detailed field sampling of habitat variables through time will provide

the most reliable and replicable HSI values. One-time sampling of habitat

variables is not recommended because the estuarine habitat is subject to

large fluctuations in water quality conditions.

The best indication of critical values for many of the habitat variables

is assumed to be the mean values derived for appropriate seasons from

1iterature sources. Much of the information necessary for the use of this

model is available from published sources or resource agencies. Calculated

HSI values are most useful when data are collected in the specific evaluation

area. Local fishery biologists should be consulted to ensure that the data

sources used accurately reflect habitat conditions in the evaluation area.

Any or all habitat variables may be estimated for preliminary

application of this model, but subjective estimates will result in decreased

reliability and duplicabilHy of the model. Subjective estimates should be

made by experienced professionals and be accompanied by full documentation of

the basis on which estimates were made. Suggested methods for measuring the variables are presented in Table 3.

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

the southern and gulf flounder HSI models.

Variable Techniques

Salinity can be measured by titration or by a refractometer, hydrometer, or conductivity meter

(American Public Health Association et ale 1975).

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 ale 1975).

Dissolved oxygen can be measured by Winkler titration or

with an oxygen meter (American Public Health Association

1975).

Substrate samples can be obtained with a coring device

such as an Ekman corer or a grab such as a Ponar

(Buchanan and Kain 1971). Substrate composition can

be determined by washing a known weight of substrate

through a series of Tyler sieves.

18
Interpreting Model Outputs

The HSI values produced by both of these models are relative and should

be used only for comparison of habitats. If two areas, or the same area at

different times, have different HSI values, the area with the higher HSI

should be interpreted as having the capacity for supporting more flounder of

the species under consideration than the area with the lower HSI value.

A flounder HSI determined by field appl ication of the model may not

reflect the actual population density of the species in the habitat being evaluated, since factors other than those related to habitat may be

significant in determining population size.

19

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21
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22

Feeding by marine fish larvae:

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fishes from Mississippi Sound.

Studholme. 1972. Activity and feeding

(Paralichthys dentatus) under controlled

Natl. Mar. Fish. Servo Fish. Bull.

1982. Food contents of six commercial

Gulf Res. Rep. 7:137-149.

Perret, W. S. 1971.

study, Louisiana.

pp.

Cooperative Gulf of Mexico estuarine inventory and

Phase IV, Biology. La. Wild Life Fish. Comm. 175

Perret, W. S., and C. W. Caillouet, Jr.

taken by trawling in Vermilion

24:52-74.

1974. Abundance and size of fishes

Bay, Louisiana. Bull. Mar. Sci.

Peters, D. S., and M. A. Kjelson. 1975. Consumption and utilization of food

by various postlarval and juvenile fishes of North Carolina estuaries.

23
Pages 448-472 in L. E. Cronin, ed. Estuarine research. Vol. 1.

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York.

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metamorphosis in the starry flounder, Platilichthys stellatus. Can. J.

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Chesapeake Sci. 18:334-339.

Powell, A. B., and F. J. Schwartz. 1979. Food of Paralichthys dentatus and Paralichth~S lethostigma (Pisces:Bothidae) in North Carolina estuaries.

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Acad. Sci. 30:25-50.

24
Tarver, J. W., and L. B. Savoie. 1976. An inventory and study of the Lake

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II. 135 pp.

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ESM 102. U.S. Fish Wildl. Serv., Washington, DC. n.p.

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nearshore habitats of the Gulf of Mexico. Texas A & M University

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Retention of three taxa of postlarval fishes in an intensively flushed

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Servo Fish. Bull. 78:419-436.

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in North Carolina estuaries: assessment of environmental factors and

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Sci. 9: 109-120.

25
OI'TION"'L FORM 272 (4-77)

(Formerly NTlS-35) * U.S. GOVERNMENT PRINTING OFFICE: 1985- 573-260 Dep.rtment of commerce

50272 101

(See ...NSI-Z39.IS)

- REPORT DOCUMENTATION i 1. REPORT NO.

Report 82(10.92)1

2

•

3. Reclo.e"t', Accession No

PAGE I Biological •• Title end Subtitle 5.. Report O.te

Habitat Suitability Index Models: Southern and Gulf Flounders June 1985 6-

7. "'uthor(.) 8. Perfonnina Ora.nization Rapt. No.

Kevin M. Enoe and Rosemarie Mulholland

9. Performln. O,.anization Name and Addr.,. 10. Project/T..k/Work Unit No.

Florida Cooperative Fish and Wildlife Research Unit

117 Newins-Ziegler Hall 11. Contr.ctlC) or Grent(G) No.

University of Florida (C)

Gainesville. Florida 32611 (G) 12. SDOnlonn. O,...nlzaUon Name and Add'•••

National Coastal Ecosystems Team 13. Type 01 Report & Period Covered

Division of Biological Services

Fish and Wildlife Service

U.S. Department of Interior u.

Washington, DC 20240

15. Supplementery Not..

·IL ......trect (Umlt: 200 words)

A review and synthesis of existing information were used to develop habitat models for

southern and gulf flounders (Paralichthys lethostigma, ~. albigutta). The models are

scaled to produce an index of habitat suitability between 0 (unsuitable) and 1.0

(optimally suitable) for southern and gulf flounder habitat in the Gulf of Mexico from

Florida to Texas. Habitat suitability indices are designed for use with the habitat

evaluation procedures previously developed by the U.S. Fish and Wildlife Service.

Guidelines for application of the models and techniques for measuring model variables

are provided.

17. Document "'ne'ysi. .0 Oescriptors

Mathematical models

Wildlife

Fishes

b.•d.ntlfi..../Open·End.d Terms

Habitat Gulf flounder

Habitat suitability index Paralichthys lethostigma

Impact assessment Paralichthys albigutta

Southern flounder

c. COSATI Field/Group

1.. Availability Statement 19. Security Clalo, (This Report) 21. No. of P••••

Unl imited Unclassified vi+ 25 -- 20. Security Ciasl (This P••e) %2. Price

Unclassified
1::r HeacJQuarter s. DIvISion ot BIol ogi cal

serv.ces, Wasnlngl on. DC

)( Eastern Energy and Land Use Team

t.eerown. WV

* National Coastal Ecosystems Team

Slidell LA

• Western Energy an<l Land Use Team

FI Collins CO

• Locations ot Regional 011Ices

..-....

'.

Hawaiian Islands

7

,

.

\

\

\

Habitat suitability index roodels. 19521

Enge, Kevin H. SK361 .U54 00.82

. I~u!I_~W~~IOOI

.. "..-

REGION 1

Regional Director

U.S. Fish and Wildlife Service

Lloyd Five Hundred Building, Suite 1692

500 N.E. Multnomah Street

Portland, Oregon 97232

REGION 4

Regional Director

U.S. Fish and Wildlife Service

Richard B. Russell Building

75 Spring Street, S.W.

Atlanta, Georgia 30303

REGION 2

Regional Director

U.S. Fish and Wildlife Service

P.O. Box 1306

Albuquerque, New Mexico 87103

REGION 5

Regional Director

U.S. Fish and Wildlife Service

One Gateway Center

Newton Corner, Massachusetts 02158

REGION 7

Regional Director

U.S. Fish and Wildlife Service

1011 E. Tudor Road

Anchorage, Alaska 99503

REGION 3

Regional Director

U.S. Fish and Wildlife Service

Federal Building, Fort Snelling

Twin Cities, Minnesota 55111

REGION 6

Regional Director

U.S. Fish and Wildlife Service

P.O. Box 25486

Denver Federal Center

Denver, Colorado 80225
u.... nllHAWIUlUn:

" .. aVICE

DEPARTMENT OF THE INTERIOR

u.s. FISH AND WILDLIFE SERVICE -- r.... '..•

As the Nation's principal conservation aleney, the Department of the Interior hu responsibility for most of our.natlonally owned public lands and natural resources. This includes

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

praservina th ••nvironmental and cultural values of our national parks and historical places.

and providlnl for the enjoyment of Iif. throush outdoor recreation. The Department ..-

...... our enef'IY and mineral resources and works toM,ute that their development is In

the best interests of all our people. The Department allo has a major responsibility for

American Indian r...rvation communities and for people who live in island territories und.r

U.S. edmlnistnltlon.