FWS/OBS-82/10,77

t\UGUST 1984

HABITAT SUITABILITY INDEX MODELS:

HARD CLAM

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,: :'. "

: .: ..

SK

361

.U54

no. 82-

10.77

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.
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 f'ol l owi nq application or review of the model. Feel

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

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

test results. Please retu rn thi s form to:

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

51 ide11, LA 70458

Thank you for your assistance.

Species _ Geographi c

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

If not, how were or could they be improved? _

No

Were the techniques suggested for

Appropriate? Yes

Clearly defined? Yes--

Easily applied? Yes

collection

No

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No--

of fi e1d data :

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

Were the model equations logical?

Appropriate?

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Yes-- No-----

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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:

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FWS/OBS-82/10.77

August 1984

HABITAT SUITABILITY INDEX MODELS:

HARD CLAM

by

Rosemarie Mulholland

Florida Cooperative Fish and Wildlife Research Unit

U.S. Fish and Wildlife Service

School of Forest Resources and Conservation

117 Newins-Ziegler

University of Florida

Gainesville, FL 32611

Project Officer

Michael S. Brody

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
This report should be cited as:

Mulholland, R. 1984. Habitat suitability index models: hard clam. U.S.

Fish Wildl. Servo FWSjOBS-82j10.77. 21 pp.
PREFACE

The habitat suitabil ity index (HSI) model in this report on the hard

clam 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 relationships, not a

statement of proven cause and effect. The model 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 and suggestions you may have on the HSI model to the

following address.

National Coastal Ecosystems Team

U.S. Fish and Wildlife Service

1010 Gause Boulevard

Slidell, LA 70458

iii

CONTENTS

PREFACE . . " • . . . " . • • . • • • . • • . . . " . . • "

ACKNOWLEDGMENTS. . • . . . . . . . . . . • . . . . . . . .

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

Embryo, Larva, Juvenile ..•.

Ad u1t . . . . . . . .. . . .

HABITAT SUITABILITY INDEX (HSI) MODEL.

Model Applicability ....•........•

Model Description . Suitability Index (SI) Graphs for Model Variables

Component Index (CI) Equation and HSI Determination.

Interpreting Model Outputs. . .•......

Field Use of Model. . . . . . ....

LITERATURE CITED..•.....

v

iii

vi

1

1

1

3

3

4

7

7

7

9

13

13

14

16
ACKNOWLEDGMENTS

Development of the habitat suitability index model for the hard clam-was

reviewed and constructively criticized by Dr. R. Winston Menzel, Oceanography

Department, Florida State University, Tallahassee; and Dr. John J. Manzi,

Marine Resources Research Institute, South Carolina Wildlife and Marine

Resources Department, Charleston. 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). Michael S.

Brody served as project officer at NCET. Supportive narrative and model

reviews also were provided by Regional personnel of the U.S. Fish and

Wildlife Service and the Florida Department of Natural Resources. Funding

for model development and publication was provided by the FWS. The cover

illustration was prepared by David S. tv!aehr, Florida Game and Fresh Water

Fish Commission.

vi
HARD CLAM (Nercena ria campechi ensis, Mercena ri a mercena ri a)

INTRODUCTION

Two species of hard clams occur along the Atlantic and Gulf of Mexico

coasts of North America: the southern hard clam, Mercenaria campechiensis

Gmelin 1791, and the northern hard clam, ~lercenaria mercenaria Linne 1758

(Wells 1957b). The latter species, also commonly kno\'m as the quahog, was

formerly named Venus mercenaria. The two species are closely related,

produce viable hybrids (Menzel and Menzel 1965), and may be a single species.

Throuqhout thei r ranges, hard cl ams support extensi ve commerci a1 and

recreational fisheries (Ritchie 1977). Additionally, because adult clams are

sedentary, they may serve as biological indicators of changing environmental

conditions.

Distribution

Mercenaria mercenaria is found in intertidal and subtidal areas along

the Atlantic and gulf coasts. Its range extends from the Gulf of St.

Lawrence to Texas (Belding 1912; Johnson 1934; Abbott 1954, 1974), but it is

most abundant from Massachusetts to Virginia (Stanley and DeWitt 1983).

Mercenaria campechiensis ranges from New Jersey (Merrill and Ropes 1967) to

St. Lucie Inlet, Florida (Godcharles and Jaap 1973) along the Atlantic coast,

and from Florida to Texas in the Gulf of Mexico (Ladd 1951).

Life History Overview

Spawning. Hard clams spawn from spring to fall depending on latitude

and temperature (Stanley and DeWitt 1983). In northwest Florida, the

spawning period extends over at least 6 months and has a peak in the spring

and a smaller peak "in the fall (R. W. Menzel, Florida State University,

Tallahassee; pers. comm.). In more northern latitudes, clams spawn from May

to August (Stanley and DeWitt 1983). Not all clams "in a given population

spawn at the same time (Loosanoff 1937); rather, spawning is spread over a

period of 8 to 10 weeks (Ansell 1967a). Water temperature is the determining

factor in final maturation of the gametes. When stimulated by the appropriate temperature , males release semen containing pheromones. Water

currents transport the pheromones to the females, which are in turn

stimulated to release eggs (Nelson and Haskin 1949). Fertilization occurs in

the water column. Carriker (1961) found that spawning occurred more

frequently during neap than spring tides and suggested that the higher

temperatures duri ng neap ti des tri ggered spawning. Bayne (1976) reported

that M. mercenaria usually spawned at or just after low tide, which was

coincfdent with maximum daily water temperature.

1
For M. mercenaria in Great South Bay, New York, Bricelj and Malouf

(l980) noted a significant corr-elation between size (length) and egg

production; 15% to 25% of the variation in fecundity was attributable to

differences in clam size. The estimated maximum production for one female

during a single spawning season was 16.8 million eggs. The sex ratio was

about 1:1. There was no evidence of a decline in egg production with

increasing age.

I9.9..: When first discharged by the female, the planktonic eggs of M.

mercenana appear grayish and granular (Carriker 1961). Kraeuter et aT.

(1982) found that survival was higher among large eggs (44 um) than among

small ones (25 um}, Eggs of hard clams differ from those of some other

lamellibranchs in being surrounded by a thick gelatinous membrane (Loosanoff

and Davis 1950). The fertilized egg reaches the two-celled stage in about 45

minutes and the four-celled stage in about 90 minutes. At 22°C (72°F), the

trochophore stage is reached about 12 hours after fertil ization (Loosanoff

and Davis 1950).

Larva. The pear-shaped trochophores actively propel themselves through

the water with a strongly ciliated velum (Loosanoff and Davis 1950). The

trochophore begins to form a primitive mouth and develop a shell gland. It

continues its planktonic existence after entering the shelled veliger stage,

which consists of two forms: straight-hinged and umboned. In the

stra i ght-hi nged phase a sma 11 thi n shell secreted by the shell gl and covers

the entire animal. The highly developed velum enables the larva to become a

proficient swimmer. The straight-hinged veliger phase lasts 1 to 3 days.

The veliger then enters the umboned veliger phase, characterized by a gently

sloping umbone projecting above the middle of the hinge line (Carriker 1961).

After 6 to 20 days the veliger reaches the pediveliger stage, in which it has

a foot and alternates between swimming in the water and crawling on the

bottom. This stage terminates when the velum is lost. The pediveliger then

enters the plantigrade benthic stages, and locomotion is limited to crawling

on the bottom. Initially the larva becomes a byssal plantigrade and affixes

itself to the substrate with a byssus. Thisis the setti ng or spatti n9

stage. For a number of weeks, until the clam is about 9 mm (0.4 inches)

long, the byssal plantigrade alternates between byssal attachment and

crawling. The larva next enters the juvenile plantigrade stage, when the

byssus gland is no longer functional and the byssus is lost; the clam then

rna i nta i ns its positi on beneath the sediment surface by means of its foot.

The siphons are fully developed at this stage (Carriker 1961). The juveniles

continue to grow and, as their siphons lengthen, they burrow deeper into the

sediment and complete their development (Carriker 1956). The adult clam

remains in much the same location for the rest of its life (Belding 1911).

Chestnut (1952) reported that adult clams moved up to 15 cm (6 inches)

1atera lly in 38 days, and Kerswi 11 (1941) observed movements of up to 30 em

(12 inches) in 2 months.

Juvenile and adult. M. mercenaria secrete shell material daily and grow

rapidly (Kennish and Loveland 1980). However, growth begins to slow at age 2

(Carriker 1961; Rhoads and Pannella 1970), when energy is diverted into

reproductive processes (Kennish and Loveland 1980).

2
Recruitment throughout the range of the two species of Mercenaria is

erratic and unpredictable (Hibbert 1976; Menzel 1976). It is probably determined by the amount of predation occurring after the juveniles settle

(Menzel 1976). A major predator of small clams is the blue crab, Call"inectes

sapidus (Haven and Andrews 1957; Menzel and Sims 1962; Castagna 1970a,1970b;

Castagna and Kraeuter 1977). Other major predators are whel ks , Busycon

spp.; moon snails, Polinices duplicatus; and stone crabs, Menipe mercenaria

(Menzel et al. 1976).

SPECIFIC HABITAT REQUIREMENTS

Embryo, Larva, Juvenile

.E.t!. Calabrese (1972) observed that the successful recruitment of !:!.

mercenaria requires that the pH of estuarine waters not fall below 7.0; he

found no significant decrease in the number of clam embryos developing

normally within the pH range of 7.0-8.75, but that number was greatly

reduced at pH 9.0. Survival of clam larvae was normal at pH 6.25-8.75, but

the range for normal growth was 6.75-8.50. Although clam larvae can survive

at pH 6.25, a pH of 7.0 is required for normal development of the embryo.

Levels of pH below 7.0 limit recruitment of the species (Calabrese 1972).

Dissolved oxygen. Morrison (1971) found that growth of shelled veligers

of M. mercenaria was normal when dissolved oxygen concentration was 4.2 mg/l

or greater. Growth essentially ceased at concentrations of 2.4 mg/l and

less. Larvae survived extended exposures (14 days) to 1 mg/l dissolved

oxygen but grew little. Prolonged exposure to levels of less than 4.0 mg/l

lengthened the clam's planktonic stage and decreased its probability of

survival. Embryos developed normally at oxygen levels as low as 0.5 mg/l;

however, 100% mortality occurred at 0.2 mg/l.

Sal inity. Sal inity appears to be most critical for!:!. mercenaria during

the egg and larval stages (Stanley and DeWitt 1983). At Long Island Sound,

New York, eggs developed into straight-hinged veligers only within the

relatively narrow salinity range of 20.0 to 32.5 parts per thousand (ppt ),

The optimum for development of clam eggs was about 26.5 to 27.5 ppt (Davis

1958). Growth of larvae, once they attained the straight-hinged stage, was

comparatively good at salinities as low as 20 ppt (Davis 1958), but Chanley

(1958) found that growth of juvenile M. mercenaria was retarded at

salinities of 22.5 ppt or lower. Castagna and Chanley (1973) found that

metamorphosis of M. mercenaria from veliger to seed clam (byssal plantigrade

stage) was inhibifed below 17.5 to 20 ppt.

Temperature. Davisand Ca 1abrese (1964) noted that 1abora tory-rea red

straight-hinged veligers of M. mercenaria were capable of ingestion, but not

digestion, at 10°C (50°F),- and consequently did not grow. Growth was

positively related to temperature at 18.0° to 30.0°C (64° to 86°F). Growth

of straight-hinged veligers of M. mercenaria was little affected by

temperature differences within the range of 20° to 30°C (68° to 86°F).

Although the optimum temperature for growth of M. mercenaria larvae was not

well defined, growth was optimum at the foTlowing temperature/salinity

3
combinations: 30°C (86°F)/22.5 ppt and higher, 27.5°C (81.5°F)/17.5 and 20.0

ppt, and 25°C (77°F)/15.0 ppt. The larvae appeared to be more sensitive to

differences in salinity than to differences in temperature (Davis and

Calabrese 1964). Kennedy et ale (1974) observed that temperature tolerance

increased with age; cleavage stages were the most sensitive to high

temperatures and straight-hinged larvae the least sensitive.

Substrate. The nature of the bottom substrate seems to be the main

factor responsible for settling of larvae and for the qualitative composition

of bottom communities (Thorson 1955). Keck et ale (1974) reported from

laboratory studies that significantly higher (P 0.05) numbers of M.

mercenaria larvae set in sand than in mud; they suggested that the addition

of organic material to the sediment may be responsible for reduced setting

because of increased bacteria levels, reduced dissolved oxygen, and increased

production of hydrogen sulfide. Carriker (1959) recommended that the

substrate be firm and free of excessive organic mud for larval clam culture;

muddy bottoms can be surfaced with shells, sand, or gravel.

Sus ended solids. Suspended solids affect both the eggs and larvae of

hard c t ams , Davis 1960) found that clam eggs did not develop normally at

silt concentrations of 3.0 or 4.0 gil. Growth of straight-hinged veligers

was normal at a silt concentration of 0.75 gil, retarded at 1.0 to 2.0 gil,

and negligible at 3.0 and 4.0 gil.

Adult

~. In mortality experiments with adult M. mercenaria, Calabrese (1972)

observed that the pH of tidal estuarine waters-should not fall below 7.0 even

though larvae can survive at lower pH levels. The species could not

reproduce successfully in waters where pH remained appreciably above 9.0

since at pH 9.50 to 9.75 there was virtually no development of embryos.

Dissolved oxygen. Fluctuations in dissolved oxygen do not affect hard

clams as much as do fluctuations in temperature and salinity (Stanley and

DeWitt 1983). The burrowing ability of ~1. mercenaria was neither severely

nor permanently impaired by exposure to reduced oxygen levels (less than 1

mgl1 seawater) for up to 3 weeks (Savage 1976). Pratt and Campbell (1956)

found no correlation between growth rates and various concentrations of

dissolved oxygen. All life stages tolerate nearly anoxic conditions for long

periods, though they may cease growing (Stanley and DeWitt 1983). Greenfield

and Crenshaw (1981) reported that M. mercenaria has evolved several metabolic

responses to anoxia; the type of response depends on the length of exposure.

Salinity. The effects of salinity and temperature on clams are difficult to interpret because of the interaction between these two factors.

Woodburn (1961, 1962) reported that near oceanic salinity (35 to 36 ppt) was

best for M. campechiensis and recommended 20 ppt as a minimum level. The

experimentally determined nonlethal minimum salinity for adult M. mercenaria

was 12.5 ppt (Castagna and Chanley 1973). The range of sallnities at a

northwest Florida site supporting both species was 26 to 35 ppt (Menzel

1961). Menzel and Sims (1962) recommend a salinity in excess of 25 ppt for

4
M. mercenaria plantings in Florida. Adult hard clams are capable of

withstanding low salinity by closing their shells. In South Carolina, ,!i.

mercenari a mortal i ty was 1ess than 5%, duri ng 2- and 3-week peri ods when

salinity was less than 10 ppt (Burrell 1977). In the laboratory, Pearse

(1936) found that adult ,!i. mercenaria could survive for 114 hours in

freshwater.

Temperature. The Joint Subcommittee on Aquaculture (1983) reported that

hard clam gonadal development begins at 8° to lOoC (46° to 50°F), spawning

occurs between 22° and 28°C (720 and 82°F), and that growth continues between

8° and 28°C (46° and 82°F). Kennish and Olsson (1975) reported 21° to 25°C

(70° to 77°F) as the preferred or required temperature range for spawning of

M. mercenaria in Barnegat Bay, New Jersey. Carriker (1961) reported spawning

at temperatures of 22° to 30°C (72° to 86°F) in New Jersey, and Mitchell

(1974) reported spawning at 18° to 20"C (64° to 68°F) in England. Menzel

(1976) reported that spawning peaks in Florida when water temperatures

approach 22° to 24°C (7r to 75°F). In northwest Florida, shell growth was

greatest in spring and fall. ~lenzel (1961,1962) demonstrated that growth of ,!i. cam echiensis was negligible during winter at temperatures approaching 10°

to 12°C 50° to 54°F). M. campechiensis continued to grow until temperatures

approached 35°C or 95°F (R. W. Menzel, Florida State University, Tallahassee;

pers. comm.). Burrowing-rate response curves for M. mercenaria suggested a

preferred range of 21° to 31°C (70° to 88°F) in Rhode Island (Savage 1976);

growth was negligible at temperatures below 10°C or 50°F (Belding 1931; Pratt

and Campbell 1956) and at temperatures above 27° to 28°C or 81° to 82°F (t~enzel 1961, 1962). In his review paper, Ansell (1967b) concluded that 20°C

(68°F) was the optimum temperature for growth of M. mercenaria and that the

rate dropped off symmetrically at 'higher and lower temperatures, ceasing

below 9°C (48°F) and above 31°C (88°F). In the laboratory, Storr et al .

(1982) observed that max inum overall shell growth of M. mercenaria occurred

at 12.8° to 15.6°C (55° to 60°F) and 23.9°C (75°F). -

Substrate. Hard clams inhabit a variety of sediment types (Joint

Subcommittee on Aquaculture 1983). Hibbert (1976) found that growth rates of

M. mercenaria were similar on bottom substrates of 3% to 93% mud. Sims and

Stokes (1967) could not establish a consistent correlation of sediment

particle size with ,!i. campechiensis population density for Tampa Bay,

Florida; however, Pratt (1953) showed that the population density of M.

mercenaria in Narragansett Bay, Rhode Island, was inversely correlated with

the particle size of the major sediment constituent. Average concentrations

of clams were greatest on predominantly muddy bottoms, less on sand bottoms,

and least on rocky bottoms. Clams were most abundant in predominantly fine

sediments, but in these sediments their abundance was generally a function of

the coarseness of the minor constituents. Clams do not grow well in silty

substrates. Pratt and Campbell (1956) found an inverse relationship between

growth of M. mercenaria and the fineness of the sediment (expressed as

percentage of silt and clay). The inferior growth was attributed to frequent

gi 11 cl eari ng, whi ch expended energy and interfered wi th feedi ng. Johnson

(1977) also reported slower growth of M. mercenaria in finer sediment due to

increased expulsion of pseudofeces. -

5
Soft sediment was given as the principal factor limiting the abundance

and diversity of benthic mollusks in bayfill canals in Tampa Bay, Florida

(Sykes and Hall 1970). Sediment types can be roughly correlated with current

velocities. A muddy bottom normally indicates calm water, whereas a coarse

bottom generally indicates more turbulent water (Thorson 1955). Wells

(1957a) found that the average density of M. mercenaria in Chincoteague Bay,

Maryland, was highest where the water current was 30 to 50 cm/s. The Joint

Subcommittee on Aquaculture (1983) reported that clams grow well where

currents average 50 cm/s or less.

Suspended solids. Little information is available on the effects of

suspended solids on adult clams. As clams filter feed, the uningestible

materials are sorted, accumulated, and expelled as pseudofeces (Pratt and

Campbell 1956). The rate of pseudofeces generation increases with turbidity

(Pratt and Campbell 1956). Menzel (1961) suggested that high turbidity may

have inhibited growth at one Florida site.

Food. Adult hard clams feed by filtering plankton and microorganisms

(Chestnut 1951). They may depend on an abundant supply of plankton before

and during spawning to provide sufficient energy to ripen the gonads (Ansell

1967a). In the laboratory, food concentrations of 300 mg carbon per liter of

seawater were optimal for feeding of M. mercenaria (Tenore and Dunstan 1973).

Robinson and Langton (1980) found that digestion was nearly continuous in a

subtidal population of M. mercenaria, regardless of time of day or tidal

stage. -

Vegetation. Studies conducted in Flortdas west coast estuaries have

reported the association of~. campechiensis with stands of turtle grass,

Thalassia testudinum (Schroeder 1924; Woodburn 1962; Sims and Stokes 1967;

Taylor and Saloman 1968, 1970; Godcharles 1971). This association with

rooted vegetation persists in inland waters but not offshore (Godcharles and

Jaap 1973; Menzel 1976). In Bogue Sound, North Carolina, Peterson (1982)

found a significantly higher (p < 0.01) average density of M. mercenaria in

partly vegetated plots than in unvegetated plots and a signlficantly higher

(P < 0.01) density in thickly vegetated than in partly vegetated plots.

Although seagrasses may provide protection from predators and stabilize the

sediment (Godcharles and Jaap 1973; Peterson 1982), they are apparently not

essential for the well-being and survival of clams.

Water depth. Hard clams seem to prefer relatively shallow water,

although they also are found in the open ocean. Along the South Carolina

coast, Anderson et ale (1978) found that 50% of all the M. mercenaria and M.

campechiensis were collected at a depth of about 2 m (7 ft), and less than

10% at depths greater than 5 m (16 f t ). Cummins (1966) found commercial

concentrations of ~. campechiensis off the South Carolina coast at depths of

7.3 to 11 m (24 to 36 ft). Along the west coast of Flori da the most

productive beds were at 5.5 to 7.3 m (18 to 24 ft). M. cam echiensis were

generally most abundant at depths of 4.7 to 9.2 m (15.4-to 30.2 ft in Tampa

Bay, Florida (Godcharles and Japp 1973). Off Woods Hole, Massachusetts,

Robinson and Langton (1980) collected M. mercenaria at 6 to 8.5 m (20 to 28

ft). In Maine, Gustafson (1955) noted-that M. mercenaria lives between tide

levels to depths of at least 15 m (49 ft). -

6
HABITAT SUITABILITY INDEX (HSI) MODELS

Model Applicability

Geographic area. The model is applicable to intertidal and subtidal

estuarine habitats as defined by Cowardin et al , (1979) along the Atlantic

and gulf coasts and can be used for both ~. mercenaria and~. campechiensis.

It is not applicable in the open ocean. The effect of pollution has not been

considered in model development; accordingly, the model is not applicable to

heavily polluted waters.

Season. The model is structured to account for seasonal variations in

habitat requirements of hard clams and, accordingly, to estimate the ability

of an area to sustain a population year-round.

Minimum habitat area. Hard clams can grow in relatively high densities.

The minimum area required for a self-sustaining population of hard clams is

not known.

Verification level. The model has not been field-tested. The

acceptable model output is an index value between 0.0 and 1.0. Dr. R.

Winston Menzel (Oceanography Department, Florida State University,

Tallahassee) and Dr. John J. Manzi (Marine Resources Research Institute,

South Carolina Wildlife and Marine Resources Department, Charleston) reviewed

and evaluated the hard clam model. Although their comments have been

incorporated, the author is responsible for the final version of this model.

Model Description

Overview. The structure of the hard clam HSI model is depicted

graphically in Figure 1. The habitat suitability index model applies to the

entire life cycle of hard clams. It uses two life requisites (water quality

and substrate-suspended solids) to evaluate an area. It is based on six

habitat variables: dissolved oxygen concentration (VI), salinity (V2), water

temperature (V3), percent silt-clay concentration in the substrate (V4),

water current (V5), and suspended solids (V6). A food component was not

included because of insufficient information on food availability in

estuaries. Vegetative cover was omitted because it is not an important habitat characteristic throughout the range of hard clams. This model does

not apply to areas where clams are incapable of burrowing in the substrate.

Water quality component. Both adult and embryonic hard clams are

capable of withstanding low dissolved oxygen concentrations The dissolved

oxygen requirements of the larval stage (V 1) are used to evaluate

suitabi 1ity. Di sso1ved oxygen is most important from April to September,

the period when most spawning occurs. Prolonged exposure to less than 4.0

mg/l dissolved oxygen decreases the probability of survival of M. mercenaria

larvae. The optimal dissolved oxygen concentration is 4.0 mg/l or higher.

Salinity (V 2) affects growth and survival of all 1ife stages. The

optimal salinity range for adult M. campechiensis is 24 to 35 ppt and 20 to

30 ppt for adult M. mercenaria:- Growth of juvenile M. mercenaria is

7
Habitat Variable Life Requisite Habitat

V3 Water temperature

VI Dissolved oxygen

Estuarine HSI

Water quality

Percentage Silt-ClaY~

Current ~Substrate-suspended solids

Sa1inity --------~

V4

V6 Suspended solids

Vs

V2

00

Figure 1. Relatior,hip of habitat variables and life requisites to the HSI for hard clams.
retarded at salinities of 22.S ppt and lower. The optimal salinity range for

hard clams throughout their range is assumed to be 22 to 3S ppt.

Water temperature (V3) requirements for all life stages are combined.

Growth of adult M. mercenaria and ~. campechiensis is negligible below 100e

(SO°F), and straight-hinged vel igers of M. mercenaria are unable to digest

food at 100e (SO°F). Accordingly, temperatures of 100e (SO°F) or less are

considered unsuitable. Growth is negligible above 28°e (82°F) for M.

mercenaria and above ss-e (9S°F) for ~. cam1echiensis (Menzel 1961, 1962,

pers. comm.). Temperatures of ss-e (9S0F and higher are considered

unsuitable. The optimal temperature range for growth is assumed to be 20° to

31°e (680 to 88°F). Water temperature shouc be measured only during the

growing season, where growing season is defined as the period of time when

the mid-depth water temperature is greater than lODe (SO°F).

The pH requirements were not included because marine and estuarine

waters normally have a high buffering capacity. An influx of pollutants,

such as hydrogen sulfide, might alter the pH, but the present model is not

designed to evaluate highly polluted areas. Water depth is not included

since hard clams are distributed throughout estuaries at various depths.

SUbstrate-suspended solids component. Hard clams are found in almost

any bottom type into which they can burrow. The percent silt and clay (V4)

in the substrate affects clam growth; as that percent increases, clams must

more frequently clear their filtering apparatus, which expends energy and

interferes with feeding.

Water currents (VS) affect the type of sediment found in an area and the

stability of the bottom; 30 to SO cmls is considered optimal.

Suspended solids (V6) affect larval clam development. Growth of larvae

is normal at silt concentrations as high as 0.7S gil, but growth decreases at

higher levels and ceases at 3.0 to 4.0 gil. Turbidity should be measured

during the spawning season from April to September.

Suitability Index (SI) Graphs for Model Variables

Graphic representations of the relationship between the habitat variables and hard clam habitat quality are given here. Optimum suitability is

indicated by an SI value of 1.0 and unsuitability by a value of 0.0. The SI

graphs are based on the assumption that the suitabil ity 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 associated with SI graphs are listed in Table 1.

9
Table 1. Variable sources and assumptions for hard clam suitability indices.

Variable and source

Morrison 1971

Chanley 1958

Davis 1958

Woodburn 1961, 1962

Taylor and Saloman 1970

Castagna and Chanley 1973

R. W. Menzel, pers. comm.

Menzel 1961, 1962

Davis and Calabrese 1964

Savage 1976

Pratt and Campbell 1956

Johnson 1977

We 11 s 1957a

Joint Subcommittee on

Aquaculture 1983

Davis 1960

Assumption

Optimal dissolved oxygen concentration for larval M. mercenaria growth

and survival is 4.0 mg/l or higher.

The optimal salinity range for growth and survival of adult M. cam~echiensis is 24 to-35 ppt while

optlmal range for adult M. mercenaria

is 20 to 30 ppt. The opTimal

salinity range for hard clams

throughout their range is assumed to

be 22 to 35 ppt ,

Growth of M. mercenaria and M.

cam echiensis adults is neglTgible

below 10°C 50°F), and M. mercenaria

veligers cannot digest food at 10°C.

Growth is negligible above 28°C

(82°F) for M. mercenaria and above

35°C (95°F)-for ~. campechiensis.

Optimal range for growth is assumed

to be 20° to 31°C (70° to 88°F).

Clams must be capable of burrowing in substrate. As percentage of

silt-clay content increases, growth

decreases.

Densities of clams are highest where

current velocities are 30 to 50 cm/s.

Larval clam growth is optimal at silt

concentrations of 0.75 gil or less.

10
Variable Descr"iption Suitabil ity Graph

VI Mean mid-depth 1.0

dissolved oxygen

concentration April 0.8 to September.

0.6

0.4

0.2

0.0

1 2 3 4 5 6

mg/l

V2 Mean annual mid- 1.{)

depth salinity. x

Q) 0.8 "0

c

.......

0.6 .....

.0 0.4 ttl

,f-)

::::1

V) 0.2

0.0

20 24 28 32 36 40

ppt

V3 Mean mid-depth water 1.0

temperature during x growing season. Q)

"0 0.8 c

.......

c-,

.....

,f-) 0.6

.....

.0 0.4 ttl

,f-)

.....

::::1

V) 0.2

0.0

10 15 20 25 30 35

°C·

11
Variable Description Suitabil ity Graph

V4 Percentage silt-clay

in substrate sample. x

(1)

"'0

C

......

>,

+-l

......

......

..0 O. ttl

+-l

......

:::J

VI

O.

0 25 50 75 100

%

1.0

V5 Mean annual

surface water x

(1)

current velocity. "'0 0.8

c

......

O. ......

......

..0 0.4 ttl

+-l

......

:::J

VI O.L

0.0

0 25 50 75 100

cm/s

V6 Mean total suspended 1.0

solids April to

<;eptember. x

(1)

"'0 0.8

c

......

>, 0.6 +-l

......

......

..0 0.4 ttl

+-l

......

:::J

VI 0.2

0.0

a 1 2 3 4

gil

12
Component Index (CI) Equation and HSI Determination

Development of an HSI for hard clams requires that the S1 values for the

habitat variables be combined into component indices (C1) for water quality

and for substrate-suspended solids. Percentage of silt-clay (SIV4) is

squared because it is considered the most important habitat va ri ab1e. The

suggested equation follows.

Component

Water quality (WQ)

SUbstrate-suspended

Equation

(SI XSI XS1 )1/3 VI V2 V3

solids (SS) (S1 v 2 X S1 V X SIV )1/4 456

HSI = (WQ X SS)I/2

Sample data sets representin~ a range of hypothetical habitat values for

hard clams are presented (Table 2). The HSI values generated are believed to

reflect the relative potential of the habitats to support hard clams.

Interpreting Model Outputs

The hard clam HS1 determined by use of these models does not necessarily

represent the population of hard clams in an area. Habitats with an HSI of 0

may contain some hard clams and habitats with a high H51 may contain only a

few. The proper interpretation of the H5I is one of comparison. On average, habitats with high H51 's would be able to support higher populations of hard

clams than habitats with low H51's. A close correlation between population size and H5I is unlikely.

Table 2. Calculations of suitability indices (51), component indices (cI) ,

and habitat suitability indices (H5I) for three hypothetical data sets on the

basis of habitat variables (V) and model equations.

Model Data set 1 Data set 2 Data set 3

component Data 51 Data 51 Data 51

VI (mg/l) 6 1.0 3 0.67 4 1.0

V2 (ppt) 20 0.0 26 1.0 35 1.0

V3 (OC) 25 1.0 15 0.5 20 1.0

V4 (%) 50 0.5 0 1.0 75 0.25

V5 (cm/s) 0 0.6 25 0.93 50 1.0

V6 (g/l ) 1 0.9 2 0.54 0.5 1.0

WQ 0.0 0.69 1.0

5S 0.6 0.84 0.5

H51 0.0 0.76 0.7

13
Field Use of Model

This model is designed for use in intertidal and subtidal estuarine

areas and not for open ocean areas. It is set up to evaluate habitat

suitabil ity for both !i. mercenari a and !i. campechi ensis. Inasmuch as

information on habitat variables was not always available for both species,

information pertaining to ~. mercenaria was applied to!. campechiensis and

vice versa. Information was merged into a range of values when information

on both species was available. This adjustment may have caused the optimal

ranges of some variables to be wider than if each species was treated

individually. Since the two species interbreed and produce viable offspring,

this use of the habitat information seems to be justified.

The reliability of the calculated HSI values can be only as good as the

data used for their calculation. Estimates of variables cannot replace

field measurements of variables. The HSI values are most useful when the

habitat variables are measured in the specific evaluation area. Existing

water quality information for the area should be used if it is available and

accurate. Adult clams are capable of withstanding suboptimal conditions by

simply closing their shells. Accordingly, temporary fluctuations in water

quality characteristics may not influence habitat suitability. Since

spawning occurs over an extended period and involves massive numbers of eggs,

temporary fluctuations in water quality may kill only a portion of the yearly

production of embryos and larvae. Accordingly, it is best to use long-term

data whenever possible to evaluate the suitability of an area for hard clams.

In the northern areas of the hard clam distribution, water temperatures may

remain below the optimal range of 20° to 31°C (68° to 88°F) throughout the

year. If hard clams are known to inhabit these areas, the temperature

suitability curve may require adjustment.

Suggested methods for measuring model variables in areas where data are

not available are described in Table 3. A valid sampling scheme must be

developed before field sampl ing is done. It is insufficient to take only

a few samples. Local fluctuations in water quality affect only portions of a

hard clam population and do not determine the overall suitability of an area.

If subjective estimates must be used, they should be made by experienced

professionals familiar with the evaluation area and be accompanied by full

documentation of the basis on which they were made. If further information

is required on hard clams, see the detailed annotated bibliography published

by McHugh et ale (1982).

14
Table 3. Suggested methods for measurement of variables used in hard clam

HSI model. For all variables, use existing data if possible.

Variable Method

Mid-depth dissolved oxygen can be

measured by using Winkler titration or

an oxygen meter (American Public Health

Association 1976). Measure from April

through September.

Mid-depth salinity can be measured by

titration, refractometer, or salinity

meter.

Mid-depth temperature can be measured

by thermometer or temperature probe

(American Public Health Association

1976).

Percentage of silt-clay can be determined

by washing a known weight of sediment

through a 63-~m (Tyler series No. 250)

sieve. Silt and clay pass through the

sieve.

Surface water current velocity can be

measured with a flowmeter.

Turbidity can be measured by direct

measurement of suspended solids

(American Public Health Association

1976). Measure from April to September.

15
LITERATURE CITED

Abbott, R. T. ]954. American seashells. Van Nostrand Co., Inc. New York:

541 pp.

Abbott, R. T. 1974.

Inc , , New York.

Ameri can seashells, 2nd ed, Van Nostrand Rei nho 1d Co.

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American Public Health Association. 1976. Standard methods for the examination of water, sewage, and wastewater, 14th ed. Am. Publ ic Health

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Anderson, W. D., W. J. Keith, F. H. Mills, M. E. Bailey, and J. L.

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S.C. Wildl. ~lar. Resour. Dep., Mar. Resour. Cent., Tech. Rep. 32. 17

pp.

Ansell, A. D. 1967a. Egg production of Mercenaria mercenaria. Limnol.

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Belding, D. L. 1911. The life history and growth of the quahaug (Venus

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Belding, D. L. 1912. A report upon the quahaug and oyster fisheries of

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Shellfish. Assoc. 70:216-229.

16
Burrell, V. G., Jr. 1977. Mortalities of oysters and hard clams associated

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Ca 1abrese, A. 1972. How some pollutants affect embryos and 1arvae of

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Carriker, M. R. 1956. Biology and propagation of young hard clams,

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Castagna, M., and P. Chanley. 1973. Salinity tolerance of some marine

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Chanley, P. E.

sal inity.

1958. Survival of some juvenile bivalves in water of low

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1952. Growth rates and movements of hard clams Venus

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17
Davis, H. C. 1958. Survival and growth of clam and oyster larvae at

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eggs and larvae of the clam (Venus (Mercenaria) mercenaria). Biol.

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Davis, H. C., and A. Calabrese. 1964. Combined effects of temperature and

salinity on development of eggs and growth of larvae of M. mercenaria

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Godcharles, M. F. 1971. A study of the effects of a commercial hydraulic

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anaerobic succinate accumulation within the central and marginal regions

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Gustafson, A. H. 1955. Growth studies in the quahog Venus mercenaria.

Proc. Natl. Shellfish. Assoc. 45:140-150.

Haven, D., and J. D. Andrews. 1957. Survival and growth of Venus

mercenaria, Venus campechiensis, and their hybrids in suspended trays

and on natural bottoms. Proc. Natl. Shellfish. Assoc. 47:43-49.

Hibbert, C. J. 1976. Biomass and production of a bivalve community on an

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J. Acad. Sci. 22(2):52. (Abstr.)

Joi nt Subcommittee on Aquacul ture.

Plan. Vol. 2. Washington, D.C.

1983. National Aquaculture Development

196 pp.

Keck, R., D. Maurer, and R. Ma 1ouf.

behavior of larval hard clams,

Shellfish. Assoc. 64:59-6/.

1974. Factors influencing the setting

Mercenaria mercenaria. Proc. Natl.

Kennedy, V. S., W. H. Roosenburg, M. Castagna, and J. A. Mihursky. 1974.

Mercenaria mercenaria (Mollusca: Bivalvia): temperature-time

relationships for survival of embryos and larvae. U.S. Natl. Mar. Fish.

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18
Kennish, M. J., and R. E. Loveland. 1980.

quahog, Mercenaria mercenaria (Li nne }.

70:230-239.

Growth models of the northern

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Kennish, M. J., and R. K. Olsson. 1975. Effects of thermal discharges on

the microstructural growth of Mercenaria mercenaria. Environ. Geol.

1:41-64.

Kerswill, C. J. 1941. Some environmental factors limiting growth and

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University of Toronto, Canada. 104 pp.

Kraeuter, J. N., M. Castagna, and R. Van Dessel. 1982. Egg size and larval

survival of Mercenaria mercenaria (L.) and Argopecten irradians

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Ladd, H. S. 1951. Brackish-water and marine assemblages of the Texas coast,

with special reference to mollusks. Publ. Inst. Mar. Sci. Univ. Tex.

2(1):125-164.

Loosanoff, V. L.

18:506-515.

1937. Spawning of Venus mercenaria (L.). Ecology

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spawning in winter and breeding its larvae in the laboratory. Biol.

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McHugh, J. L., M. W. Summer, P. J. Flagg, D. W. Lipton, and W. J. Behrens.

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Menzel, R. W. 1961. Seasonal growth of the northern quahog, Mercenaria

mercenaria and the southern quahog, ~. campechiensis, in Alligator

Harbor, Florida. Proc. Natl. Shellfish. Assoc. 52:37-46.

Menzel, R. W. 1962. Seasonal growth of northern and southern quahogs,

Mercenaria mercenaria and ~. campechiensis, and their hybrids in

Florida. Proc. Natl. Shellfish. Assoc. 53:111-119.

Menzel, R. W. 1976. Comprehensive report on the quahog clam (Mercenaria

spp.) industry in Florida. School of Marine Science, University of

Delaware, Newark. Ms. 20 pp.

Menzel, R. W., and M. Y. Menzel. 1965. Chromosomes of two species of

quahogs and their hybrids. Biol. Bull. (Woods Hole) 129:181-188.

Menzel, R. W., and H. W. Sims.

Mercenari a mercena ri a, in

53:103-109.

1962. Experimental farming of hard clams,

Florida. Proc. Natl. Shellfish. Assoc.

19
Menzel, R. W., E. W. Cake, M. L. Haines, R. E. Martin, and L. A. Olsen.

1976. Clam mariculture in northwest Florida: field study on predation.

Proc. Natl. Shellfish. Assoc. 65:59-62.

Merrill, A. S., and J. W. Ropes.

the middle Atlantic coast.

29(4):62-64.

1967. Distribution of southern quahogs off

U.S. Bur. Commer. Fish. Commer. Fish. Rev.

Mitchell, R. 1974. Aspects of the ecology of the lamellibranch Mercenaria

mercenaria (L.) in British water. Hydrobiol. Bull. 8:124-138.

Morrison, G. 1971. Dissolved oxygen requirements for embryonic and larval

development of the hard-shelled clam, Mercenaria mercenaria. J. Fish.

Res. Board Can. 28:379-381.

Nelson, T. C., and H. H. Haskin. 1949. On the spawning behaviour of oysters

and of Venus mercena ri a with especi a1 reference to the effects of

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Pearse, A. S. 1936. Estuarine animals at Beaufort, North Carolina. J.

Elisha Mitchell Sci. Soc. 52:174-222.

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growth in Ven~ mercenaria. Limnol. Oceanogr. 1:2-17.

Rhoads, D. C., and G. Pannella. 1970. The use of molluscan shell growth

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106 pp.

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Savage, N. B. 1976. Burrowing activity in ~lercenaria mercenaria (L.) and

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962. 74 pp.

20
Sims, H. W., Jr., and R. J. Stokes. 1967. A survey of the hard shell clam

(Mercenaria campechiensis) (Gmelin) population in Tampa Bay, Florida.

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Stanley, J. G., and R. DeWitt. 1983. Species profiles: life histories and

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Army Corps of Engineers, TR EL-82-4. 19 pp.

Storr, J. F., A. L. Costa, and D. A. Prawel. 1982. Effects of temperature

on calcium deposition in the hard-shell clam, Mercenaria mercenaria. J.

Therm. Biol. 7:57-61.

Sykes, J. E., and J. R. Hall. 1970. Comparative distribution of mollusks in

dredged and undredged portions of an estuary, with a systematic list of

species. U.S. Natl. Mar. Fish. Servo Fish. Bull. 68:299-306.

Taylor, J. L., and C. H. Saloman. 1968. Benthic project. Pages 3-5 in J. E.

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Wildl. Servo Circ. 290.

Taylor, J. L., and C. H. Saloman. 1970. Benthic project. Pages 3-10 .tn

J. E. Sykes, ed. Report of the Bureau of Commercial Fisheries

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U.S. Fish Wildl. Servo Circ. 342.

Tenore, K. R., and W. M. Dunstan. 1973. Comparison of feeding and biodeposition of three bivalves at different food levels. Mar. Biol. 21: 190-

195.

Thorson, G. 1955. Modern aspects of marine level-bottom animal communities.

J. Mar. Res. 14:387-397.

U. S. Fish and Wildlife Service. 1980. Habitat evaluation procedures (HEP).

ESM 102. Wshington, D.C. n.p.

Wells, H. W. 1957a. Abundance of the hard clam Mercenaria mercenaria in

relationship to environmental factors. Ecology 38:123-128.

Wells, H. W. 1957b. Status of the name Venus. Ecology 38:160-161.

Woodburn, K. D. 1961. Survival and growth of 1aboratory-reared northern

clams (Mercenaria mercenaria) and hybrids (M. mercenaria x M.

campechiensis) in Florida waters. Proc. Natl. Shellfish. Assoc. 52:31-

36.

Woodburn, K. D. 1962. Clams and oysters in Charlotte County and vicinity.

Fla. State Board Conserve Mar. Lab., Mimeo Rep. No. 62-12. 29 pp.

21
OPTIONAL FORM 27 (

(Formerly NT15-35)

Department of Commerce

50272 1 T

(See ANSI-Z39.18)

- 0

REPORT DOCUMENTATION II. REPORT NO.

1

2

.

3. Recipient's Accession No.

PAGE -- FWS/OBS-82/10.77 •. Title and Subtitle \ 5. Report Date

Habitat Suitability Index Models: Hard Clam ____ t AUJlust

1984

-- 7. Author(.) ~rl"'"'.' 0"an " at,on , ept. "" R. Mulholland

9. Performing Organization Name and Address 10. ProJect/Task/Work UnIt No.

Florida Cooperative Fish and Wil dl ife Research Unit. USFWS

School of Forest Resources and Conservation I~Cont,"ct{Cl or Grant(G) No

117 Newins-Ziegler

University of Florida «»

Gainesville. FL 32611 (G)

12. spOn"'Orin~OreanizationName and Address 13. Type of Report & Period Covered

Nationa oastal Ecosystems Team U.S. Department of Interior

Division of Biological Services Washington, DC 20240

Research and Development 14.

Fish and Wil dl ife Service ---

IS. Supplementary Notes

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

16. Abstract (Limit: 200 'Words)

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

evaluating estuarine habitat for hard clams (Mercenaria mercenaria and Mercenaria

campechiensis). The model is scaled to produce an index of habitat suitability between

o (unsuitable habitat) and 1 (optimally suitable habitat) for the Atlantic and Gulf

coasts of the United States. Habitat suitabil ity indices are des i gned for use wi th

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

Servi ce.

17. Docurnent Analysis a. De scrfptors

Mathematical models

Estuaries

Mollusca

Habitat

b. Iderrtifie rs yOoen-End ed Terms

Bivalves Mercenaria campechiensis

Habitat Suitability Index Venus mercenaria

Hard clam Quahog Mercenaria mercenaria

c . COSATI Field/Group

119. Security Crass (This Report)

-r-r18. AlJailability Statement i 21. No. of Pages

Unl imited ! Uncl assified : vi + 21 ~---~----------- ----:-----------

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UGION 1

Rqiunlll Director

O.S. Fish lind Wildlife Service

Uoyd Five Hundred Building, Suite 1692

500N.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

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REGION 2

Regional Director

U.S. Fish and Wildlife Service

P.O. Box 1306

Albuquerque, New Mexico 87103

REGION S

Regional Director

U.S. Fish and Wildlife Service

One Gateway Center

Newton Comer, Massachusetts 02158

REGION 7

Regional Director

U.S. Fish and Wildlife Service

10II E. Tudor Road

Anchorage, Alaska 99503

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REGION 3

Regional Director

U.S. Fish and Wildlife Service

Federal Building, Fort Snelling

Twin Cities, Minnesota 5SIII

REGION 6

Regional Director

U.S. Fish and Wildlife Service

P.O. Box 25486

Denver Federal Center

Denver, Colorado 80225
DEPARTMENT OF THE INTERIOR

u.s. FISH AND WILDLIFE SERVICE

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

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

preserving th.environmental and cultural'values of our national parks and historical places,

and providing for the enjoyment of life throulh outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in

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

American Indian reservation communities and for people who live in island territories under

U.S. administration.