! 1992 Rhode Island Sea Grant ISBN 0-938412-33-7 This publicationis sponsoredin part by RhodeIsland Sea Grant under NOAA Grant No. NA 89 AA-D-SG-082. The views expressedherein are those of the author and do not necessarilyreflect the views of NOAA or any of its sub-agencies.This publicationis alsosponsored by RhodeIsland Cooperative Extension System under Grant No. Si 402, and the Northeast RegionalAquaculture Center, U.S. Depart- ment of Agriculture. This is RhodeIsland Sea Grant publicationNo, RIU-B-92-001 P1276! and publicationNo. 2674 of the RhodeIsland Agricultural Experiment Station,College of ResourceDevelopment, University of RhodeIsland. Additional copiesof this publicationare available from Rhode Island Sea Grant, University of RhodeIsland Bay Campus, Narragansett, RI 02882-1197.

~ c;a~+ . Printedonrecyeied paper. The NortherniNI ~ Quahog:

The Biology of Mercnusria mercenaria

Michael A. Rice AssistantProfessor of Fisheriesand Aquaculture Departmentof Fisheries,Animal, and Veterinary Science University of Rhode Island Kingston, Rhode Island 02881

Edited by CaroleJaworski and Malia Schwartz Rhode Island Sea Grant Information 0%ce Design by Donna Palumbo O'Neill Rhode Island Sea Grant Information Of6ce Drawn from living clam .04mm less than one- fiftieth of sn inch! in length. S, siphons, two tubes, one of which conducts water bearing food and oxygen to the body within the shell, the other conducting a stream containing waste matter to the exterior. F, foot, the organ of locomotion.B, byssus, a delicate thread for attachment, which is not presentin the adult. From Bulleti n of the ¹w York State hfuseum, le. 43, Vol, tt8, April 190I. INTRODUCTION

GENERAL BIOLOGY 1 Taxonomic Position and Common Names 1 GeographicDistribution and RecognizedSubspecies Similar Bivalve Species Anatomy 25 27 Respiration and Circulation Filter Feeding and Digestion Reproductive Biology and Life Cycle 8 Growth 11

ECOLOGY AND ENVIRONMENT 13 Ecology 13 Factors Influencing Growth of Adult Populations 13 Factors Influencing Larval Settlement and Juvenile Survival 14 Quahogs as Prey 16 Structure of Unexploitedand ExploitedQuahog Populations 18 Quahog Assemblages as Modifiers of the Environment 20 Effects of Environmental Factors 20 Effects of Temperature 20 Effects of Salinity 21 Effects of Oxygen 22 Effects of Pollutants 23

FISHERIES 27 Fishing Methods and Gear 27 Fishery Management Regulations 32 %ster Quality and the Fisheries 32 Economics of the Fishery 34

AQUACULTURE $7 Hatchery Methods 37 Nursery Methods 39 Growout Methods 40 Aquaculture Diseases and Pathology 40

CONCLUSION

REFERENCES Acknowledgements

I wish to acknowledge a number of individuals and orlmnixations who were very helpful in the production of this book, Elizabeth Watkins and Vincent Kncena provided many of the line drawings. Recognition is due to William Brown Publishers, Macmillan Publishers, the Joiirnal of Shellfish Reaearch,Trunsactions of the American Fisheries Society, and Scientific American for granting permission to reproduce previously published mate- rial. I especially thank Dr. Joseph DeAlteris, Dr. Lindy Eyster, Dr. Jan A. Pechenik, Dr. Conrad Recksiek, Ms. Kathleen Castro, Mr. Gef Hlmlin, Ms. Carole Jaworski, and Mr. Robert Rheault for providing valuable comments on the manuscript.

Intvoduction

Thenorthern quahog, Mercenaria mercenaria Linnaeus!, isof interest to manypeople, especially to thoseliving along the eastern seaboard ofthe UnitedStates. This species supports valuable commercial and recreational shellfisheriesallalong the Atlantic coast. Seafood connoisseurs eqjoya host ! ofdelectable recipes alldependent onits delicate Qavor. Inaddition, some ! marinescientists find the quahog tobe the ideal organism tostudy a hostof ! fundamentalscientific problems. Theimportance ofthe quahog tomany peoplehas not been overlooked bythe legislature ofthe state of Rhode Island, whichdesignated the quahoghs the oKcial statemollusk. Thisbook is patterned after a previousRhode Island Sea Grant publica- tionon the lobster J. StanleyCobb's 976! TheAmericun Lobster: The BiolagyofHonmrus americanus! andis intended tobe a companionvolume. Theaim of this book is to providean overview of someof theinformation availableabout the quahog, and to provide a startingpoint for those who wishto delve further into the Mercenarta literature. Although the references sectionof this book contains a numberof recent citations, the serious re- searchershould be aware of an excellent annotated bibliography 74! con- tainir~ over 2,200 citations.

Photographby DonnaPalgznbo O'A'eili

GenemlBiology

Taxonomic Position and Common Names Menenaria mercenaria Linnaeus, 1758!is a molluskof the class Bivalvia formerlyPelecypoda!, subclass Lamellibranchiata, order Heterodonta,and family Veneridae. Thus, its shellhas two valves,it has sheet-likegills, it is clam-hkewith large hinge teeth, and is in thefamily of hard-shelledclams. In theliterature prior to theearly 1960s, the quahog is knownas Venus mercenaria L., 1758.The accepted o%cial malacological commonname the nameused by molluskan scientistsand shell enthusiasts! for M. mercenariais the northernquahog, but it is locallyknown as hard clam,hard-shell clam, round clam, or quahog quahaug!. In additionto these regional names, there are difFerent names for arumals of different sizes. In RhodeIsland, the smallestlegally sold quahogs approximately 48 milli- metersvalve length! are known as "littlenecks."Intermediate-sized hm length!and large >75mmlength! are known as "cherrystones and "chowder quahogs,"respectively. In someother localities, the term cherrystone"refers to the smallestanimals that can be legally harvested.Other market names indude topnecks and "topcherries forintermediate-sizedaninmls. In NewEngland, quahogs have been harvested since pre-colonial times. Accordingto the OxfordEnglish Dictionary, RogerWilliams, the flrst colonial governorof the ProvidencePlantation Colony, wrote in 1643,"Poquauhock, this the English caOHens, a little thick shel-fis sic!, which the Indians wade deepe sic! and dive for." Our modernword "quahog"is derived&om "poquauhock"in the languageof the NarragansettIndians living in whatis nowRhode Island!. They ate the meatand used the shellsto makewampum beads,which were a trading currency.The wampumbeads made &om the

Phocognrph by Michaei A. Rice purple shell margins were especially valuable. Indeed, the speciesname "M. mercenaria" refers to their former value as a tradn~ currency.

Geographic Distribution and Recognized Subspecies The quahog inhabits shallow coastal waters from The Gulf of St. Lawrence in Canada to Florida 81!. It has been introduced into Europe ; 115; 116; 117! and California 5; 56!. One recognized subspeciesis Mercenaria mercenaria notata, which is characterized by chestnut-colored, often chevron-shaped markings on the shell exterior 2; 96!. Field and laboratory studies suggest that the M. m. notata subspeciesoccurs with a frequency of 1 to 2 percent or less in the southern Atlantic states 6; 120!. The subspeciesMercenaria mercenaria texana is native to the northern coast of the Gulf of Mexico ; 55!.

Similar Bivalve Species The southern quahog, Mercenaria campechiensis,is a closely related speciesthat has more prominent shell sculpturing and attains larger sizes. M. campechiensisis found in the more southern regions of the range of M. mercenaria and the two species are reported to form hybrids 8; 176!. The false quahog, Pitar morrA,uanus formerly Cailocardia!, is found in the geographic range of M mercenaria 96!. Individuals of the false quahog, P. morrhaanus, reach a maximum size of about 60mm valve length, have thinner shells than Mercenaria, and are dull gray with smoother shells. There is evidence that Pitar prefers a muddier substrate than does Mercenaria 00!.

Anatomy The quahog, like all other clams, has its soR tissues surrounded by a shell consisting of two halves or valves. The quahog shell consists of calcium carbonate in a crystalline form aragonite! held in a network of complex organicmolecules 05!. The valvesare held togetherby a tough,but pliable, hinge ligament alongthe top or dorsalsection of the animal, called the hinge plate. The valve hinge, which consists of intermeshing teeth, forms the joint between the valves. The hinge allows for opening and closing of the shell. In close proximity to the hinge is the umbo or what is commonly known as the "beak."If you notethe roughly triangular shapeof the quahog,and orient the quahogso that the hinge and the umbo are at the top or dorsal,the widest edgeof the shell or the part that is oppositethe umbo is the ventral shell margin. The umbo is the oldest section of the shell, with subsequent shell growth radiating out from it. The concentricrings on the external surfaceof crenulated shell margin

anterior posterior adductor adductor muscle musde scar

erlor ot hinge actor

anterior foot posterior retractor adductor musde labial palps excurrenl siphon

anterior adductor ncurrent muscle siphon

Figure1. Grossanatomy ofthe quahog, Mcrcenonamercenaria, Thesoft tissues aredis- sectedfree from the left valve toreveal the pallial lineand adductor musde scars. From Shuster, C,N.1966. "A three-ply representation ofthe majororgan systems ofa quahog."National Shell6shSanitation Program, U.S. Department foot inner outer ofHealth, Education, and Welfare. Pechenik, demibranch J,A.1991, Biokgy ofthe invertebrates nded.!. demibranch WilliamC. BrownPublishers. Used with permission. the shell indicate the generalgrowth pattern of the animal. The mantle is responsiblefor the formationof newshell material. New shell forms at the ventral shell edgeby secretionof the network of organicmolecules proteina- ceousmatrix! and calciumcarbonate by the mantle 85; 248;254; 255!. The extrapallialfluid KPF!is a liquidsimilar to seawaterand the bloodof the quahogand is foundbetween the mantleand shell. This fluid playsan importantrole in the shellformation process 7!. Featuresassociated with the inner surfacesof the shell include the pallial line, which is the point of attachment for the mantle. Muscle scars are the depressions near the front anterior!and rear pasterior!ends of the valves.These muscle scars are the pointsof attachmentfor the posteriorand anterior adductor muscles Fig. 1!. The valvesare closedby the posteriorand anterior adductormuscles. Theprimary function of the two adductormuscles is to keepthe valvesclosed in responseto predatorsar adverseenvironmental conditions. In eachof the adductormuscles there are two fiber types arrangedin distmct regionsof the muscle.These regions are pinkish in colorfor the fast Gbersand white for the catch muscle fibers. The fast muscle is responsible for the rapid closure af the shell; its pinkish colorreflects the presenceof a pigmented,iron- cantaizmgprotein calledmyoglobin 38!. The function of myoglobinis to bind and storeoxygen &om the bloodand releaseit as the musclerequires moreoxygen for metabolicactivity. Catch muscles can maintain the sheDsin a dosedposition for longperiods of time with little or nofatigue ll; 53;157!. A largemuscular foot that canbe extended beyond the ventralshell marginallows for burrowing Fig. 1!.Key muscles controDing the footare the anterior and posteriorfoot retractor musclesand the musclesof the foot itself. Theseallow for rapid.retraction of the extendedfoot and locomotion both vertically and horizontally in the sediments. Other key featuresof the grossanatomy of a quahoginclude the incurrent incomingwater! and excurrent siphons outgoing water! Fig. 1!.The incur- rent and excurrentsiphons have sometimes been called "necks." Since quahagsare normally buried in the sediments,the siphansextend into the water columnabove, so that water canbe taken into and expelledby the animal.The water brought into the quahogvia the incurrentsiphon contains oxygenfor respiration and tiny food organisms. Metabolic wastes are expeDed in the out-flawingwater of the excurrentsiphon. Lateral cilia microscopic hair-likeappendages! on the filamentsof the gills actto propelthe water throughthe animal08; 129!.The gills actto filter outfood particles in the incurrent water and providea surfacefor gasexchange, Labial palps at the anterior sectionof the gills providefor foodsorting prior to ingestionvia the mouth. Details of filter feedingand gasexchange wiH be discussedlater. hingeligament pericardium foot

rectum

anus anterior toot retractor posterior mouth adductor musde

labial excurrent palps siphon

anterior Incurrent adductor siphon musde

digestive foot intestine gonad gland

Figure X, Diagrammaticrepresentation of the Most of the digestive,circulatory, excretory,and reproductiveorgans are nugororgan systems in the visceralmass of the containedwithin the major body mass visceralmass! lying abovethe foot quahog,j&rcenario mercenaria.Pechenik, J.A. Fig. 2!.A two-chamberedheart is surroundedby a fluid-filledsac called the 1991.Biofogy af the Invertebratesnd ed.!.Wg- liarn C. Brown Publishers.Used with permission. pericardium.Closely associated with the pericardiumis the kidney. The digestivesystem consists of a shortesophagus leading to the stomach.Leav- ingthe stomach, the intestine passes through extensive gonadal tissue and thendorsally through the heart. The rectum is situatedin a positionabove theposterior adductor muscle. The anus opens into a chamberthat leads directly to the excurTentsiphon Fig. 2!.

Respirationand Circulation Water is drawn by gill cilia into the quahogthrough the incurrent siphon andpassed across the giHs, which provide a surfacefor gas exchaxige Fig. 3! Theremay also be considerable gas exchange across the mantlesurface of bivalves 84; 85!. Bloodvessels within the gills provide a meansfor trans- 'portingoxygen to sitesof metabolicneed. The blood, called hemolymph, is afferent inner blood vessel demibranch

efferent blood vessel outer emibranch ascending lamella

descend cilia lamell

mantl foot deliveredby the arterial systemfrum the heart to the gills, where oxygenis pickedup, and then transportedto the outer tissues.The quahoghas an opencirculatory systemso once in the outer tissues,the hemolymphis releasedfrom the bloodvessels into opensinuses to directly bathe the tissues and to deliver oxygen.The hemolymphpasses through the kidney as it returns toward the heart and collectsin the openspace of the pericardium.It is then drawn into the two chambers of the heart to start the ~atory cycleover again.The hemolymphof quahogsdoes not carry any specialized proteins respiratory pigments!to aid in the transfer of oxygen,Gasses are dissolveddirectly in the hemolymph.The hemolymphis chemicallysimilar to, but not identical to, the extrapaHial fluid 7; 209!.

Filter Feeding and Digestion The processof foodacquisition by the quahogis dependenton the pump- ing of water 49! Fig. 3!. As water passesbetween the gill filaments,par- ticulatematter, suchas silt and phytoplankton,is trapped on mucoussheets on the outer surfaceof the gills 1!. Specializedcells in the giH are respon- sible for the productionof the large quantity of mucousnecessary for tra p- ping foodparticles in the internal water stream,The filtration efftciency the amount of particlesretained per unit volumeof water pumped!of quahog gills is known to decreaseas particle concentrationin the water increases 45!. This decreasein filtration efficiencyserves to regulatethe amountof food availableto be passedalong to the digestivetract. Frontal cilia alongthe outer surfaceof the gill moveall trapped particulate material foodas well as non-food items such as silt! in the mucous to food grooves ciliated tracts similar to a conveyerbelt! at the ventral edgeof eachof the gills Fig. 3!.

Figure S. Diagrammatic mid-body transverse Oncethe particulate matter trapped in mucousreaches the foodgroove, it is section of a 92ameihbranchbivalve showing the carriedalong by ciliary motionanteriorly toward the labial palpsand mouth. position of the gills and water flow during active The labial palps act to sort and further regulatethe amount of foodingested pumpingand particie filtration. From Russell- 4!. Rejectedparticulate matter, suchas silt or excessphytoplankton, is Hunter, W.D, 1979. Life of the Invertebrates, Macmillan Publishing Company. Used with per- droppedonto the mantle surface,eventually to be releasedas mucous-coated mission. balls resembling feces pseudofeces!2!. Ingestedfood particles are passedthrough the mouth and esophagusto a multi-chamberedstomach with numerouspassageways and dead-endsa& digestivediverticula! 03!. One of the key organsassociated with the stomachof the quahogis the crystallinestyle, a thinglass&ear organ that is own mistaken for a wortn,but actually containsdigestive enzymes. The styleis locatedin the stylesac, which is closeto the stomach.During diges- tion, the styleis rotatedby ciTialocated along the wallsof the stylesac to releasedigestive enzymes and natural "detergents" emulsifiers!, which aid in the digestionof fats 04; 221!.The style may alsohave a grinding Figure 4 Life cyde of the quahog, Merccnaria function as it rotates like a mortar and pestleagainst the stiff gastric shield merccnaria; a! unfertilized egg aud sperm; h! 3!. Although quahogsdo not haveenzymes to digestthe silica tests, or fertilized eggand polar body formation; c! firstcell "skeletons"of diatoms,diatoms are a keyfood source. The anatomyof the division;d! four~ll embryo;e! eight-cellembryo showing spiral cleavage;f! morula; I! early stomachsuggests that there is somemechanical crushing of the cells.Thede trochophorelarva post-gastrulation!; h! fully is alsoevidence that someingested silt may enhancethis mechanical developedtrochophore larva; i! D-hingeveliger crushingof cells3!. Thestomach and associated digestive glands may also larva; j! umbonateveliger larva; k! pediveliger larva; and l! developed post-set juvenile. secretesome digestive enzymes 20!. Much of the uptake of digestednutri- Drawings by V.C. Encena. ents occursin the digestivediverticula numerousblind sacs!associated with the stomach and the intestine. A number of studies suggest that quahogs feedperiodically with a rhythmthat correspondsto dailyfluctuations in food availability even in continuallysubmerged populations 2; 213!.In addi- tion to feedingon filtered particulate matter, quahogsmay derivepart of their nutrition from direct absorptionof dissolvedorganic nutrients across soft-tissue surfaces 09; 252!.

ReproductiveBiology and Life Cycle Like many other bivalves,juvenile quahogsare typically males61!. In successiveyears they may changesex and produceeggs a characteristic calledprotandric hermaphrodism. Sperm cells are much smaller than eggs and,as such, require much less metabolic energy to produce.Production of spermcells by predominantlysmaller and younger individuals allows for earlierreproductive potential among quahogs. In older,larger, and slower growing animals, more metabolic energy can be devoted to production of sex cells, thus the switch to the larger eggs. By the time quahogs reach legally harvestable size about 2.5 inches long!, there is about a 1 1 ratio of males to females 1!. In natural populations, gonads begin to produce ripe sperm and eggs during the late spring and early summer months. This corresponds to water temperature rising above 10 C 0'F! and the beginnirig of feedirig on available phytoplankton. Once the water temperature exceeds20'C S'F!, "ripe" adults begin to spawn 49!. Quahogs do not exhibit reproduc- tive senescenceor de~aeM gamete production with age 1; 190; 191!, A large >90mm valve length! female may be 40 or even 50 years old and still produce 10 to 30 million viable eggs.The eggs of quahogs are 70 to 73 irrn lim = 1/25,400 inch! in diameter and are surrounded by a gelatinous membranewhich is 50 pm thick. Both eggsand spermof adults axeexpelled in the water current of the excurrent siphon; fertilization prexeds externally in the water column. The embryonic stages which include the fertilized egg through early cell divisions,the hollow ball of cellsstage or blastula, to the 6rst larval stage trochophore! take &om 18 to 48 hoursto complete,de- pendingon the temperature,The eggmembrane often surroundsthe devel- opmgembryo well into the blastula stage53!. Followingthe trochophore stage,successive larval stagm include the straight-hingeveliger, the um- bonate veliger, and the pediveliger Fig. 4!. The various veliger stages are characterizedby the presenceof the ciliated velum, a largesail-shaped organ fine growth lines

annual increment

translucet zones

Figurv: 5. Aging quahogs using shell growth bands: a! yearly growth bands of hfercenario can be counted by sectioning the shell from the umbo to the ventral shell margin with a lapidary saw; b! darker "winter break" bands are visible along the polished cut; and c! diagramnmtic dose-up of tbe ventral shell margin. Prom McManamon, FP. and J.W, Bradley. 'The Indian Neck Ossuary," Scieniitic American 258!:9S- AII104.rights Copyrightreserved. 1988,Used Scientificwithpermission. American Inc. extending &omthe smaHlarvalshell thatcaptures particulate food,gener-

ates a respiratory current, and allows for some movement of the animal in the water column. The straight-hinge veliger also known as the D -hinge veliger! is given its name becauseof the shape of the larval shell. The larv~ shell of the umbonate veliger has taken on the characteristic triangular shapeof quahogs,with a prominentumbo. Most of the larval stagestend t swim toward light or opposite the force of gravity!, so they tend to be conco- trated in the surface water. This is the period of dispersion by wind, waves and currents,The pedivebgerstage is the final stageprior to settlementad eventual metamorphosis to juveniles. Pediveligers are characterized by a well-developedfoot that extendsfrom the shell, and they eventuaHybegin o settle. Depending on water temperature, the time &om trochophore to settlement may last &om eight days to two weeks 55!. 2.0

E o E ~ a ZC9 80 ig oft 9 OO Z 1.6 z 60P- o Nt: ~ West Passage G Greenwich Cove 1.6 0

l.4 10 20 30 -0.3 0.3 0,7 1.1 1.5

ANNUAL GROWTH RINGS LOG WEIGHT

Growth Figure 6 leg!. The valve lengths of quahogs As previously noted, the shell grows by deposition of calcium carbonate in &am three study sites in Narragansett Bay, R.I., a network of complex organic rnolecules at the ventral edges.In addition, as are plotted as a functionof estimated age.Grawth curves were fit to the Greenwich Cove and West quahogs age, there is a thickening of the shell by deposition of shell material ~ data. Fram Rice et al. 989!. Used with on the inner valve surfaces. Shell growth is temperature dependent with pernnssion. deposition owning mainly between the temperatures of 10oC0'F! and 25'C 7'F!, with growth ceasing below 9'C 8 F! and above 31'C 88 F! ; 128!. Most rapid growth is reported to be at 20'C 8 F! 8!. In mast areas Figure 7 righth Logarithmic plot of Aferaenaria within the geographical range of quahogs, the winter water temperature meraenarra valve length and weight of the soft drops below 9'C, so growth ceasesand there is an annual winter break" tissues. There is a high degree of correlation r = 0.973! between valve length and the weight of recorded as a translucent layer in the shell; thus, by sectioning and polishing soft tissues.Data and figurefram Rice et al 989!. shells, the age of the animal can be determined Fig. 5!. In addition to the Used with permission, annual winter break, there are possible growth lines that correspond to slowing of growth due to excessivelywarm summertemperatures 98! and daily or sub-dailygrowth lines correspondingto periodsof non-feedingand valve closure 95; 133!.The growth and ageof quahogshas beendetermined in Massachusetts 05!, New Jersey 98; 133!, Maryland and Virginia 90!, North Carolina 96!, New York 16!, Georgia 43!, and Rhode Island 28, 210!. The rate of quahog linear growth tends to decreasewith age Fig, 6!, and can be best described mathematicallyby von BertalanfIy'snegative exponentialgrowth equation28!. In general,the growth of soft tissues closely follows shell growth Fig. 7!, but there is a seasonality in the size and weight of somatic and gonadal tissues 17; 149; 193!. ~ 5>~ I

~No

! 'i~k,P'

~~ p ~ Ecologyand Enwirurument

Ecology Factors In8uencing Grovvtb of Adult Populations The character of the bottom sediment may irdluence the growth of quahogsin the field 00; 201; 205!.In thesestudies, quahogs tended to grow faster in sand as opposedto silt/clay sediments.The quantity and quality of foodhas an effecton bivalve growth rate 5; 80; 244;249!. In addition, other investigatorshave shownthat increasingcurrent speedincreases the growth rate of quahogs02; 111;134!, which has beeninterpreted to be a result of increasedrate of fooddehvery to the animals.The relationship betweenthe various factors of sediment type, current speed, and food availability is complex.It appearsthat excessivelyhigh current speedscan be so disruptive to normal feedixxgthat they can inhibit quahoggrowth 83!. A recentstudy was designedto determinethe relative contributionsof foodquantity, cur- rent speed,and bottom sedimenttype on the growth of quahogsin the field 00!. It found that the combination of food concentration and current speed seston flux! is the major determinate of quahog growth. Sediment composi- tion had a very xninor effect on growth. Effects of sediment composition on growth, as reportedin earlier studies,were reinterpretedto be a secondary effect of current speed on the distribution of sediment types. It is the higher averagecurrent speedsthat usually result in both sedimentswith larger averagegrain size and in higher rates of quahoggrowth, A statistical model was developedthat predicts quahoggrowth under a variety of foodconcen- trations, current, and sedimentconditions 99!. The xnodeltates into account that: a! food availability is the major determinateof quahoggrowth; b!

Photograph oourtorryof R.I. Division of Tourism I sedimentgrain sizesare occasionallypoorly correlatedwith current speed C. Browning [e.g,,New Jerseycoastal lagoon 0!; Narragansett Bay%est Passageand GreenwichBay 10!]; and c! there may be inhibitory effectsof very high ~nt speeds on growth 83!. The seston flux hypothesis implies that if the flow of seawater past quahogsis artificially increased,it is possibleto increasetheir growth rates, There are a number of aquaculture systems that rely on this pumping raw seawater rather than adding supplemental food 69!. Food limitations can explain reducedgrowth rates of quahogsseeded at, very high densitiesin aquaculture plots 9; 83; 242!. It may be possible to predict stocking densities of quahogsof varioussizes in a numberof aquacultureapplications by know- ing food availability, current speed,and particle filtration rate of the quahogs at various sizes.A theoretical "care ing capacity" model has been described for mussels in suspendedculture 23; 124; 158!. The model relates the particle filtration rate of a population of mussels to the food available due to seston flux. The filtration of phytoplankton by quahogs has been studied 12; 245!, and it is well known that filtration rate its in relation to the weight of the soft tissues 07!. From this, it can be concludedthat optimal stocking densities for clams in an aquaculture plot are better estimated by using biomass or biovolume per unit area rather than simple numbers of animals. To illustrate this point, there is an unexploited population of quahogs in the Greenwich Cove portion of Narragansett Hay, R.L, with a density of 190 animals per square meter and an average valve length of 62mm 10!. Individuals in the population are slow growing, characterized by "blunt" valve margins faster-growing quahogs are called "sharps"!. Suspecting that there may be densityAependent stunting or food-limited growth in this natural assemblageof quahogs, the standing cmp biomass of this assemblagewas compared with recommendedaquaculture stocking densities. In an aquacul- ture application, it has been recommendedthat quahogs with a valve length of 20mm should not be stocked at densities exceeding1,000 animals per square meter or reduced growth will occur 9!. This stocking rate converts to 440 grams of shell-free meat biomass! weight of quahogs per square meter. In Greenwich Cove, the shell-free biomass works out to be over 2,000 grams per square meter conversion data in Fig. 7!. Since filtration rate is a function of body weight 07!, it is clear that the 190 quahogs per square meter in Green- wich Cove would be filtering three to four times as much water as the 1,000 20mm animals. Thus, it is likely that these quahogs in a densenatural assemblageare stunted due to food limitations.

Factors Influencing Larval Settlement and Juvenile Survival One of the factors that determines the eventual distrIbution of adult quahogs is the successof larval settlement and metamorphosis. Spawning of Mercenaria appears to be triggered by water temperatures approaching 20'C. In Rhode Island and the Great South Bay of Long Island, spawning occursin June and July 31; 143!.In the more southerlyareas of the quahog range, spawnmgcan occurin early May 82!. Sincethe larval period of Mercenariacan last approximatelytwo weeks,tidal currents and wind- generatedsurface waves can effectivelydisperse the larvae to areasmany kilometers distant from the paxent stock ; 250!. The dispersal of the larvae by wind, waves,and currents is facilitated by their tendencyto seeklight and float high in the water column.In nature, the survival of the planktonic larval stagesis highly variable, but on average,only 2 percentof the early larval stagessurvive through to the pediveligerstage 8!. As the larval stagesprogress and pediveligersdevelop, they reacha stagein which they are capableof settlementand physicalchange metamorphosis!,enabling them to live in the sediments. It is likely that the events of quahog settle- ment and metamorphosisare similar to thosedocumented. for other bivalves 9; 188!.Once pediveligers become capable of settling, there are distinct changesin behavior.One of thesebehavior changes is a switchkom light- seekingto dark-seekingbehavior, causing the larvae to swim toward the bottom !. Many of the experimentsdone to study the behaviorof larvae at the time of settlement have not ruled out other possible larval responses.It is possiblethat there may be a gravity responseby settling larvae 7!. As the settling larvae reachthe bottom, there is a substrate-seekingbehavior in which the larvae touches down on the substrate and appears to crawl around 8!. If a preferredsubstrate is not found, the larvae may return to the water column.This delay of settlementand metamorphosiscan last for several days,but the selectivityfor preferredsubstrates may decrease 88!. Thus, astime pammds,larvae may select less than optimumsubstrates for settle- ment. One study suggeststhat the substratepreference of quahogsis sand rather than finer silts or clays.In addition, there is a strongattraction to sedimentswith added"clam liquor" quahogblood!, suggesting that there maybe a chemicalcue pheromone!that larvaeactively seek out during settlement site selection 32!. Once the larvae metamorphose, the velum, whichis the key organfor larval swimmingand feeding, is droppedfree. The larvaebecome fully adaptedfor life onthe bottomand there is nofurther return to the watercolumn. This periodof settlementand metamorphosis is oneof the most critical in the moHuskanlife cycle,and a largenumber of larvae do not survive the transition.- Preferred settlement locations appear to beimportant for minimizingsubsequent post-settlement predation losses. In addition to the mechanismof active substrateselection and delayof metamorphosis,near-bottom water currents have a considerableimpact on the eventuallocation of larval settlement5!, In the caseof Mercenaria, settlementis enhancedby the relativelylow currents in seagrassbeds 92!. Settlementis generallylower in opentidal channelsof coastallagoons with higher current speeds 97!. The implication is that bottom currents are capableof sweepinglarvae into areasof relatively low current prior to settle- ment. In a reviewof this topic, Butman 8! points out that, of the factors that havean impacton the final settlementsites of bivalves,hydrodynaxnic factors,such as curxents,are dominant on large spatial scalesfrom tens of metersto tens of kilometers;but on smaller scalesof centimetersto meters, activehabitat selectionby settling larvae may be domixiant So,the currents appearto get the larvae into the proper neighborhoodfor settlement,but the final site choice may be by active substrate selection. In addition to larval settlementand successof metamorphosis,post-set survival of quahogsis a critical factor determiningadult distribution, Immedi- atelyafter settlementand metamorphosis, quahogs lack incurrent and excur- rent siphons, so they must reside on the sediment surface for a tixne. Turnover of sedimentsby deposit-feedinginvertebrates may ~t in burial or disrup- tion of filter-feeding,resulting in lossof the post-setjuveniles 84; 206!. Predationlosses also account for post-settlementloss of quahogjuveniles. Early work on the distribution of adult quahogsfocused on the characterof estuarinesediments and associatedbiota. Quahogswere least abundantin areasthat had a highclay and silt content00!. Maxixnumquahog abun- dancewas in areasof GreenwichBay, RI., which had moderatenumbers of the tube-dwellingamphipod, Ampelisca 31!. Verydense assemblages of Ampelisca called "tapemud" by somefishermen~ffectively exclude quahogs.A studyin RhodeIsland's Providence River showed that quahog abundanceswere significantly higher in areasthat hadsediment particles in excessof 2mm 19!. Other studies,including a recentone in Florida, show similar trends of increased quahog abundance in areas with sediments containingshell f'ragments 97!. Studiesthat have directly testedthe survival of juvenile post-setquahogs in varioussubstrates have concluded that large sedimentgrain sizeoffers protection from predators,thus allowingfor greater survival 9; 26; 50; 51; 159!.Seagrass beds may alsoprovide protection from predators 89; 192; 195!.

Quahogs as Prey Post-set predation loss is the key determinant of eventual distribution of adult quahogs60!. Onestudy found that loweredadult quahogpopulations are found in areaswith abundantjuvenile clam predators.Poisoning of the predatorsincreased seven- to eight-foldthe numbersof quahogssurviving to adult size59!. Recognizingthat predationis of suchimportance to the eventualsize of quahogpopulations, one stu.dy concluded that management approachesdirected at increasingjuvenile survivorship would be the most Figure L Representativepredators on qnahogs: a! the blue crab, CoQiaectes sopidus; b! the rock crab, Coaorri rrorotus;c! the greencrab, Curci aus masnus;d! the mnd crab, ¹opaaops sp.; e! the oysterdrill, Umsalpinxcinerea; f! themoon snail, Poiioicssduplicatus; g! the seastar, Asterias sp, Drawings by E. Watkins. effective in increasing productivity of quahog fisheries 64!. Aquaculturists are well awareof the necessityto protectjuvenile stages from predationlosses. Smaller juvenile sizes <20mm valve length! are known to be much more susceptibleto predationthan larger animals7!. Successfulprotection of juveniles until the stageat which they are resistant to most predatorscan makethe differencebetween a profitableand nonprofltable aquaculture business 9; 140!, The predatorson juvenile Mercenariaare numerous.They include crabs such as the blue crab, Calli nectessapidus Fig. Sa! 9!; Cancer spp. crabs Fig. Sb!9!; the greencrab, Carci nus maenus Fig. Sc!46!; the stonecrab, Menippemercenaria 70!; mudcrabs of the genusNeopanope Fig. Sd! and related panopeidgenera 44!; and the horseshoecrab, Limulus polyphemus. Gastropodpredators include whelks of the genusBusycon 89!; the oyster drill, Urosalpinxcinerea Fig. Se!;the whelk, Thais lapillus; and the moon snails,Polinices dupluxrtus Fig. Sf! and Lunatia heros.Oyster drills and moonsnails prey upon smaller quahogsby being ableto bore a holethrough the shell. Empty quahogshells with a singlestraight-sided hole the sizeof a pencil point are signs of oyster drill predation.Holes bored by moonsnails appearto be "counter-sunk. Seastars of the genus~rias Fig, 8g! are also known quahogpredators 9; 70!. Seastars are the most important predator on adult quahogs,The large size and very thick shell make adult quahogs very resistant to most predators,but Asteriascan effectivelyopen them. Predatoryfish include the black drum, Pogoniascromis, and rays, Rhinopteraspp., Dusyatis spp., and Myliobatis spp.1!. Other flsh that are known predatorson juvenile quahogsinclude the tautog, Tautogaonitis, and the winter flounder,Pseudopleuronectes americanus. In addition, there are a number of ducks and shore birds that prey on small quahogs. This listing of known quahogpredators is certainly not exhaustive,but it doesgive repre- sentatives of key taxa.

Structure of Unexploitedand ExploitedQuahog Populations A number of studies have been undertaken to assessthe population densitiesof quahogsin areasthat sustain fisheriesor are areasfor potential fishery expansion,Walker 43! has summarizedthe findings of these studies,The distribution of quahogsis typically very patchy,with areasof high abundanceand areas of low abundance30; 159;218; 219!. The overall densitiesof quahogsin most of the areasstudied typically range&om 5 to 20 arumalsper squaremeter. The highestreported quahog densities in stable adult populationsare 556 per squaremeter in the ColoradoLagoon of south- ern California 6!, and 190 to 215 per square meter in the Greenwich Bay regionof RhodeIsland 10; 232!.Mature populationsof quahogsare charac- 98

86

74 E E Z 62 l

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26

40 320 0 40 FREQUENCY

Figure S. Valve length-&SquenCydistributiOnS terized by having a predominance of very large adults Fig, 9a and 9c!. of quahogsfrom three locationsin Narragansett Although recruitment in some areas may be sporadic, or recruitment rates Bay, R.l.: a! Greenwich Cove, b! Greenwich Bay, and c! South Ferry. Data and 6gures irom may be very low, mature populations have size-&equency distributions such Rice et al. 989l. Used with permission. as those in Fig. 9a and 9c! that are uniform with a single peak and with relatively few juvenile quahogs present 62; 243!. It appearsthat large populations of matuxe quahogs can efFectivelyexclude recnntment of younger individuals. This exclusion of recruitment into areas with large adult populations has also been found to occur in dense assemblagesof the European cockle Cardium edule 09!. Explanations for the lack of recruit- mentinto large assemblagesof a'dultsinclude the following. starvation of post set juveniles due to intense competition for food; greater losses from predation among post-setjuveniles due to slowed gmwth becauseof food limitations; an external cue from adults preventing settlement; or direct filtration of larvae from the water column by the adults 75!. A more recent study suggests that the mass water flow of water plumes from excurrent siphons of bivalves in dense assemblagesmay effectively prevent larval settlement 7!. In heavily 6shed areas, large quahogs are effectively cropped from the population Fig. 9b!, and younger animals dominate 43!. With the removal of the adults, there is an increase in the absolute number of smaller

19 animals,perhaps resulting from the removalof competitionfor spaceand foodby the adults.Other explanations for this enhancementof smallersize classesin areas with few adults is that back eddies forming at the base of excurrentsiphonal plumes of solitary adult quahogsin scattereddistribu- tions may actually enhancethe settlexnentof larvae 81!, or that in some way, the action of fishing may modify the physicalenvixxinment sediments! in such a way that post-set predation is minixxxized.

Quahog Assemblagesas Modifiers of the Enviri:ifimexit Since bivalves such as Mercenaria are active filter feeders, and dense assemblagescan potentially filter a largequantity of particulatematter from the overlyingwater column, a numberof authorshave postulated that dense assemblagesof filter-feeding bivalves can act asa majorintermediary in nutrient transfer between the water column and sediment environments 1; 135;142!. In this respect,one study suggests that filter-feedixxgbivalves can controlthe overloadingof estuarieswith excessorganic material eutrophica- tion! by filtering out excessphytoplankton stimulated by nutrient loading 86!. Thus, the quahogbeds can, in somecases, act as a natural pollution control.As an exampleof this phenomenon,communities of estuarinemus- selswere found to efFectivelyclear the waters of a smallMassachusetts estuary of phytoplankton,with onebivalve species Geukensia demissa! beingvery e8ective at filteringbacteria-sized plankton 51!. Otherstudies carried out in mesocosms large seawater tanks simulating Narragansett Bay exaxninedthe ratesof particulatecarbon deposition sedimentation! by quahogassemblages, aswell asthe ratesaf nutrientrelease by the sedi- ments 1; 72; 73!. The results of thesestudies showed that quahogswere able to increme the rate of transfer of particulate material from the water to the sedimentsthrough their filtering activity. In addition, the in~ in organicmaterial delivered to the sedimentsstimulated greater release of nutrients natural fertilizers! from the sediments. This release of extra nutrients &om the sediments to the water column in turn further stunulated phytoplanktonproduction. The overall productivity amount of total bioxnass production!was higher in the mesocosmscontaixung quahogs. In brief,the quahogswere able to stixnulateprocesses in theentire ecosystem by provid- ing an importantlink betweenthe sediments and the overlyingwater.

E8ects of Environmental Factors EKects of Temperature Larvaeof quahogshave been successMly grown through to metamorpho- sis betweenthe teinperaturesof 18'C 4'F! and 30'C 86'F! 56!. Above 70

Z 60 g 50 40

30

Figure 10. Growth of quahogs at different temperatures A! and sajinities g3!. Redrawn from Davis 969!. 10

12.5 17.5 22.5 27.5 32.5 7.5 12.5 17.5 22.5 27.5 TEMPERATURE C SALINITY PPT

33 C 91.4'F! and below 15'C 9'F!, larvae do not develop properly and there is high mortality Fig. 10a!. Temperature also a6'ectsthe rates of shell opening and growth of adults 51; 239!. Overall metabolic rate increases with increasing temperature 37!. Activity ceasesat temperatures below 10'C 0'F! and above 31'C 88 F! !. See previous section on growth.!

Effectsof Salinity Salinity refers to the relative salt content in waters and is typically mea- sured in parts per thousand ppt!; that is, ~ of salts per kilogram of pure water. Typically, meshwater will have a salinity of 0 ppt and seawater will be 34 to 35 ppt. In a few instances,natural waters may have salinities higher than seawater, such as desert salt lakes and somecoastal embayments in very arid regions. Estuaries bodies of water in which &esh and seawaterjoin and mix- are the habitat for quahogs.These range in salinity between the fresh water and seawater extremes.Quahog embryos developoptimally at a salinity of 27.5 ppt. Early-stage larvae cannot developin salinities below 17.5 ppt. The upper salinity limit for Mercenaria larvae is 35 ppt, but only few larvae can developnormally at this saiinity. Veliger larvae can survive and grow in 17.5 ppt salinity, but below 15 ppt settlement and metamorphosisis inhibited 2! Fig. 10b!. Adult quahog densities are generally highest in estuarine waters be- tweensalinities of 18 ppt and 32 ppt. There is someevidence that metabolic rates of quahogs are higher in lower salinity waters 37!. Within the estua- rine environment, organisms can be exposedto periodic salinity fluctuations thatmay exceed the normal physiological tolerance limits. One response of thequahog to theseperiodic salinity shifts may be simple valve closure, whichwould efFectively prevent exposure to thetransient salinity extreme 22; 223!.Quahogs can maintain valve closure for severaldays by respiring anaerobically.If alteredsalinity regimes are prolonged, quahogs have the capacityfor somephysiological adjustment to the altered osmoticconditions. The various salts and other small moleculesdissolved in the cells havethe tendencyto drawwater. As seawatersalinities drop, more water is drawn intothe cells because ofthe relative difl'erences in dissolved materials, and thecells begin to sweH.In verylow salinities, the cell swelling can be so severethat cells burst and the quahog is killed.Intracellular volume regula- tion is carriedout by adjustingthe amountof osmoticallyactive solutes mostly free amino acids,which are simple organicmo1ecules that can be usedas the buildingblocks for proteins!in the cells.In adaptingto lower salinity,free amino acids are released by quahogsto the externalenviron- ment,so that waterfollows by diflusionand the cells shrink to theiroriginal size09!. Anothermechanism for intracellularvolume regulation by quahogsmay be increasedmetabolic oxidation of the free amino acidsand increasedexcretion of ammonia4; 113!.In conditionsof increasingsalini- ties, the oppositeproblem occurs. The cellstend to shrink becausewater is drawn out of the cells into the more concentratedsalts in the seawater. Adaptation to increasedsalinities may involve use of amino acidsderived from protein in the diet, or biosynthesisof thesesmall moleculesfrom metabolicprecursors 5!, or by uptake of the amino acidsfrom the external environment 09!.

ERectsof Oxygen Mostmarine organisms require oxygen to carryon metabolicactivity. Oxygenin naturalwaters is typica11yproduced by photosynthesizingplank- ton or otheraquatic plants, or diflusesinto waterfrom the atmosphere. Typically,well-oxygenated seawater will bein excessof 5 milligramsoxygen per liter mg O~!. Low oxygenconditions hypoxia!can occurin estuariesas a resultof eutrophicationand the decompositionof organicwastes. Embry- onic and larval quahogsappear sensitive to low levelsof dissolvedoxygen in the watercolumn 82!, with all individualsdying if heldin 0.2mg WL. Larvae can survive at levelsabove 0.5 mg O~. Optimum growth occurs when oxygenconcentrations are above4.2 rng O~. Growth is severely impaired at dissolvedoxygen concentrations below 2.4 mg O~. Whether by valve closurefor protectionfrom salinity extremes,pollut ants, or low oxygen in the water, adult bivalves such as Merce~~ known to have a high capacityfor being able to carry on metabolicac«v ity in the presence of greatly reduced or zero levels of oxygen anaerobic metabolism! 6; 67; 106; 107!. In Mercenaria, lactic and succinic acids are produced as products of anaerobic metabolism 8; 103!. The production of these organic acids during anaerobic metabolism leads to lowered pH in the mantle cavity. The pH shift is buffered by the quahog through dissolution of the shell, which releases calcium carbonate 8; 74!. For this reason, quahogs are known to have thin or eroded shells when collected from areas that are proneto low oxygen hypoxia!.Periodic valve closurescorresponding to daily cyclesof inactivity have alsobeen shown to result in shell dissolution,visible in the shell as microscopic daily or sub-daily bands 95!. There is evidence that under conditions of very low oxygen, different tissues, such as muscle and mantle tissue, can utilize entirely different biochemical reactions for generatingenergy through anaerobicmetabolism 39!. Theseadditional biochemicalpathways exist in quahogsto allow anaerobicmetabolism to occur without producing acidic end products. These are very important biochemical pathways in that they allow much longer-term metabolism in reduced oxygen without the pH balance problems. Enzymes that direct these metabolic reactions alanopine and strombine dehydrogenases!have been isolated Irom gill and foot muscle tissues of Mercenaria 87!. There is evidence that even in adequately high oxygen concentrations, the various anaerobic biochemical systems of Mercenaria are actively producing metabolicenergy 04!. The processof filter feedingby quahogsis closely associatedwith oxygenuptake. It has beensuggested that oxygendemand by Mercenariamay be the controlling factor determiningthe rate of water pumpingby the gills 08!, but this remains controversial38!.

Effects of Pollutants Bivalve mollusks,including Mercenaria,have been used extensively as researchmodels for pollution studies0!. Bivalvespossess a number of characteristics that make them attractive as pollution indicators and re- searchmodels. First, they inhabit estuarine and coastalmarine areas,which are proneto pollution. Second,they are sessile,so they cannotmigrate away from the pollution source.Third, bivalvesgenerally live for a longtime, makinglong-term studies feasible. Fourth, broad geographic distribution of somespecies aids in the comparisonof widelyseparated geographic areas- Fifth,the physiology and ecology of commercially important species are weH- studied,providing valuable baseline information. Finally, many species of bivalvesare easily collected and therefore readily available to researchers. It is known that variousheavy metals, such as copper,lead, mercury, cadmium,and zinc in the watercolumn, are much more toxic to embryonic andlarval quahogs than to theadults 2!, Concentrationsofheavy metals at sublethal concentrationscause slowed larval growth and development.In addition to metals,there is evidencethat pesticideresidues and petroleuxn- derivedhydrocarbons may be very toxic to quahoglarvae 9; 63;122!. Lossesof larvae through acute toxic effects or by sublethal decreasesin fitnessof larvae leadingto predationor diseaselosses! are possiblereasons for reduced recruitment into benthic populations 1!, A number of studies document the concentration of heavy xnetals in adult quahog soft tissues and the rates of accumulationand release depuration! under varyixxgenvironmental conditions.It has been gener- ally shownthat quahogscan accumulate metals frown seawater, but areslow to releasethexn when placedin cleanerwater 9; 88; 146;215!. Heavy xnetal concentrationsfound in quahogtissues tend to be highly variable, depending onsalinity, temperature, season, age, and the degree to whichthe gonadsare developed.8; 215!.Some studies have focused on uptake of metalsby quahogsin contaminatedsedixnents. These studies have yieMed conflicting results, with somestudies 9; 236! showingtransfer of metals from sedi- ments to quahogsoft tissue, and another17! showingno transfer of metals from contaminatedsediments during 100days of exposuxe,It is highly likely that theseconflicting results can be explainedby differing sedimentchem- istry characteristicsbetween the sites.Rates of metal releaseor mobilization and bioavailability of the metals xnaybe quite different in different sediment types 21!. There are three generalphysiological effects of metal poisoningin marine organisms.First, functionsof variousenzymes proteins that catalyzebio- chemicalreactions! may be affected.For example,replacement of metals in metaIloenzymesmay severelyimpair their function 4; 214!.Also, the function of sulfur-containing sulfhydryl! enzymesmay be similarly impaired 27!. Onemechanisxn to counterthe toxic effectsof metals on proteins is for the animal to synthesizereplacement proteins. The chronicdemand for proteinturnover is costlyin termsof metabolicenergy and can interfere with growth 99; 207!.Second, enzymes and/or binding proteins such as metallothioneins!are inducedto providea meansfor detoxificationof the xnetals6; 137!.These metal-binding proteins have a highcapacity for scavengingxnetals and act to protectother proteins from damageby the metals.Finally, damageto external tissue layers epithelia!, or gill cilia in the caseof bivalves,may interfere with respiratory and feedingxnechanisms 45; 214!, A number of studies have been undertaken to assess the toxicities, uptake,and depuration of varioushydrocarbons associated with petroleum products!. Of the varioushydrocarbons associated with petroleum,the polycyclicaromatic hydrocarbons PAHs! and lighter volatilestraight-chain aliphatic! hydrocarbonsappear to be the most toxic to bivalves80; 211!. Bivalvesaccumulate hydrocarkmns from both particulate and dissolvedforms in the water column,and it has beenreported that someclasses of hydmm- bons,especially shorter-chain hydrocarbons, are rapidly releasedonce the animal has been placed in clean seawater 47!. Studies with Mercenana have shownthat a numberof petroleum-derivedhydroc u4>ns can be accumulated&om the environment8; 86!. Many of theseaccumulated hydrocarbons,especiaHy longer-chain and cyclic hydrocarbons, are very persistentin soft tissues8!. Petroleumhydrocarbons and other pollutants areknown to impair the immunologicaldisease-fightin capability of quahogs!, as weHas causepathologic lesions 33!. 4 lli I

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' '4 l ' r f I Fisheries

Thequahog fishery is oneof the largestalong the Atlanticcoast of the United States.According to the National Marine FisheriesService statistics for 1987,11.4 million poundsof quahogswere landed, with a docksidevalue moneypaid to the fishermen!of about $49.6million. The value of all fisher- ies alongthe Atlantic coast excludingthe Gulf of Mexico!in 1987was $847.8 million,with quahogsmalnng up 5.9 percent of the total value. The only Atlanticcoast fisheries surpassing the quahogfishery in valuewere the lobsterfishery with 12.7percent, and. the blue crab fishery with 8.3percent of the total value.The increasing demand for quahogsby thepublic is reflectedin the generaltrend of its increasingdockside value Fig. 11!.The overallquantity of quahogmeats landed on an annual basis has been highly variable, and there is no generallong-term trend of increasedharvests since the 1960s.It is likely that naturalstocks are now being exploited at rates that arelikely to beclose to the niaximumallowable without depleting the stocks called the maximum sustainableyield MSY! 72; 173!. As previouslynoted, quahogs have been harvested in NewEngland since pre-colonialtimes. By the turn of the 20thcentury, the fisheryin NewEn- glandwas well developed,and state governments were considering measures to managethe stocks0!. Presently,commercial and recreational quahog fisheriesare significant in all Atbmtic coastalstates fmm Massachusettsto Florida

Fishing Methodsand Gear It is generallyaccepted that the earliestmethod of harvestingquahogs consistedof handcoOection, or useof simpleharvesting tools, such as shovels Photqgmph from the R'ide Island Historieul 8ocieCy and rakes, alongthe shorebetween the high and low tide marks intertidal 26

24

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cd 20 Cd 0 4! 40 E 0 C6 18 CE 0 0 D. 30 16 o UJ E 14 Z', 20 O 12 Z

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68 70 72 74 76 78 80 82 84 86 88 90

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Figure ll. Landings of quahogs 1967 to 1989. Open circlesrepresent landings in shuckedmeat weights and closedcircles represent the value in millions of doHars.Data Rom NMF8 statistics. Figure 12 rsIfht!. Hand tongs for harvest of HAND TONGS quahogsin shallow sub-tidal water,w r, Dra awing ' bby E. Watkjns.

Figure 18 belaee!. Patent tongs allow for har- vest in waters deeper than 15 feet.

zone!. Since the late 1800s, tongs have been used to harv e in ' zone Fig. 12!. Due to the scisso dl fth gs an d th e p h ysical'cal'cal limit ' ' as to how ow wide the tong handles could be openedn y b the e arm spans of the fishermen, theey are limi imited to working depths of about 12 to 15 feet of water 0 !. . Al though tongs are limited in worki 'ng d ep,th th ey are the g earoice of ch by some"traditionalists" who claixn that to ngs are the most efFicienthand-o -operated ra tool for shallower waters. Thee patentpa n ngs to Fi Fig. . 13! oFer a means foror fis hing ' in deeper waters PATENT TONGS 6!. The patent tongsngs are tetetheredteth ed and dro p from a boat.A catchmecha- ows or c osure of the tongs as theey hit bobottom, and then they are retrieved b yth e teth er rope. Patent to ngs s arear now very rarely seenin the quahogog fishe s ery, because ause most fishermen prefer bullrakeses for or harv esting quahogs in deeper waters. Bullrake

Figure 14. The bullrake is the most popular of the The hand-operated harvesting device most widely used for quahog hand-operated iishing implements in the quahog fishing is the bullrake Fig. 14!. The bullrake is said to have been invented in fishery of the Northeast. Typically, the stales handles! are 50 feet or longer and allow harve st in the late 1940s by a shellfisherman &om Long Island, N.Y. 0!. In the mid- 25 feet of water. The float near the "T-bar" handle 1950s,the bullrake was adopted by a few fishermen in the Greenwich Bay allows the stale tofloat astherake head is retrieved region of Rhode Island. By the late 1960s,bullrakes had becomethe industry irom the bottom. standard in all of New England, New York, and south to the southern Atlan- tic states 94!. Figure 15. Mechanical dredges, such as the rock- The bullrake head has evenly spacedteeth to extract quahogs from the ing chair dredge, are very efficient for harvesting sedimentand an integral basketto hold the quahogsas they are hauled to quahogs, but their use is restricted in many states the surface. The earliest bullrakes had wooden stales handles! that were because their efficiency is often responsible for overexploitation of stocks. Drawing by E, Watkins, often 80- to 40-feet long, which allowed fishing in about 20 feet of water. Individual sectionsof the oM woodenstales were held togetherby iron rings. As a generalrule, a bullrake can harvest in waters roughly half as deep as the length of the stale because the optimum leverage is reached when the rake is at an angle,not perpendicularto the estuarybottom. Introduction of aluminum stalesin the 1960sallowed for longer polesand harvestingin about 30 feet of water. Currently usedbullrakes have speciallydesigned heads for vying bottom type. A variant of the standard bullrake is the "short-stick"bullrake also known as the "donkeyrake" in somelocalities!, which has a greatlyshortened stale for harvestingquahogs in shallowwater bya wadingfisherman. Aluminum stales are currently the mostpopular, but up-to-datefiberglass and graphite composite materials have been used by someto makestales stronger, lighter, and longer. Soxne fishermen working RhodeIsland's NarragpmsettBay are known to fish with 90 feet of stale, allowing them to harvest in excessof 45 feet of water. Theuse of divingequipment in the quahogfishery has allowed for exploi- tation of waters that are inaccessibleto implementssuch as tongsand rakes. Typically, a metal diving basket will be tethered to the diver's boat. The diver will then fil the basket with quahogsand a tender may haul the basket aboard.Another alternative used by divers is to fill mesh bags with quahogsand to tie them to buoyedlines. After the day of fishing is com- pleted, the buoyedline with the attached quahogbags is retrieved. The direct collectionof quahogsfrom the sedimentsis usuallyby useof simpl,e hand tools. A number of fisherxnen will use an air line attached to a small coxnpressoraboard their boat.The compressedair is usedto blow o8'a top layer of sedimentto exposethe quahogs,after which they are collectedby hand. Somedivers will work in the winter months by using dry suits to protect themselves from the cold water. Mechanical harvesting gear has been used in some areas as an effective meansfor harvestingquahogs. One method, called clam kicking," is prac- ticed in North Carolina 97!. The claxn kicking technique is used in shallow estuarine sandy-bottoxnareas or seagrassbeds and involves the use of a boat with an outboardmotor. The outboardmotor is usedto stir up sedimentsand dislodgequahogs. The quahogsare then simply raked up from the disturbed sedimentsand brought aboardthe boat. Other quahogharvesting methods includemechanical and hydrauhc dredges 18!. A mechanicaldredge, such as the "rocking chair" dredge Fig. 15!, is pulled alongthe bottoxnand is designed to dig into the sediments with an up-and-down motion. As the dredgeproceeds, quahogs are kicked into a trailing chain bag.Hydraulic dredgesare sixnilarto the mechanicaldredges, but work more efficiently. The hydraulic dredgeswork by having a water pump on board a boatthat is connectedby a hoseto multiple water jet nozzles manifold! on the dredge. Water sprayedfroxn the manifold acts to liquify the sedimentsas the dredge passesso that the quahogscan be quite efhciently pickedup and passedto the trailing chain bag. Mechanical and hydraulic dredges have been used in some states for harvesting quahogs 0!, but have now been banned because they are too scient, possibly posing a threat to the sustainability of the fishery.

Fishery Management Regulations In most instances, fishery managexnent regulations are designed to prevent overexploitation of the quahog resource see previous discussion of population structure of exploited and unexploited quahog populations!. Authority for managexnent of quahog fisheries varies from state to state. In some states, the xnanagenient of the fishery falls under the jurisdiction of state officials; but in other states, shellfishery management is largely under the control of local xnunicipalities. A number of regulations are designed to limit the fishing eÃort in a given area. These regulations include: a! Restrictions on gear type b! Limitations on time of day for harvesting c! Seasonal limitations on harvesting d! Daily catch limits e! Licensing requirements and limited entry to the fishery Other regulations are designed to protect the adult quahog brood stock. These regulations include: a! Minimum sizes for harvest -inch hinge width in xnost states of the Northeast! b! Regulations on bar spacings in harvest gear teeth spacing on rakes and tongs, mesh sizes on dredge bags, bar spacing on diver's baskets! c! Maintenance of closed areas as "spawner sanctuaries" 71!

%ater Quahty and the Fisheries Since quahogs are sedentary filter feeders, there is concern that they will potentially accumulate pollutants or pathogens disease-causingorganisms! froxn the water, potentially posing a health threat to the public 4!. For example, varying amouxits of petroleum hydrocarbons can be passed on to the public if quahogs are harvested in polluted waters 02!. Current efforts by the U.S. Environmental Protection Agency have focused on developing models to predict public health risk from the consumption of seafood contaminated with various metallic and hydrocarbon contaminants 36!. Shellfish accumulating pathogenic microorganisms associated with sewage have been a major threat to public health. In response to repeated

32 outbreaksoftyphoid fever and cholera associated with bivalves, the Natiord ShellfishSanitation Program NSSP! was established in 1925to setnatior widewater quality and post-harvest handling procedures tosafeguard pukic health.The NSSP was incorporated into the Interstate Shellfish Sanitatioi Conference ISSC!, a groupof public health officials &om various shellfish- producingstates and o%cials kom the U.S. Food and Drug Administration Forstates to engage ininterstate commerce ofbivalve sheHfish, including Mercenariamercenary, they must comply with thevarious sanitation provisionsof theISSC 25; 126!.Sanitation regulations include microbio- logicalstandards for shellfish-producing waters, microbiological standards forshellfish meats at harvest, proper handling ofshellfish atharvest, and properhandling of sheHfish asthey pass through the marketing channels. In mostinstances, quahogs are harvested koxn. waters that arecertifiec asclean by state agencies under ISSC guidelines. The ISSC guidelines do, however,allow for harvest ofsheHfish &om moderately poHuted waters, providedthat some form of post-harvest disinfection is undertaken. The met commonforms of post-harvest disinfection are transplant, relay, and depuration.Transplant involves the harvest of juveniles Rom seed beds the maybe in pollutedwaters, and trmmferring them to certifiedclean waters k finalgrow out to adult size. A keyfeatuxe oftransplant isthat the final gro outmay take several months to years. Under ISSC guidelines, transplant stockcan originate in moderately toheavily polluted waters, because very longcleansing periods in certified clean waters are involved. Relay differs &omtransplant in that quahogsof legaHy harvestable size are harvested fromlightly polluted waters and transferxed to certifiedclean waters for periodsof time ranging&om a fewweeks to a fewmonths. Because of the shortertime periods for cleansing, quality standards for sourcewaters of relaysheHflsh are stricter than the transplant standards. Relay may involve thereplanting ofquahogs into the sediments ofcertified waters 93!, or the holdingof quahogs in bags or enclosures off-bottom 40!. In depuration, quahogsare held in shoxe-basedtanks supplied with clean seawater 92!. Therates of elimination ofbacterial indicators bythe quahog are affected b~ variousenvironmental and seasonal factors 0; 114!As a result,usually C to72 hours depuration time is allowedin suf6cientlywarm and oxygenated water for reductionsof bacterial indicatorsin shellfish meatsto occur. Thereis evidencethat virusesmay take considerably longer than the standardtwo ta three day depuration period to be eliminated by quahogs. Coliphages viruses that attack bacteria are commonly found in sewage Theyare not harmful to humans, but they are retained in thedigestive trad ofquahogs very xnuch like x~ thatcan cause human disease. The colipb ageS-13 virus can take weeks to be fuHy elimixxated under optimum condi- tions 3!. Recent work with the MS-2 coliphage indicates that depuration of this virus from Mereenaria may take several weeks to months 6!. These results suggestthat in the interests of public health, specialcare must be taken to assuresanitary quality of harvestingareas. Depuration can add an extra margin of safety to bivalve sheHfish, but it is not a substitute for envi- ronmental pollution abatement. Toxins ass+~ted with somespecies of phytoplanktonare a potential health threat to the shellfish-consunungpublic. This is becausefilter-feeding aHowsfor accumulationof toxins in bivalve st tissues.Shumway 24! provides a recent review of the impacts of toxic algal blooms on sheHfisheries and aquaculture, The dinofiageHate,Akxandrium fundyense formerly Profogonyaulax Azmarensis!,responsible for red tides," produces the toxin responsible for paralytic shellfish poisoning PSP!. During one major red tide event in Massachusetts, various speciesof bivalves were shown to be very toxic, includmg mussels, My6l us edulis; soft-sheHedclams, Mya arenaria; and scallops,Argopecten irradians, but quahogsremained completely &ee of the toxins. Laboratorystudies have shownthat quahogswiH retract their siphons and completely close their shells if there are bloom concentrations of Akxandrium fundyensein the water 25!. However,another laboratory study suggeststhat quahogscan accumulatealgal toxins if dinofiageHates are in mixtures with other phytoplankton,as is likely in the natural aquatic environment 4!. Quahogs might accumulate some of the toxins if there is a red tide event;however, they will tend to releasethe toxins in a fewdays 4!, The exact conditions for the initiation of an AEexandrium bloom are not well understood, but they are much more common during the warm summer months. States such as Maine, with major shellfish industries, have pro- grams to closely monitor the appearance of red tides 26!. Toxins associated with the phytoplankton Dinopkysis are responsible for diarrhetic shellfish poisoning DSP!. DSP is manifested as mild to moderate gastroenteritis, and there have been no known fatalities. Quahogs can accumulate the DSP toxin. Like PSP, the incidence of DSP is higher durmg the warm-water summer months 89!.

Economics of the Fishery Key economicaspects of the quahog fishery indude sustainability of yields and market demand considerations. Holmsen 18! provides an excel- lent analysisof the various factorsinfluencing the RhodeIsland quahog fishery, which consists mainly of bullraking by individuals in smaH boats. Since the 1960s,the landings of quahogs have been variable, but there has been no general increasing or decreasing trend in catches Fig. 11!. One key aspectof the industry is that during pooreconomic times in other sectors,the number of shellfishermenincxeases, thereby increasing fishing effort 19!. An economicanalysis by Gates 94! suggeststhat increasingdemand for bivalve shellfish, including quahogs,coupled with a fairly constantfishery supply has resulted in increasingprices. Increasing demand for the product has spurred increased imports of clam meats from abroad. It has been suggestedthat fishermens'cooperatives can providefor greater stabilization of market prices and stabihzation of fishexmens' incomes. This is due to the powerof cooperativesto limit the supply of perishableproduct entering the marketat a giventime, and the ability of cooperativesto encourageinnova- tion and more efficient methods of harvest 94!.

AqU:multure

Increasingmarket demandfor quahogshas providedthe impetus for greater interest in quahogs as an aquaculture species.The feasibility of aquaculturalproduction of quahogshas beendiscussed &om the 1930s50!, and the methods for artificial propagation of quahogs have been developed at the National Marine Fisheries Service Laboratory in Milford, Conn. 4; 155!. As the culture techniques were developing, culturists divided them into three distinct phases with difFering functions 66!. The first phase of clam aquaculture,or the hatchery phase,focuses on the productionof ripe bmei stock, sperm and egg cells, fertilization, and the culture of the larvae through settlementand metamorphosis.The second,or nursery phase, focuseson the rearing of post-set juveniles to a size in which they become somewhat resistant to predators. The final phase is the grow out phase in which predator resistant-sized animals am grown to market size. A recent book by Manzi and Castagna 67! provides an in-depth review of the fisher- ies and aquaculture literature as it applies to Mercenaria mercenaria and other clam species.

Hatchery Methods The hatchery production of seedclams is well estabhshed. Most hatcheries divide their operations into four distinct functions: a! algal culture, b! brood-stock maintenance and conditioning, c! larval culture, and d! post-set juvenile maintenance. Hatchery operations depend on the production of phytoplanldan for food for the brood stock, larval, and juvenile clams. There have been a number of excellent reviews outlunng the procedures of algal culture 91; 101; 230; 235!. In commercial hatcheries, two basic algal culture methods are used. The WeHs-Glancymethod relies on the bloomsof naturaHyoccurrmg algal spe- ciesin nutrient-enrichedseawater after initial filtration or centrifugationto removelarger zooplankton47!. The Wells-Glancymethod is attractive in that it is simple and inexpensive, but it suffers from the drawbacks of un- knownalgal species being of variablefood quality and being prone to algal die-ofFor "crashes,"Alternatively, if natural foodconcentrations are high, goodlarval and juvenile growth can be obtainedby pumping large quantities of "raw" seawaterpast the animals.The Milford method52; 153!relies on isolatedspecies of phytoplanktonin bacteria-free axenic!cultures, or cul- tures with few contaminants oligoxenic!.The Milford methodof algal pro- duction is more labor intensive than the WeHs-Glancymethod, but there is greater control on foodquality 9; 244! and stability of the cultures.Most, but not all, hatcheriesnow use the Milfordmethod of algalproduction. A number of hatcheriesmay use the Wells-Glancymethod for productionof foodfor brood-stockmaintenance and use the Milfordmethod for producing larval food, Prior to spawning,quahogs must be fed sufficiently sothat gonadgrowth and maturation can occur,In the natural environment,rising water temperatureduring the spring months brings algal bloomsand the onsetof quahogfeeding. In a hatcherysituation it is own desirableto spawnthe animals well beforethe natural spawnirg period sothat o8'springcan be set out in the environmentearly to take advantageof a longer growing period. To conditionthe broodstock, water temperature must, be gradually raised &om a maintenance temperature of 12'C 4 F! to about 18 to 20'C 4 to 68'F! and fed an amplesupply of phytoplankton.By using a combinationof coolstorage and conditioningtechniques, spawnable brood stock can be made available throughout the year 53; 154!. Spawningof Mercenariacan usually be inducedby raising the water temperatureto between28 to 30'C 82 to 86 F!. If this temperatureshock doesnot work, addition of a suspensionof eggsor spermwill &equently induce spawning1!. Aker spawning,eggs and spermare mixed to allow fertilization. Followingearly stagesof development,larvae are transferred to a larval culture vessel,Straight-hinge veligers and all subsequentlarval stagesare fed on phytoplankton.Larval culture vesselswill range in size from a fewliters to thousandsof hters, dependingon the sizeof the facility, Most &equently, commercial hatcheries use 400 to 1,200-liter circular tanks with conicalbottoms. Vessels of this sizeare generallyeasier to maintain than larger vessels, and it is often wiser to have more medium-sized culture vesselsthan a fewlarger onesbecause many potential diseasescan be more easily confined and treated. Typically, larvae are stocked in culture vessels at densitiesof 1 to 10 larvae/mLand fed algal suspensionsat concentrationsof 10,000 to 100,000 cells/mUday. Proper maintenance of larval cultures in- volves the occasional drain-down of each of the tanks, sieving the larvae to select for the fastest-growing individuals, and cleaning the tanks. At a typical larval-rearing temperature of 25 C 7 F! and with adequate feeding, larvae will be capable of metamorphosis in about eight days. The typical size of metamorphosing Mercenarie mercenana larvae are 190 to 230 pm in length. Under conditions of reduced rations and educed temperature, metamorphosis can be delayed for up to 20 days 55!. Frequently, commer- cial hatcheries will maintain post-set juveniles for a few weeks in the larval tanks to feed them higher concentrations of algae. Under these conditions, the quahogs wiH reach 600 pm valve length in two to three weeks. At this time, the quahogs are generally transferred to nursery facilities

Nur.~ Methods Nursery culture systems provide controlled or semi-controlled conditions for intermediate growth between hatcheries and grow out. As the quahogs grow, their requirement for food grows in proportion to their weight. At the 500 to 600 pm size, it is usually not economically attractive to produce the large quantities of phytoplankton required to maintain the animals in static systems such as the hatchery. The post-set juveniles cannot be planted directly into unprotected Geld gnaw out areas without nearly 100 percent loss due to predation 40; 141!. There are basically two types of nursery systems: onshore and field. Onshore facilities require the pum ping of seawater to deliver suf5cient food to the juveniles. Field-based systems usually consist of enclosures and other predator exclusion devices to protect the seedquahogs. Most &equently, smaller juvenile quahog seed are held in some type of onshore nursery system. In one such system, seawater enters one end, passesthe length of the racewayover the quahogs,and exits through a standpipeor spillway drain at the other end. This systemis a simple,yet e8ective, means for rearing early juveniles 1; 102!. Another nursery system is calledthe "upweller" 6; 163;169! because water flow is actively or pas- sively forcedupward through a containerholding seedquahogs Fig. 16!. Upwellersare attractive in that they are spaceeKcient. Additionally, the upflow of water from the bottom of the seedquahogs allows for the efficient removal of feces and pseudofeces,thus increasing time between clearungs. There are a number of Geld-basednursery systems that have been used successfully.Gravel has beenused to protect quahogseed in somebottom nurseries 0; 51!, Trays with plastic mesh covers provide excellent protec- tion &om predators, but require considerable maintenance due to fouling 7; 148!. Off-bottom nursery culture systems utilize a number of devices including rafts, suspended trays, lantern nets, and pearl nets. Recent sea sea wate water inlet outlet

Figure 16. Diatpujnmatic representationof an developments in nursery culture systems include a tidal-flow upweHer upweller system for the nursery culture of seed system on a raft 79! and enclosures suspended from floating docks in quahogs, smaH-boat marinas 08!.

Grow Out Methods The technology for raising quahogs from larval to market size in closed recirculating systems has been developed and used in a number of nutrition and other studies 9!. One such system utilized treated wastewater 65!. For commercial applications, such shore-basedfacilities have not been economically viable. Most commercial grow out methods currently used are based on field planting intertidally or subtidally with some form of predator protection. Methods of predator exclusion include the use of crushed stone, pens,and net tents 0; 141!.Grow out of quahogsin trays providesexcellent protection and is used widely 12; 168!. It is also known that quahogs can reach market size in shorter tune if raised in heated efHuents 98!.

Aquaculture Diseases and Pathology One of the main diseasesposed by the culturist is larval vibriosis a disease caused by bacteria in the genus Vibrio. Larval vibriosis can cause 100percent mortality in larval cultures 5; 78; 234!.Cell destructionand

40 tissue death necrosis! can occur after four to five hours of initial exposure to Vibrio. Complete mortality of the larnyx culture can be within 18 hours, Diagnosis of vibriosis is by culturing on Vibrio-specific agar plates, microscopic emunixMxtion,or a tarpan-blue dye exclusion test. Several antibiotics can be used to control vibriosis, but becauseof the rapid onset of this disease, most hatcheries will simply destroy all infected cultures and carefully disinfect the hatchery 28!. Another disease problem associated with quahog hatcheries is larval mycosis. Larval xnycosisis a systemic infection of larvae with the fungus Sirolpidium zoophthorum 5; 241!. Symptoms of larval mycosis include clumping of larvae and the presence of fungal filaments hyphae! upon microscopic exaxnination. No known treatment is known for this disease, so infected larval cultures must be discarded. Sinderman and Lightner 29! review the curren.t techniques for diseasecontrol in aquaculture systems. Adult quahogs are susceptible to some other infections. Chlamydia-like organisms have been found in quahogs &om Chesapeake Bay and the Great South Bay of New York 10; 177!, but none of these infections have been associatedwith severe diseasethus far. One study reports a stress response of quahogs associated with pollution and/or shell invasion by the annelid worm Polydorn 27!. It has been reported that some clams may be suscep- tible to parasites such as Perkinsus marinus, Hylakossia, Nematopsis, and Trichodina 66!; however, direct surveys of cultured and wild Menenana rnercenaria have shown very low incidences of parasitism and histological abnormalities 78!. A gonadal neoplasia cancer! has been desex%edin quahogs &om NaxnxgaxxsettBay, R.I. 53!, but incidences of the disease and mortahties have been low 3! It has been suggestedthat these gonadal neoplasms may be the result of environmental pollution 3!. The mecha- nism for induction of these neoplasms may be a depression of the immune response ! and subsequent invasion by a viral agent similar to the virus responsible for neoplasms in the softshell clam Mya arenaria 87!. Although some of the diseasesof quahogs xesemble human diseases, there is little danger of transmission to the seafood-consumingpublic. DieM.sesamong shellfish are of greatest concern when they becomewide- spreadenough to affectthe market supply.So far, the diseasesof quahogs have not been widespread or severe enough to be of concern !P r

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I Conclusion

It is hopedthat the information presentedin this bookwiH be a useful startixig point for thoseinterested in understandingthe biologyand economic value of the northern quahog.The quahoghas beenimportant historically, particularly in the Northeast where it has beena foodsource since pre- colonial times. In Rhode Island, it has even been designated as the oKcial state mollusk, It is highly unlikely that the quahogwill becomeany less important as a foodsource in the future, now that an emphasishas been placedan promotionof seafoodsfor their nutritional value. Quahogs,as 6lter feeders,rely on the natural productivity of estuarinewaters to derive their nutrition. They actively 6lter the abundantphytoplankton for their foodand, under proper conditions,grow quickly thereby meetingthe demandsof a growing seafoodmarket. Aquaculturists are interestedin quahogsbecause supplementalfeeds are usually not necessaryonce they are seededinto estuarinenursery and grow out areas.Fishermen exploit the fast growth of fllter-feeiing quahogsby being ableto harvest productivenatural bedson a constantbasis Assuxningthat fishing e6ort is not too excessive,or the stocks are well managed,the quahogfishery can be productivewell into the future. There is evidence that filter-feeding quahogs are a "natural 6lter," removing excessphytoplankton produced as a result of increasedsewage loading in estuaries But as filter-feeders,quahogs are quite capableof filtering out and accumulatingmany of the toxic or disease-causingwaste productsof our modernindustrial society.Maintenance of estuarinewater quality is not only goodfor the quahog,but alsofor the livelihoodof the captureflsheries and the growing aquacultureindustries that dependon it. Water quality and the shellflsheries also have important implications to public health. This is Photographcourtesy of the ProvidenceJouraal- becausethe health of the estuaries,and indeedthe quality of life for people Bulletia living near them, may be reflectedby the health of the she116sheries. Refnenees

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52 133! Kennish, M.J. 980!. Shell microgrowth analysis: Mereenaria mercenaria as an example for research in population dynamics. Pp. 255-294. In. D.C. Rhoads and R.A. Lutz eds.!, Skeletal Groxvth of Aquatic Organisms. Plenuxn Press, New York. 134! Kerswill, C.J. 949!. Effects of water circulation on the growth of quahaugs and oysters. Journal of the Fisheries Research Board of Canada 7:545-451. 135!Kitchell, J.F., R.V. O'Neill, D. Webb,G.W. Gailepp, S.M. Bartell, J.F. Koonceand B.S. Ausmus. 979!, Consumer regulatibn of nutrient cycling. Bioscience29:28 33. 136!Kipp, K 991!. Shellfish safety:human health implications of toxic contaxni- nation in Narragansett Bay. Pp. 29 41. In: MA. Rice, M, Grady, and M.L. Schwartz eds.!,Proceedings of the First RhodeIsland ShellfisheriesConference, Rhode Island Sea Grant, University of Rhode Island, Narragansett. 137! Kohler, K and H.U. Ripped. 982!. 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Chxomosomesof two species of quahog clams and their hybrids. Biological Bulletin 129:181 188. 177! Meyers, T.R. 979!. Preliminary studies on a chlamydial agent in the diges- tive diverticular epithelium of hard clams Memenaria mercenaria L.! kom Great South Bay, New York. Journal of Furh Disease 1:179-189. 178! Meyers, T.R. 981!. Endenuc diseases in cultuxed shellfish of Long Island, New York: Adult and juvenile American oysters, Crassostrea virginica, and hard clams, Mercenaria rnercenaria. Aquacultune 22:305 330. 179! Mook, W. 990!. Seed production in a hatchery/nursery in Maine. Envi ronrnenta/ Audi tor 2:7. 180! Moore, S.F, and R,L. Dwyer, 974!, EQects of oil on marine organisms: a critical assessxnent of published data. Water Research 8:81~27. 181! Morris, PA. 973!. A Field Guide to Shells of the Atlantic and Gulf Coastsand the WestIndies rd edition!. Houghton MiBlin, Boston. 330pp, 182!Morrison, G. 971!. 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