Aspects of the Biology of the Northern Quahog, Mercenaria ...

Aspects of the Biology of the Northern Quahog, Mercenaria mereenaria,with Emphasis on Growth and Survival during Early Life History

V. MONICA BRICELJMarine Sciences Research Center

State University of New YorkStony Brook, NY 11794

Introduction

The biology of northern quahogs, Mercenariamercenaria, has been the subject of several earlier e,g, Belding, 1931! and more recent literaturereviews Pmtt et al., 1992; Rice and Pechenik,1992!. Therefore this paper does not attempt anexhaustive review, but rather, will highlightsome salient features of this species' life historywhich are of significance in managing wild

stocks. Processes operating during early lifehistory stages Figure 1! are emphasized,because recruitment success into the fisheryappears to be largely predetermined during theclams' first one to two years of life Malinowski,1985; Wallace, 1991!. Poorly understood aspectsof the species' biology will also be stressed, inorder to suggest avenues for future research.

Abstract. Key features of the biology of Mercenaria mercenaria are reviewed with emphasis on early lifehistory processes. Predatory mortality during juvenile stages of the northern quahog is identifr ed as aprimary factor controlling recrui tment of natural populations. Predation rates are shown to be stronglymodulated both by substrate preference and prey-size selectivity of major predators crabs and carnivorousgastropods!. Smaller xanthid crabs prefer heterogeneous substrates gravel and shell bottoms!, andconsume quahogs at a higher rate in these substrates, whereas larger, portuni d crabs prefer and foragemost electively in homogeneous substrates. In contrast to predictions of optimal foraging theory, evenlarger crabs preferentially consume smaller quahogs, when a wide range of prey sizes is available, thusincreasing predation pressure on smaller quahog size classes.

Under field conditions, at near-optimum temperatures, j uvenile M. rnercenaria exhibit mean shellgrowth rates of 0.8 mm week- t maximum = 1 mm wk ~!. Native populations along the east coast exhibitcomparatively lower and higher than average lifetime growth rates at the species' northern Prince EdwardIsland, Canada! and southern Florida! distributional limits, respectively. These extremes correlate withthe length of the gro~ing season, which is strong Ly temperature-dependent. Thus, the time to attain Legalmarket-size ranges from 1.9 to ! 6years and averages three to four years in the mid-portion of thenorthern quahog 's latitudinal ranges Massachusetts to Virginia!. Up to a two- to three-fold variation ingrowth rates is typically observed within a single estuary, Three toxic/noxious algal species are identifr edas potentially harmful to M. mercenaria under bloom conditions: the chrysophyte Aureococcusanophagefferens, the chlorophyte Nannochloris atomus, and the dinoflageLLate Alexandrium fundyense.Management implications and suggested fruitful directions for future research are discussed throughoutthe text.

Figure 1. Schematic diagram of major factorscontrolling Mercenaria mercerLaria recruitment to a size shel'I length of 20-25 mm! when hard clams attai n sizerefuge from many of their common predators. See textfor discussion.

ReproductionMercenaria mercenaria is a relatively slow-

growing, long-lived, dioecious bivalve,characterized by iteroparity multiplereproductions over its lifespan!, high fecundities,production of planktotrophic larvae that typicallyremain in the plankton for one to two weeks Camker, 1961!, and high juvenile relative toadult survival Malinowski and Whitlach, 1988!.Important life history characteristics of thisspecies are summarized in Table 1. Agmgtechniques rely on the presence of annual growthchecks in the shell, which are typically producedduring the winter in the northern and centralportion of the northern quahog's geographicrange, and in the summer and early fall insoutheastern states North Carolina, Georgia,and Florida! Fritz and Haven, 1983; Grizzle andLutz, 1988; and references therein!. Longevityestimates for the species range widely between23 and 46 years because of the difficulty in agingolder specimens, which show crowding ofgrowth rings and numerous spurious growthchecks. Maxiinum size ranges between 110-111inm in shell length Rice et al., 1989; Jones etal., 1989! and 135 mm Walker and Tenore,1984!. A long lifespan, and the coexistence ofmultiple year classes, will tend to buffer hardclam populations from sudden population crashescaused by sporadic recruitment failure.

Fecundity, as determined by repeatedspawning induction of mature individuals in thelaboratory, is positively correlated with bodysize, but highly variable among individuals of thesaine size Table 1!. M. tttercenaria shows noevidence of reproductive senility, or decline inreproductive output or gamete viability withagelsize Bricelj and Malouf, 1980!, since olderclams produce gametes at a level predicted by thepower curve relating gonad mass to body size inyounger individuals isometric growth! Peterson, 1986!. Bricelj and Malouf �980!showed that mature eggs spawned at one time bya single female are characterized by a bimodalsize-frequency distribution, with modal peaks at67 and 81 pan in diameter range = ca. 50 to 97pm!, This was confirmed by Gallager and Mann�986!, who found that quahog eggs separatedinto thee distinct bands following densitygradient centrifugation. The significance of thiswide range in egg sizes has not been determined.Since egg size in M mercenana is known to bepositively correlated with egg lipid content Gallager and Mann, 1986!, eggs of differentsizes may be characterized by differentdevelopment times Clarke, 1982! or differentialviability. Thus Kraeuter et al. �982! found thatsinaller eggs < 35 pm! had significantly lowersurvival than eggs > 44 pm.

Spawning of quahog populations is lesssynchronous and starts earlier in the year withdecreasing latitude Table 3.3 in Eversole, 1989!.The length of' the spawning season and thefrequency of peak spawning periods also tend toincrease with decreasing latitude. A single,annual spawning peak, occurring in the summer,is characteristic of northern and rnid-Atlantic

waters e.g. Connecticut, New York, andDelaware!, whereas two spawning peaks in thespring and fall! occur in North and SouthCarolina reviewed by Eversole, 1989!, and athird winter spawning may occur in Georgia Heffernan et al., 1989! and in Florida Hesselman et aL, 1989!. Quahogs may retain arelatively high condition index after spawning Ansell et ai., 1964; Keck et al., 1975!, andconsequently do not experience the largefluctuations in meat quality and marketability

associated with changes in the reproductive cyclewhich are typically observed in oysters,Crassostrea spp. e.g. Purdue et al., 1981!.

Settlement of quahog larvae is highlygregarious, and is stimulated by the presence ofconspecifics e,g. 3 mm juveniles! or other clamspecies such as Gemma gemma, which oftenocul at high densities in Mercenaria habitat Ahn, 1990!. This attraction appears to be

chemically mediated Keck et al., 1974; Ahn,1990!. In the field, larval settlement and/orretention of postlarvae may be enhanced in shell-covered sediment, which could provide a suitableattachment substrate and/or refuge from predators Carriker, 1961!, but this effect has not beenrigorously tested under field or laboratoryconditions. Flume studies show that selectioncapabihties of quahog larvae for a suitable

31

settlement substrate i.e. preference for sand vs.mud! are affected by flow conditions Butman etal., 1988!, but the relevance of this finding tofield conditions has not been demonstrated.

Studies of settlement success and post-settlementsurvival of quahogs have been hindered primarilyby the difficulty in efficiently segregatingpostlarvae from sediinent grains of comparablesize. Differential settlement was successfullyused by Ahn �990! in small-scale experiments,but may not be practical for large-scale samplingof a patchy natural environment.

Interactions between adult, benthicpopulations, through suspension-feeding activityor reworking and destabilization of sediinents,and quahog larvae are poorly understood.Kurkowski �981! demonstrated that adultquahogs can readily consume young veligerlarvae < 120 elm in laboratory experiments, andthat larvae do not survive entrapment inpseudofeces. A negative interaction betweenadult Mercenaria stocks and settlement was also

suggested by Rice et al. �989!, whodocumented much higher densities of juvenilequahogs in areas of Narragansett Bay, RhodeIsland, with low adult densities.

Successful metamorphosis and post-settlement recruitment of hatchery-producedquahog larvae is known to be influenced by eggand larval quality, as measured by their lipidcontent Gallager et al., 1986; Gallager andMarin, 1986!. These authors found that survivalof quahog and oyster larvae to the pediveligerstage was invariably poor when egg lipid levelswere low < 18% of the ash-free dry weight!, butthat high egg lipid content could result in bothhigh and low survival. A similar relationship wasdescribed between larva1 lipid content andsurvival through metamorphosis Gallager et aL,1986!. These results indicate that lipid contentalone is not always a reliable predictor of gametequality and larval survival. It also remains to bedetermined whether naturally occurring egg andlarval populations commonly experience lipidlevels below the minimum threshold which was

established as prerequisite for larval survival inthe laboratory.

Natural Mortality: PredationPredation is often considered the most

significant source of natural mortality, andthereby the dominant factor controllingrecruitment success of naturally occurringbivalves, including Mercenariamercenaria e,g.Virnstein, 1977; Malinowski, 1985!.Vulnerability to predation is known to bestrongly size-dependent, with smallest quahogs < ca. 20 rum in shell length! suffering greatestmortalities MacKenzie, 1977; Malinowski,1985!. Furthermore, modeling efforts byMalinowski and Whitlach �988! demonstratedthat population growth rates of quahogs weretwo to four orders of magnitude more sensitive tochanges in juvenile survivorship, than to those inadult survival or fecundity. These authorstherefore suggested that stock enhancementmeasures would be most effective when directedtowards enhancing juvenile survival e.g.through predator control!. In this context,Peterson �990! recently argued convincingly forthe need to apply experimental data on sizeselectivity and habitat preference of bivalvepredators to fishery management and resourceenhancement efforts,

Effect of Prey SizeNewly settled clams are expected to be highly

vulnerable to predation because they areasiphonate and must feed at the sediment-waterinterface. Information on predation of early post-settlement stages is extremely liinited, however,and largely qualitative reviewed by Gibbons andBlogoslawski, 1989!. Losses during later,juvenile stages are better documented, but haverarely been determined by sampling of naturalpopulations because quahogs < 20 mm in lengthare inefficiently captured by most commonlyused sampling gear, e.g. quahog shell buckets.MacKenzie �977!, however, was able toprovide strong evidence of high predatorymortalities at smaller sizes in Great South Bay,New York and Horseshoe Cove, New Jersey, bydetermining the relative proportion of dead andlive quahogs collected with a diver-operatedhydraulic suction dredge. He also used

32

distinctive shell markings to attribute deaths tospecific predator groups various gastropods,crabs, and starfish!. This approach, when usedin commerciaHy exploited areas, is obviouslyonly reliable to determine natural mortality ratesof quahogs below legal harvestable size. It alsotends to underestimate the number of dead

quahogs, because crabs often break sheHs,especially of smaller prey, to irretrievablefragments. Evidence of greater predationpressure on smaller sizes is therefore mostlygenerated from field plantings of quahogs ofvarying sizes e.g. Malinowski, 1985; Flagg andMalouf, 1983; Peterson, 1990!.

Crabs, carnivorous gastropods, and starfishare the three most important groups of predatorsof quahogs, although finfish e.g. rays! areknown to be significant predators south ofDelaware Bay Kraeuter and Castagna, 1980!.M. niercenaria attains complete size refuge fromoyster drills, and most crabs, including spidercrabs, rock crabs, green crabs, and mud crabs Dyspanopeus sayi j at a shell length p 30 mm Figure 1!. Qoahogs 40 mm remain vulnerableto two of the larger crabs species, the mud crabPanoepus herbstii and blue crab, CalDnecressapidus, which attain maximum sizes of ca. 62mm and 227 mm in carapace width CW!respectively Williams, 1984!. Susceptibility topredation is inversely related to quahog sizebecause both the number of potential predators Figure 2!, and the number of prey consumed byany given predator size class e.g. Peterson,1990! decline with increasing prey size.

Burrowing, predatory gastropods such aswhelks and moon snails are the most importantpredators of adult quahogs > 40 rnm in shelllength Figure 1!. Gastropods including oysterdrills! are higMy specialized predators that feedalmost exclusively on bivalves, and leave distinctmarkings on their shells. However, they arerelatively slow moving and thus cannot rapidlyinvade an area following natural e.g. salinitydisturbance! or man-induced eradication. Widedispersal is further limited by the lack of a free-swimming early developmental stage, except inthe case of moon snails. Gastropods also exhibitlong prey handling times, and consumption rates

for quahogs that are typically two to three ordersof magnitude lower than those of crabs [e.g. g 1quahog week 1 for whelks and moon snails Carriker, 1951; Greene, 1978!]. Whelkspieferentially feed on larger quahogs ! 40 mm!,and both whelks and moon snails show

preference for thin-shelled bivalves, whenalternate prey is available Cairiker, 1951!.Nevertheless, whelks are known to be majorpredators of adult Mercenaria in North Carolina Peterson, 1982; Irlandi and Peterson, 1991!,and can account for up to 13 percent annuallosses of the quahog population in Great SouthBay, New York Greene, 1978!. Starfish canonly prey on large quahogs in aggregation Doering, 1981!, and are more effectivepiedators of epifaunal than infaunal bivalves e.g. oysters, mussels, and scallops!.

Due to their motility, high predation rates,and high relative abundance, crabs are deemedthe inost serious predators of smaller quahogs.They are generally able to consume quahogs ofshell lengths up to 30 percent of their CW,although Panopeus herbstii, which feeds onquahogs up to 65 percent of its CW, provides anexception to this rule of thumb Mac Kenzie,1977; Whetstone and Eversole, 1981; Gibbonsand Blogoslawski, 1989!. The high vulnerabilityto piedation of quahogs <20-25 mm in length isaggravated by the fact that even larger crabs, thatare not mechanicaHy constrained to feed on smallprey, select smaHer quahogs when a wide sizerange is available. For example, large blue crabs > 125 mrn in CW! prefetentiaUy prey on 10-25min M. mercenaria, when offered quahogsranging in size from 5 to 35 mm, both in thepresence and absence of sediment Peterson,1990!. Their consumption rates number of preyeaten per unit time! for 30-35 mm quahogs are 5times lower than those for 10-15 mm quahogs. Asimilar preference for smaHer quahogs wasshown for the large mud crab, Panopeus herbstii Whetstone and Eversole, 1981!, althoughenergy intake tissue weight of quahogsconsumed per unit time!, was maximized atlarger sizes.

Selection for smaller prey appears to be ageneral phenomenon among crustaceans

33

Figure 2. Maximum size shell length in mmj of hard clams consumed by twelve common predators of Mercenaria

MacKenzie, 1977Carriker, 1951, Greene, 1978Carriker, 1951, Greene, 1978Doering, 1981

34

metceuaria in east coast esfuanes.

Sources:P. longicarpus hermit crab!Libinia sp. spider crab!D. Neoism~! sayi mud crab!C. uT0rates rock crab!O. occllatus caMo crab!C. maes' green crab!

P. berbstii mud crab!C. sapidus blue crab!Eupleura caudata k Umsalpinx

ctuerea oyster drills!P. duplicatus moon snail!Busycon canaticulatum whelk!Asterias forbesi

primarily crabs! feeding on hard-shelledmollusks, and occurs even when larger prey aremore profitable in terms of energy yield per unittime Juanes, 1992!. This discrepancy betweeneinpirical data and classical optimal foraging

Gibbons, 1984Gibbons h Blogoslawski, 1989Carriker, 1961; Gibbons, 1984MacKenzie, 1977Gibbons 4 Blogoslawski, 1989MacPhail et al. 1955 k Taxiarchis, 1955

in Gibbons d Blogoslawski, 1989Whetstone k Eversole, 1981Arnold, 1984

theory, which predicts preference for prey thatmaximize the predators' net energy gain, may berelated to greater claw damage and Qtness costsassociated with handling af larger prey Juanesand Hartwick, 1990!.

Effect of Substrate TypeCharacterization of the predator assemblage at

a given site, and an understanding of the effectsof environmental factors e.g. temperature andsubstrate type! on feeding rates of key predatorspecies are important in explaining and predictingsite-specific differences in population abundanceof quahogs, and in implementing resourceenhancement management strategies e.g.habitat manipulation!. The effects of sedimenttype on quahog predatory mortalities have onlybeen studied in the laboratory or smaH-scalefield trials; their outcome depends to a largeextent on the species composition, abundance,size structure, and substrate preference ofexisting predators.

Seagrasses were shown to enhance survivalof infaunal prey that can burro~ beneath the root-rhyzome mat, such as quahogs, by reducing theforaging effectiveness of whelks Peterson,1982!. Whelks are also generally absent fromshell-covered bottoms, which inhibit theirburrowing activity WAPORA, 1981!. Crabstypically show highest predation rates in theirpreferred substrates. This generalization will beillustrated below for two major groups of quahogpredators, the large swimming crabs Portunidae!, such as blue crabs Callirtectessapidus! and calico crabs Ovalipes ocellatus!,and for the smaller mud crabs Xanthidae!. Theformer prefer homogeneous substrates sand ormud/sand combinations! to crushed shell orgravel �0 to 50 inm in diameter! Figure 3A!.Given an equal density of quahogs amongsubstrates, they also prey most heavily in theirpreferred substrate Figure 3B!.

Reduced foraging efficiency of 0. oceQatusin gravel was related to increased searching timeand handling of non-prey items in this substrate Sponaugle and Lawton, 1990!. This behavioralresponse provides the basis for the recommendeduse of gravel or crushed stone aggregate inquahog growout sites in southeastern states suchas Virginia, where blue crabs are prevalent Castagna and Kraeuter, 1977!. Abundance andtherefore predation intensity by large, highlymobile portunid crabs e.g, blue crabs! isexpected to be temporally much more variable,

especially in areas that ee at the limit of theirdistributional range, than that of mud crabs,which form less mobile, gregarious populationsthat persist from year to year.

Mud crabs are often the numerically dominantcrabs in east coast estuaries. Dyspanopeus sayi ismost abundant north of Delaware Bay, attainingdensities of up to 100 crabs m 2 WAPORA,1981!, whereas Fttrypanopetts depresstts andPanoperts herbstii are prevalent in ChesapeakeBay and the Carolinas respectively Day, 1987!.Field surveys reveal that the three species aa:found at highest densities in heterogeneoussubstrates gravel, or bottoms with shell,eelgrass or Spartr'tta cover! WAPORA, 1981;

Figure 3. A! Results of blue crab Callinectessapidus! substrate preference rests, in which the locationof a crab with respect to sedunent type was determined ina series of laboratory paired �-choice! comparisons drawnPorn Arnold, l984!.

B! Predation rate mean ~ standard error! of blue crabsand calico crabs on Merceiiaria mercetiaria in varioussubstrates, determined in the laboratory drawn fromSponaugle 8 Lawton, 1990! see sources for furtherderails!.

35

Day and Lawton, 1988!. In agreement with fielddata, and in contrast to the larger portunid crabs,laboratory trials show that mud crabs D. sayi, E.depressus and P. herbsriQ prefer heterogeneoussubstrates, especially crushed shell, to sand ormud Day, 1987!. Predation rates of D. sayi onjuvenile quahogs were found to be significantlygreater in smail <17 mm diameter! or large �0mm! gravel than in sand Day, 1987!, therebylending support to the observation that substratepreference and predation pressure are positivelycorrelated, and illustrating that habitat structuralcomplexity is not always associated with areduction in foraging efficiency. Flagg andMalouf �983! also found that survival of field-planted juvenile quahogs in areas where mudcrabs were abundant, was inversely correlatedwith gravel size ranging between 6 and 32 mmin diameter.

This preference for substrates with a complextopography appears to be related to the mudcrabs' small adult size e,g. D. sayi attains only28 mrn in CW!, and thus requirement for refugefrom top predators bottom-feeding fish!.Consumption of juvenile quahogs by D. sayi,for example, is strongly inhibited by increasedpredatory risk in the presence of toadfish,Opsanus tau Day, 1987!. Introduction ofthis fish species has therefore been suggestedas a method of biological control of predationin quahog growout sites Gibbons andCastagna, 1985!.

Effect of Prey DensityA strong, predator-mediated, negative

correlation between population density andsurvivorship of M mercenaria has beendemonstrated during juvenile but not adult stages Malinowski, 1985!. Average seasonalsurvivorship in eastern Long Island was fourtimes greater at a density of 100 juvenile quahogsm 2 than at 1200 quahogs m 2. In this study,quahog density had a greater effect in explainingjuvenile survival at two sites where crustaceanswere the dominant predators, than all othercombined variables tested quahog size, time ofyear, and location!. Predation during juvenilestages was thus attributed a dominant role in

maintaining the low densities characteristic ofquahog populations.

Low density may thus provide infaunal preypopulations with a mechanism for persistenceeven when subject to intense predation Eggleston et al., 1992!. Sponaugle and Lawton�990! suggested that juvenile quahogs achieve arelative refuge from predation by calico crabs atlow densities, in heterogeneous sand/shell!substrate, but not in sand. In contrast, Peterson�982! found no low density refuge, over therange seven to 28 quahogs m 2, for adultquahogs preyed upon by whelks. Thus refugevalue at low densities may be predator- andhabitat-specific.

Growth

The time required for seed clams to achievesize refuge from most predators will beeffectively determined by their growth rate.Growth data for clams < one year old, sampledfrom natural populations, may be biased by size-selective predation or sampling efficiency, andare thus more readily derived from land-basedculture systems or field enclosures that excludepredators. Growth rates of seed clams reared inthe laboratory under optimal temperature andfood conditions average about 0.58 mm week Table 2.12 in Malouf and Bricelj, 1989!. Evenhigher growth rates, of up to one rnm wk-1 canbe ieahzed with natural seston at near-optimumtemperatures �7 to 28oC! Table 2!. However,lower values e,g. 0.45 to 0.62 rnm week 1 aretypically obtained when averaged over the entiregrowing season e.g. Eldridge et al., 1979!.

Table 3 lists some of the factors which havebeen shown to significantly influence growth ofquahogs see Rice and Pechenik, 1992 for amore extensive review!. Temperature andsediment type are two of the environmentalvariables most frequently correlated with growthof M. mercenaria. Growth is generally greater incoarse-grained sand or silty sand! than in fine-grained sediments. However, the effects ofsubstrate type per se cannot be readily decoupledfrom the effects of flow velocity, and the qualityand quantity of the overlying seston. Growth rateis consistently reduced at high suspended

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Table 2

Growth rates of juvenile Mercenaria rnercenaria exposed to natural seston Lo and Lf =initial and final shell length in mm!. Maximum subtidal and density-independent!growth rates were selected where several conditions were tested. Growth rates weredetermined in enclosures in the natural environment unless otherwise noted.

SourceGrowth

Rate rnmlwit!Ye inp. oC!

Shell Length Lo - Lg!

Site;Period

Bricelj 4Borrero,unpubl.

22- 28 0.96Napeague Harbor, 10.3 - 14.2NY; July-Aug.

Bricelj,unpubl.27 0.54Great South Bay, 10.5 - 15.4NY; Oct.-Nov. raceways!

Appelmans, 1989Fishers Island, 4.6 - 5.7NY; Aug. land-based upwellers!

22 1.05

Flagg kMalouf, 1983

Hadley & Manzi,1984

0.62Shinnecock Bays, 7.9 - 15.4NY; July-Oct.

Folly River, SC;Feb.-Aug.May raceways!

8-32

21-26

0.48

1 lla3.9 - 16.9

Eldridge etaL, 1979

Walker, 1984

Clark Sound,~ SC; 13.0 - 26.9May-Dec.

0.45

1.08Wassaw Sound~, GA 6.1 - 28.3 intertidal!

17- 26 0.84 Menzel, 1963Alligator Harbor, 5.4 - 9.0FL; April

Mean= 0.83

Magnum seasonal growth rateClams rown in substrate.

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sediment loads Q 23 to 44 mg dry weight 1-1!,whether these result from bioturbation Murphy,1985!, or physical disturbance wave action! Turner and Miller, 1991!.

Studies of the effects of seagrass habitatrelative to unvegetated substrate on growth ofMercenaria mercenaria reviewed in Table 3!have yielded conflicting results which may betelated to site-specific differences in the flow

regime, food concentrations, and structure ofsubtnerged aquatic vegetation. Positive effectshave been attributed to increased near-bottom

food supply to the benthos through enhancedparticle settlement Peterson et al., 1984!, andresuspension or in situ production of benthic orepiphytic algae within the seagrass bed Judge etal., in press!, reduced siphon nipping activity byfinfish Coen and Heck, 1991!, enhanced

Table 3.

Factors influencing growth of Mercellria Ntercerraria TOM = total organic maatte; Loand Lf = initial and final shell length!. Positive or negative effects on growth areindicated, as well as the magnitude of growth inhibition, where appropriate.

SourceVariable Effect Magnitude

�!�!�!

24%~36%<8%

Sand ! MudSand ! MudSand ! Mud

Sediment type % silt-clayor TOM!

16% at 44 mgDW 1-138% at 23 mgDW l-l38% at 193 mgDW 1 1

�! S!�!

-! -! -!

SuspendedSediments

�,7!Max, growth at intermediate1eveis

Seston Flux

�! 8!

+! Diatoms�5 pm +! Chlorophyll a

Phyto planktonConcentration

9,12!�0,11,12!�2!

-! +!no effect

Presence ofSeagrass

�3!�1!

Biologicaldisturbance:siphon nipping

Range for +! growth = 9-31oCOptimum = 20-25oC

�,14,15!Temperature

18% 80 clams m-2. �2!Lo � � 5.8 crn!

22% �90 clams m 2; �6!Lf-6.2ctn!

g19% �159 clams m �7!Lo � -1.3cmLf=4.6-5.7ctn!

33% �027 clams m-2 �8!Lo= 1.7,Lf=3.3cm!

PopulationDensity

~ Population density parameters indude average length data.

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Sources: �! Prau, 1953; �! Pratt k Campbell, 1956; �! Grizzle k. Morin, 1989; �! Bricelj et al., 1984a; �! Murphy,1985; �! Turner k Miller, �! Grizzle & Lutz, 1989; 8! Evjen, 1985; 9! Kerswill, 1949; �0! Peterson et al., 1984; �1!Irlandi k Peterson, 1991; �2! Peterson k Beal, 1989; �3! Coen k Heck, 1991; �4! Ansell, 1968; �5! Laing et al., 1987;�6! Rice et al., 1989; �7! Eldridge et al., 1979;�8! Walker,1984,

sediment stability and reduced sedimentresuspension postulated by Irlandi andPeterson,1991!. Adverse effects may result froma reduction in horizontal seston flux the productof seston concentration and current velocity!due to baffling of currents within the seagrasscanopy lrlandi and Peterson, 1991 andreferences in Table 3!.

Differences in growth rate of M. merceiiariaalong the east coast of North America weredescribed by Ansell �968!, who found no clearlatitudinal pattern or coiielation of growth rateswith temperature in comparing populationsbetween Massachusetts and Florida. Reducedgrowth occurs, however, near the species'northern distributional limit Prince Edwardbland, Canada!, where lower temperatures resultin a shorter growing season, and quahogs requiresix years or more to attain legal market size. Thehighest growth rates time to market size = 2.2years! have been recently reported forpopulations in Florida, where they are attributedto continued growth during the winter, and thuslengthening of the annual growing season at thislatitude Jones et aL, 1990!. These results weresubstantiated by Arnold et aL �991! who notedthat the mean m growth parameter the product ofk and asymptotic size in the von Bertalanffygrowth equation! was twice as high in the IndianRiver, FlOrid, than in NarraganSett Bay, RhOdeIsland. Therefore M, mercenaria can achieve

growth rates approaching those of faster growingclam species such as Spisula solidissima surfclams! and Mya arenaria softsheH clams! Fig.2.5 in Malouf and Bricelj, 1989! only in thesouthern portion of its geographic range.

Table 4 compares growth rates of M.mercenaria populations, as reflected in the timerequired to attain legal market size. Ansell's�968! data are extended or replaced where inorecurrent information is available, Minimum legalsize is here assumed to be 25.4 mm in shell

thickness, the New York state limit!,corresponding to a shell length of 48 mm basedon morphometrics of quahog populations inGreat South Bay Greene, 1978!, although theratio of length to thickness may vary somewhatbetween locations. Populations between Maine

and Georgia grow at comparable rates, typicaHyrequiring 3.0 to 4.4 years to attain market sizeg able 4!. Differences in growth rate among siteswithin an estuary are often greater than thoseamong latitudes over a broader geographic scale[e.g. two-fold variation in Great South Bay Greene, 1978!, three-fold variation in CapeLookout, North Carolina Peterson and Beal,1989!, and 1.7- to two-fold variation in the mparameter in the Indian River, Florida Arnold etaL, 1991! and Narragansett Bay, Rhode Island Jones et al., 1989! respectively].

Population density of quahogs is generallynot a significant factor influencing growth ratesof natural populations e.g. Malinowski, 1985!.Density-dependent growth inhibition, generallyonly occurs during growout of cultured clamsplanted at densities two to three orders ofmagnitude greater than those found in nature Table 3!. Stunting of adult clams was found,however, in uncertified waters in GreenwichCove, Narragansett Bay, at unusually highnatural densities of 190 clams m 2 Rice et al.,1989!. Similarly, reduction in the condition indexof adult clams occurred at experimental densitiesof 203 clams m-2 Malinowski, 1985!.

Bulk measures of food quantity orphytoplankton biomass chlorophyH aconcentration! may not necessarily provide agood predictor of bivalve growth, except underconditions of food limitation, which are nottypically encountered in shallow, eutrophicestuaries. Thus, during virtually monospecificblooms of noxious unpalatable, toxic, orindigestible! algae, bivalve populations mayexhibit severe growth depression which may notbe reflected in low or abnormal chlorophyll levels e.g. Cosper et al., 1987! Correlations betweenfood availability and growth can be improved byincorporating relevant measures of foodquality e.g. biomass of phytoplankton speciesknown to support bivalve growth! Pratt andCampbell, 1956!.

Effect of Noxious Algal BloomsAlgal species which may adversely affect

quahog populations under bloom conditionsinclude: the chlorophyte Nannochloris atones,

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Table 4.

Average time in years! to attain legal market size [= 48 mm in shell Length see text!!of Mercenariu mercennria natural populations along the species' latitudinal range, fromnorth to south. Range is shown between brackets; unless indicated, time to market sizeis calculated from fitted von Bertalanffy, Gompertz or Logarlthjnic growth equations.

Location SourceTime

yrs.!

Prince Edward Island,Canada

Maine

6.0

4.4

Monotnoy Point,Massachusetts

3.2

'Jones et al., 1989Narragansett Bay,Rhode Island

4.0

�.0 - 4.8!

Great South Bay,New York

Barnegat Bay,New Jersey

York River, Virginia

Core Sound, NC2.4

Wassaw Sound, Georgia Walker and Tenore, 1984 intertidal!

3.0- 4.0

Kings Bay, southern GA 3Jones et al., 19902.0

3Jones et al., 1990Arnold et al., 1991

Indian River, Atlanticcoast of Florida

2.2 - 2.3

2.1

�.9 - 2.5!

Gulf Coast, Florida Fig. 5 in Ansell, 19682.6

1. Shell height H! converted to length using an H/L ratio = 0.933;

2, Assuming that age in years = number of annual bands;3. Using a H/L conversion factor = 0.91,

3.5

�.0 - 4.0!�.5 - 5.0!3.0

4.3

�,8 - 4.6!4.4

Fig. 5 in Ansell, 1968

Fig. 5 in Ansell, 1968

Fig. 5 in Ansell, 1968

Appendix 4 in Buckner,1984

Greene, 1978lKennish and Loveland, 1980From Table 5 in Kennish,1980

From Fig. 3 in Loesch andHaven, 1973>Peterson et al., 1983

the chrysophyte Aureococcus anophagefferens,and the dinoflageHate Alexandrium fundyense,the causative agents of "green," "brown," and"red" tides respectively, The first two species arepicoplanktonic algae circa 2 pm in diameter!,which, due to their small size, are expected to bepoorly retained by the clams' feeding apparatus.[Particle retention efficiency in M mercenariadecreases steeply with decreasing particle sizebelow a size of about four mm Riisgkrd, 1988!].

Summer blooms of N. atomus were

documented in Long Island's southern bays inthe 1950's reviewed by Ryther, 1989!.Laboratory studies subsequently demonstratedthat monospecific cultus of this alga do notsupport growth of quahogs in either larval Tiu etal., 1989! or juvenile stages Bass et al., 1990!,and cause growth inhibition when combined withother algae of high nutritional value. Lack ofgrowth on a monospecific diet of N. atomus wasattributed to the quahogs' short gut retention andlow absorption efficiency of ingested organicsfor this alga Bricelj et al., 1984b!.

Aureococcus anophagegerens first occurredin Narragansett Bay Sieburth et al., 1988! and ineastern and southern Long Island bays in 1985 Cosper et aL, 1987!, and has reappeared in NewYork waters in past years. This alga causessevere inhibition of quahog filtration rates Tracey, 1988!, and inhibition of ciliary beat ingill tissue excised from quahogs Gainey andShumway, 1991!. The mechanism of this alga'stoxigenic action is not yet clearly understood.Preliminary data suggest that although quahogsare less sensitive to the effects of A.anophagefferens than mussels, Mytilus edulis,they may still experience growth reduction ateven moderate field concentrations of A.

anophagefferens �.1 x 108 to 3.2 x 10 cellsliter 1! Bricelj and Borrero, unpublished data!.

Finally, Alexandriutn fundyense and relatedspecies ate responsible for the accumulation ofparalytic shellfish poisoning PSP! neurotoxinsin suspension-feeding bivalves. Mercenariamercenaria was found to accumulate low levels

of PSP toxins during New England red tideoutbreaks in 1972, when other similarly exposedbivalve species became highly toxic, presumably

because blooms of highly toxic forms of thisdinoflagellate elicit feeding depression and shellclosure in this species Twarog and Yainaguchi,1974!. Laboratory toxification experimentsshow, however, that M. mercenaria is capable ofacquiring high levels of PSP toxins � to 3 ordersof magnitude above the regulatory level forsheOfish closures!, when exposed to a LongIsland, low-toxicity strain of A. fundyense, or aNew England, high-toxicity dinoflagellate strainin coinbination with non-toxic phytoplankton Bricelj et aL, 1991!. In conclusion, although theeffects of blooms of these three algal species onnaturally occurring quahog populations have notyet been determine, the experimental evidenceindicates that they are at least capable of causingsevere growth reduction, and could potentiallycause inortalities of some life history stagesunder prolonged exposure.

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Questions and Aaswers

Q. Mr. George DeBlois, shellfisherman! Havethere been any studies to show how manyspawners are needed to electively repopulate anarea, given predation, fishing, and otherfactors?A. Dr. Monica Bricelj, SUNY-Stony Brook!There has been some interest in trying todetermine the minimum amount of stock

necessary to sustain recruitment into the fishery.I know that there was a plan to do this kind ofstudy in New Jersey, but I don't know if the planwas actually carried out. There is no publishedinformation at this time about minimum requiredbroodstock, but there is some indirect evidencethat rrught be considered, In the Great South Bayof Long Island, there has been a steady decline ofthe adult population, In spite of this, there hasbeen no noticeable effect on the abundance of

sublegal-sized clams new recruits! between1986 and 1989 in eastern Great South Bay,where survey data are available. Fishing has notappeared to lower the adult population below thecritical minimum requited to sustain recruitment.A decrease in the number of sublegal clams hasbeen observed, however, in the last few years�990 through 1992!.

In terms of quahog growth, most studieshave shown that the density of quahogs is not tooimportant in hmiting growth. In most areas,densities are somewhere between two to 15

animah per square meter. Generally it is ratherrare to find natural populations of quahogs indensities of hundreds per square meter.Greenwich Cove is one of the exceptional areaswith very high adult densities. In these very highdensities, lower growth or stunting has beenshown, Density may have a major affect oniecruitment, but this needs further study.

Q. DeBlois! I have read that cherrystonesproduce many more eggs than the smallerlittlenecks Is there any evidence of a cessationof egg production as quahogs age?A. Bricelj! There is no evidence for thisreproductive senecence in quahogs. Scallops arethe only group of bivalves that I am aware of thathave a reduction of gamete production with age.There is one important thing to be aware of. Thestudies which have shown that chowder quahogsare the most fecund are based upon laboratoryexamination of the number of garnetes producedduring induced spawning. We do not know howthis reflects what is going on in the real world, inthe sense that "How often do chowders spawn innature?" But from the laboratory spawningexperiments, there is no difference in the viabilityof eggs from littlenecks or chowders.

Q. Prof. Dennis ¹xon, URI! One of ourobjectives is increasing the stock, and myquestion is about predator controL About ahundred years ago there used to be a statute inRhode Island that set a bounty on starftsh,because of their recognized impact on sheliftshpopulations. Do you believe that attempts atpredator control in an open fi shery such asinNarragansett Bay coukfbe of any value?A. Bricelj! Well, Clyde MacKenzie of theNational Marine Fisheries Service has suggestedjust this in the past, but people have balked atprograms that would clear large areas such as theGreat South Bay of predators. Economically it isnot a very feasible solution. Perhaps in thecontext of smaller-scale areas, enhancementprograms might work. Some type of habitatmanipulation might be undertaken when weknow which predator is most troublesome andwhat features of the substrate or other

characteristics ate important, Predator control iscertainly important in nursery and growoutphases of aquaculture. Predator control in openfisheries has not been tested except forMacKenzie's work back in the mid-1970s. He

did some predator eradication in relatively smallplots and showed that there was a positiveresponse, but there has been little further testingof this in the field. Eradication needs to be tested

on a variety of scales to show just where it iseffective. Eradication measures may be moteeffective at controlling moon snails or whelks,because they move slowly into an area If youhave highly motile predators such as blue crabs,it is doubtful that any eradication measures wouldwork because they can come into an area soquickly. In brief, I think that any kind oferadication program must address predator typeand scale.

Q. Mr. John Finneran, shellfisherman! Is thereany evuknce showing that some sediments aremore conducive to larval seolement than others?

In other words, are there sediments that have a"better flavor" to settling kuvae?A. Bricelj! Yes, there have been somelaboratory studies on this. Keck and co-workersshowed that if you treat sediment with "clamjuice," you will get an attractant response andincreased larval settlement. This is a similar result

to the data I presented which showed thatjuvenile clams exhibit an attractant response. Itmust be a chemosensory response, becausesediments that were "pretreated" by placement ofclams that were subsequently removed, alsoenhanced settlement. Physical factors may alsoplay a role, since additions of gravel or cleanclamshell to the sediment also increased

settlement in this study.

Q. Finneran! Are there any inorganics thatmight act as an attractant? I have noted that thereare often large quahog assemblages in sedimentsthat are high in coal ash.A. Bricelj! No, but one of the possibleexplanations for areas of high abundance in GreatSouth Bay, might be the presence of shellfragments that modify the bottom topographymaking the area better for larval settlement andsurvival of juveniles. There has been no testingof this though. This is very difficult work. Withoysters it is relatively easy, because they settle onhard surfaces like shells. Postset quahogs are inthe size range of sediment particles, so nobodyreally wants to do this kind of tedious work. Thiskind of work is not intractable though, justdifficult. One of the key things I want to

47

emphasize is that we need many more studies onthe early life history stages of quahogs ratherthan adults. Year-class strength is beingdetermined by post-larval and juvenile survival.These stages need more tcsearch attention.

48

Overview of Quahog Management Studies in Narragansett Bay,1946 to 19921

MICHAEL A. RICEDefertment of Fisheries, Animal, and Veterinary Science

University of Rhode IslandKingston, Rl 02881

Introduction

It has been recognized by fisheriesmanagers that a number of factors describing afishery stock must be determined before onecan begin to make any rational predictionsabout what levels of fishing effort aredesirable for maintenance of a sustainablefishery e.g. Royce, 1984!. In simple terms,the inputs to a fishery stock are the recruitmentof new individuals into the fishery and growthof individuals previously recruited into thefishery Figure 1!. Likewise, the factorswhich tend to reduce the size of fishery stocksare natural mortality of the stocks and thefishing mortality � a combination of thefishing catch plus associated mortality due todamage by gear etc. These basic principles ofdynamic fishery stock models have been mostfrequently used for the management of finfishstocks, but with care can be applied to bivalvemolluscan fisheries Caddy, 1989!,

Figure 1. The four key factors that determine quahogstock size,

One key aspect that distinguishesassessment of bivalve fisheries, such as thequahog fishery of Narragansett Bay, fromassessment of finfisheries is that one is dealingwith a sessile popu1ation of juveniles andadults. Additionally, quahogs and otherbivalves are highly fecund, so that parentalstock size is considered less important forrecruitment than is available space, suitable

49

Abstract. There have been a considerable number of studies on quahog populations in Narragansett Bay

that have provided valuable informatt'on for fisheries managers. Addi'tionaNy, there have been a few socio-economic studies that have characterized the labor force in the quahogfishery and have provided

information pertinent to levels offishing effort. Most ecological studies have focused on the populationstructure and standing crop densities of quahogs in Narragansett Bay. 1he age and growth rates ofquahogs in diferent parts of Narragansett Bay are well known, but there is a dearth of information aboutthe patterns of quahog recruitment. As with most fisheries that require little capital requirements, the levelsof fishing effort in Narragansett Bay are known to increase or decrease with relative ease as conditions inthe fishery or the general economy change. It is recommended that available socioeconomic data aboutthe fishery be updated, and that studies be undertaken to assess the relative impacts of currently usedfishing gear.

conditions for settlement, and postsettlementpredation loss prior to recruitment into thefishery Hancock, l 973; Kassner and Malouf,1982; Malinowski and Whitlatch 1988;Malinowski, this volume!. The end result ofthe various physical and biological factorsdetermining quahog distribution is that fisheryrecruits are found in patches or clumps e.g.Kassner et al., 1991.!. As a consequence of thepatchiness in distribution of quahogs, specialcare must be taken to design appropriatesampling protocols that utilize appropriatestatistical methods e, g. Saila and Gaucher,1966; Gardefors and Orrhage, 1968; Ludwigand Reynolds, 1988; Murawski and Serchuk,1989!. Techniques that may be appropriate forthe sampling of quahogs in Narragansett Bayinclude transect or quadrat methodologies incoves and inlets e.g. Rice et al,, 1989! orstratified random sampling methods over widerareas e.g, Russell, 1972; Murawski andSerchuk, 1989!.

Recognizing that the sampling andstatistical methods f' or quahogs may bedifferent from those used for finfish stockassessments, it is the aim of this paper tooutline key studies related to shellfish stockassessment that have been carried out inNarragansett Bay over the last 45 years, andto provide some insights into the at@as wheredata is lacking. It should be noted that a recentreport has been prepared that outlines ingreater detail many of the studies covered inthis paper Pratt et al., 1992!.

Quahog Age and GrowthOne of the key factors of interest in

quahog stock assessment is the rate at whichquahogs grow. There have been a number ofstudies since the 1950s that have investigatedthe growth rate of quahogs. Pratt �953!estimated the growth of quahogs bymeasuring changes in the length-frequencydistributions of experimental populations quahogs planted into bottom enclosures! overtime, Using a protocol of marking quahogs atvarious stations arotmd Narragansett Bay andineasuring their growth after recapture, Pratt

Figure 2. Locations of stations in ¹rragansett Bayfrom which quahogs vere sampled for studies of growthrate. Triangles: Pratt and Campbell �956!; Squares:Jones et al. �989!; Circles: Rice et aL �989!,

and Campbell �956! were able to collect verydetailed information about the growth ofquahogs from a number of locations aroundNarragansett Bay Figure 2!,

More recent studies have used the techniqueof sclerochronology the assessment of age byquantification of periodic increments in theshell! for determining the age of quahogs e.g.Rhoads and Panella, 1970; Kennish, 1980!.Use of sclerochronology allows for theidentification of individual year classes, whichis otherwise difficult because quahogs arespawning throughout the summer andindividual growth rates ate highly variable.

By quantifying annual growth rings ofquahogs from 10 stations in Narragansett Bay Figure 3!, Jones et al. �989! were able togive a very detailed description of quahoggrowth as related to average annual watertemperatures. One of their main findings is

50

Figure 3. Growth curves for quahogs fromNarragansea Bay based on 100 individualsPom l0stations. The top graph represents average size-at-agemeasurements ~ I SD for each year of growth.?atebottom graph represents modeled growth based on thevon Bertalanfry equation. Curve fittingfolloeed hvoapproaches a! faring one curve to the averaged shelhu-age measurements on the top graph, and b! fitting aseparate curve to each of the No quahogs and averagingthe resultant von Bertalang growth parameters Joneset al. J989!.

that quahog growth is best described by thevon Bertalanffy negative exponential growthfunction Figure 3!. Jones et al. �989! usedshell height SH! � umbo to ventral valvemargin measurement � as their standardmeasurement. %heir average von Bertalanffygrowth parameters from the 10 NarragansettBay stations were:

SH = 73.32 mm valve height!lt = 0.21

and t p = 4.57

Although there are differences in the growthrates of quahogs from various sites inNarragansett Bay, the average quahog reaches

minimum legal size �5.4 mrn, 1 inch wide!by the end of its third growing season, andremains in the littleneck size category �5.4 to38 mm wide! for four years after recruitmentinto the fishery. A recent comparison ofgrowth data collected by Pratt and Campbell�956! and the data of Jones et al. �989!showed that although the methods weredifferent and the studies were 33 years apart,the average growth rates of quahogs inNarragansett Bay were quite similar Pratt etal., 1992!. Additionally, a study done inNorth Carolina has validated the annual

periodicity of bands in quahogs by use of amark-and-recapture study Peterson et al..1983!. Rice et al. �989! also usedsclerochronology to determine the growthrates of quahogs in dense adult assemblages inGreenwich Cove and West Passage Figure2!, They found that quahogs in very denseassemblages grow at considerably lower ratesthan quahogs in less dense assemblagestypified by the other growth studies. See Riceand Pechenik �992! for a review of factorsthat may affect the growth of quahogs.

Spawning and RecruitmentThe spawning of quahogs is known to be

temperature dependent, and appears to be trig-gered as water temperatures approach 20o C Loosanoff, 1937!. Details of quahogfecundity and spawning are provided byBricelj this volume!. A study based onplankton net tows in Narragansett Bay from1950-1952 showed that the maximum

numbers of quahog larvae in the water columnwere found during the summer months Juneto August Landers, 1954!. Since the larvalperiod of quahogs can last approximately twoweeks Loosanoff et al., 1951!, it is mostprobable that tidal currents and wind-generated surface waves can effectivelydisperse quahog larvae throughoutNarragansett Bay Wood and Hargis, 1971;Andrews, 1983!. Thus, there need not be anecessity for a close proximity of broodstockto increase the level larval settlement and

subsequent recruitment into the fishery.

51

Successful recruitment of quahogs ishighly dependent on postsettlement survival ofjuveniles rather than absolute numbers ofspawners producing gametes Hancock, 1973;Malinowski and Whitlatch, 1988;Malinowski, this volume!. Kassner andMalouf �982! evaluated the practice ofaugmenting the numbers of broodstock in theGreat South Bay, Long Island and found thatthere was no significant increase in quahogrecruitment. Indeed, MacKenzie �979! urgedthat the most effective means for increasingquahog recruitment is to protect juvenilequahogs from predation losses.

There have been few studies carried outin Narragansett Bay in which the rates ofquahog recruitment have been determined.Juvenile quahogs have been quantified insome benthic surveys e.g. Pratt, 1977!, butthese studies were not fonowed up todetermine the rates at which postset juvenilesreach legally fishable size. There have beentwo studies aimed at investigating the effectsof fishing gear on the recruitment ofquahogs. We first of these studies comparedthe relative impact of power dredging versusbullraking on the recruitment of quahogs Glude arid Landers, 1953!. In the study areachosen for this study, there was littlesettlement or recruitment in control and testareas. Likewise, in a recent study by Sparsisand DeAlteris details in this volume!designed to test the effects of bottomcultivation on the settlement and recruitmentof quahogs, little settlement or recruitment ofquahogs was noted in their test or controlplots, The failure to find settlement andrecruitment of quahogs in these studiesduring 1949-1950 and 1990-1991 suggeststhat there may be some paucity of quahogrecruitmeiit in some areas of NarragansettBay. In another area of Narragansett Bay-Greenwich Bay, which is known to be one ofthe most productive areas for the quahogfishery � benthic studies suggest that there isannual recruitment af quahogs Stickney andStringer, 1957; Rice et al., 1989!.

Assessments of Standing Stocks ofQuahog

There have been a number of' studies in the

last 40 years aimed at mapping the location ofquahog stocks and providing estimates ofstanding stock densities, but most have beenconfined to coves, inlets, and small portionsof Narragansett Bay. A survey by Pratt �953!carried out in 1949-1950 included 123 stationsin Narragansett Bay, but the data were notmapped. The first Narragansett Bay-widestudy that mapped the distribution of quahogswas based on a dredge survey between 1956and 1957, undertaken in response to calls forthe construction of mid-Bay hurricane barriers Stringer, 1959!. In this study, nearly 2,800samples were taken on a 900-foot grid.Although the data from this study Figure 4!were collected 35 years ago, the quahogdistributions roughly approximate the generalposition of known stocks today. The lastNarragansett Bay-wide study of quahogdistributions was conducted jointly by theU.S. Environmental Protection Agency andthe R.I. Division of Fish and Wildlife �974!.Maps of shellfish distribution were made, butthere was no information provided as to stockabundance. The reason why large-scalesurveys of quahog populations have beencarried out only on an occasional basis is thatthese studies tend to be quite expensive.

Most quahog population studies havefocused on coves or other subsections ofNarragansett Bay. Quahog population studieswere carried out by grab sampling inGreenwich Bay from 1951- 1957 by the U.S.Fish and Wildlife Service and the R.I.Division of Fish and Game Stickney andStringer, 1957!. At that time, maximumdensities of quahogs were found in mixedsand and silt bottoms near the mouth ofGreenwich Cove and near Mary's Creek Figure 5!. For a comparison, divers in 1988using quadrat sampling methods found anaverage density of 190 quahogs/m2 at themouth of Greenwich Cove Rice et al., 1989!.Quantitative stock surveys have been carTied

Pigare 4, Distribution of quaItogs in Narragansett Bay from a quantitative survey 1955-l956 Stringer, 1959!.

53

out in the closed areas of Providence Riverand the Upper Bay in 1956, 1965, 1976, and1985 Stringer, 1959; Campbell, n.d.;Canario and Kovach, 1965a; Saila et al.,1967; Sisson, 1977; Pratt et aL, 1987!. Ineach of these studies, the population ofquahogs in these areas was dominated by thepresence of large adults, which typifies thepopulation structure of quahogs in areas thathave been closed to shellfishing for a longtime Figure 6!.

In comparison to the numerous publishedstudies which have focused on stocks in areas

closed to shellfishing, there have beenrelatively few published studies on shellfishstocks in the areas of Narragansett Bay opento shellfishing. In addition to the previouslycited Narragansett Bay-wide studies, Russell�972! surveyed quahog populations in theWest Passage of Narragansett Bay usingdredge sampling techniques. A number ofR.I. Department of Environmental

54

Figure 5. Distribution of quahogs over 25 mm longin Greenwich Bay during l952. Data were gathered from226 stations using a 0.46 m2 grab sampler and a 12 mmmesh sieve Stickney and Stringer, 1957!.

Figure 6. Overall length-frequency distribution ofquahogs in the Providence River and Mount HopeBay. Both areas are closed to shellftshing. Quahogswere collected November 1985 Pratt et al., 1987!.

Management R,I,D.E,M.!-sponsored studiesare documented in the R.I. Division of Fish

and Wildlife Leaflet Series. These R.I.D.E.M,

surveys include the northern portion ofGreenwich Bay Campbell, 1959a!; thePotowomut River Campbell, 19S9b!; theKickamuit River Campbell 1959c; Canario,1963!; and the East Passage Canario andKovach, 1965b!. In general, the populationstructure of quahogs in actively fished areas ofNarragansett Bay differs from closed areas inthat the fished areas have a predominance ofyounger, smaller quahogs. This distinction isbest illustrated in a comparisoti study betweenactively fished and closed areas in Greenwich

Figure 7. Quahogs were collected by divers from 30quadrats �.25 m2>in each of three sites in NarraganseaBay, The sites are: A! Greenwich Cove, B'l GreenwichBay, and C! South Ferry, West Passage. Histogramsrepresent total numbers of quahogs in size classes of 3nim increments. The indicated valve lengths are size classmidpoints. The dashed line represents the Rhode islandlegal size limit for quahogs that is a one-inch hinge width,which corresponds to a 48 nun valve length Rice et al.,1989!

Bay and West Passage Figure 7! Rice etal., 1989!.

Estimates of Natural MortalityThe first of the factors which remove

individuals from quahog stocks is naturalmortality. Natural mortality includes lossesdue to predation and diseases. It is known thatpre-recruit juvenile quahogs are, high1ysusceptible to predation losses. However,once quahog reach the size at which theyrecruit into the fishery, they am relativelyresistant to most predators e.g.. Whetstoneand Eversole, 1978; MacKenzie, 1979;Boulding and Hay, 1984!.

Caddy �989! outlines a method forestimating the natural mortality of bivalvesby successively sampling the number ofdead shells paired valves! present in closedareas in relation to the number of live

animals present. In the various studies ofstock abundance in closed areas of

Narragansett Bay, there has been noquantitative data collected as the levels ofnatural mortality in those areas. It is likelythat natural mortality is relatively higher in

quahogs in excess of 8S rnm valve lengthbecause of their reduced ability to reburrow Rice et al., 1989!. 'Dere may be periodicincreases in natural mortality of quahogscorresponding with cyclic fluctuations ofstarfish Asterias forbesir'! populations inNarragansett Bay Pratt et al., 1992!.

Estimates of Mortality Caused byFishing

Estimates of the mortality of quahogscaused by fishing can be made by usingestimates of standing crop abundances ofquahogs and the number of quahogs caught.This method of estimating fishing mortalityis highly dependent upon knowledge of theselectivity of the harvest gear. Using a"rocking chair" dredge, with catchefficiencies known for various substrate

types, Russell �972! estimated the fishingmortality of quahogs in the West Passageduring one season of the commercial dredgefishery. During the dredge season, thestanding crop of quahogs in his study areadeclined from 18,148 2 5,704 bu to 7,23S 42,167 bu � bu = approximately 80 poundsor 31.5 kg!. Breakage is important toconsider as it may be a source of bias infishing mortality estimates if catch alone isthe sole basis of these estimates. One studyin Narragansett Bay compared the effects ofharvesting gear and handling on thebreakage of quahogs Glude and Landers,1953!. Broken quahogs caught in a "rockingchair" dredge ranged from 0.7% to 1.2% ofthe total catch. In rocky sediments, 2.9% ofthe remaining quahogs were found brokenon the bottom, but in sand/silt areas, 1.0%of the remaining quahogs were found to bebroken. There was no evidence of breakageof quahogs <60 mrn valve length by thedredge, and there was no evidence that thedredge smothered quahogs by coveringthem with sediments. There was essentiallyno damage to quahogs directly by bullrakm,but handling of quahogs by bullrakersaboard the boat caused some breakage�0,1% to 0.3% of the total landed.

Estimates of Fishing Effort andFishing Intensity

The number of fishermen in the quahogfishery and the average catch per fishermanare important data which can be useful forestimating fishing effort. Me NationalMarine Fisheries Service NMFS! makesannual estimates of the numbers of

shellfrshermen in Rhode Island. Based on

the number of licenses issued and the

number of boats registered toshelifishermen, NMFS estimates there to bebetween 1,000 and 1,300 full-timeshellfishermen in recent years. This estimateis considerably higher than the estimate of700-800 full-time sheilfishermen currentlyused by Rhode Island Department ofEnvironmental Management fisheriesscientists see Pratt et al., 1992 fordiscussion of estimation methods!.

There have been two surveys ofshellfishermen to gather data about the levelsof fishing effort among shellfishermen.Holmsen �966! and Holmsen and Horsley�981! conducted mail surveys of all shellfishlicense holders and made estimates of thenumbers of those deriving differentproportions of their income from quahogging Table 1!.

55

56

The proportion of fishermen obtaining atleast 75% of their income from quahoggingincreased from 21% to 36% in these twosurveys. Holmsen �966! also found anincrease in full-time fishermen from the mid-1950s to 1961, indicating a trend ofprofessionalization during periods of incliningas well as decreasing landings. One keyconclusion of Holmsen's surveys is that oneof the main characteristics of the Rhode Islandquahog fishery is the relative ease ofincreasing or decreasing fishing effort asconditions change in the fishery or thegeneral economy.

At present, the three commercial methodsof quahogging in Rhode Island waters arewith the use of tongs, bullrakes, and bycommercial diving. Commercial diving hasincreased in importance as a method ofquahog harvesting since the 1981 study ofHolmsen and Horsley, so there is littleinformation about the amount of fishing effortattributable to divers. Boyd �991! providesan excellent historical overview of tonging andbullraking in Narragansett Bay. Over the yearssince the late 1940s, there has been a gradualshift in the use of gear from a predominanceof tongs to a predominance of bullrakes.

The location of tong and bullrake fishingeffort was mapped intermittently between June1955 and August 1960 Campbell and Dalpe1960; Campbell, 1961!. Maps generated bythis project Figure 8! showed that the majorfishing effort was confined to the middle andupper Narragansett Bay, with tong fishermenconfined to the shallower near-shore areas.

By use of a mail-survey questionnaire,Hoimsen and Horsley �981! showedsimilarly that most of the fishing effort wasconfined to the middle and upper NarragansettBay Figure 9!. There is little publishedinformation about the location of fishing effortby shellfish divers.

Relative Gear Efficiencies andEnvironmental Impacts

There has been only one study inNarragansett Bay that compares the reiative

Figure S. Location of tong and bullrake fi shennen inNarragansett Bay recorded between Septeniber 1959 andAugust 1960 Campbell, 1961!.

Figure 9, Percent of quahogging effort in selectedareas of ¹rragansett Bay Hobnsen and Horsley,l981!.

impacts and efficiencies of different gear types Glude and Landers, 1953!. In this studycarried out in 3-acre plots, the efficiency ratiosfor the bullrake and dredge were determined.The bullrakes used were able to most

efficiently retain quahogs greater than 55 rnmvalve length, but some quahogs 35 to 5S inmin valve length were able to be retained. Thedredge was able to most efficiently retainquahogs above 70 mm in valve length. Therewere no significant differences in the physicalor biological composition of raked or dredgedbottoms, but both had fewer living forms thanthe unfished control area. These authorsconcluded that there was no biological basisfor prohibiting either bullraking or dredging.

A recent study on the environmentalimpact of bottom cultivation and removal ofadult quahogs on the set and recruitment ofquahogs was recently concluded Sparsis etal., this volume!. They found that after threemonths there were no significant differencesin the physical, chemical, and biologicalparameters between fished, cultivated, andcontrol plots because of high environmentalvariability.

Conclusions and RecommendationsThere have been a considerable number of

studies since 1946 focusing on the biologyand fishery of quahogs in Narragansett Bay.The information base about the growth ratesof quahogs m Narragansett Bay seems to bequite good. There appears to be a shortage ofinformation about quahog recruitinent. Somestudies appear to indicate that there is a paucityof recruitment into some areas, yet thereappears to be annual recruitment into otherareas, particularly Greenwich Bay and upperNarragansett Bay. A systematic study ofquahog recruitment patterns throughoutNarragansett Bay is warranted. Estimates offishing effort and fishing mortality in areasthroughout Narragansett Bay are lacking, andmuch of the information that is available isnow 10 years out-of-date. It is recommendedthat a socio-economic study similar to that ofHolmsen and Horsley �981! be undertaken in

order to update the fishing effort information.Additionally, given the current mix of harvesttechnologies, a study of the relative harvestefficiencies of tongs, builrakes, and handcollecting by commercial divers isiecommended. In conclusion, there is a greatwealth af information available to fisheriesinanagers about quahogs in Narragansett Bay.It is hoped that this review will provide astarting point for the assessment of where wehave been in terms of the fisheriesmanagement studies, and that it will be usefulin planning management strategies for RhodeIsland's most important fishery resource.

References

Andrews, J.D. �983!. Transport of bivalvelarvae in James River, Virginia. Journal ofShellfish Research 3:29-40.

Boulding, E.G. and T.K. Hay. �984!. Crabresponse to prey density can result indensity-dependent inortality in clams.Canadian Journal of Fisheries and AquaticSciences 41:521-525.

Boyd, J.R. �991!. The Narragansett Bayshellfish industry: a historical perspectiveand an overview of problems of the1990s. Pp. 2-10. In: M.A. Rice, M.Grady and M.L. Schwartz eds.!,Proceedings of the First Rhode IslandShellfisheries Conference. Report No.RIU-W-90-003, Rhode Island Sea Grant,University of Rhode Island, Narragansett.

Caddy, J.F. �989!. Recent developments inresearch and management for wild stocksof bivalves and gastropods. pp. 665-700.In: J.F. Caddy ed.! Marine InvertebrateFisheries: Their Assessment andManagement. Wiley Interscience, NewYork, 752pp.

Campbell, R. no date!. An inventory of thequahog population of the ProvidenceRiver and Mount Hope Bay. RhodeIsland Division of Fish and Game.unnumbered leaflet.

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Game. Leaflet no 1.

Campbell, R. �959b!. Quahog investigations:Potowomut River. Rhode Island Divisionof Fish and Game. Leaflet no. 2.

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Campbell, R, �961!. A summary report onthe fleet plotting program in NarragansettBay. Rhode Island Division of Fish andGame. Leaflet no. 7.

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Holmsen, A.A. and S. Horsley. �981!.Characteristics of the labor force inquahog handraking, Rhode Island SeaGrant Marine Memorandum No. 66,University of Rhode Island, Narragansett.

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Kassner, J and R.E. Malouf, �982!. Anevaluation of "spawner transplants" as amanagement tool in Long Island's hardclam fishery. Journal of Shellj7shResearch 2:165-172.

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Kennish, M.J. �980!. Shell microgrowthanalysis: Mercenaria mercenaria as a typeexample for research on populationdynamics. pp. 255-294. In: D.C. Rhoadsand R.A. Lutz eds.!, Skeletal Growthof Aquatic Organisms. Plenum Press,New York.

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MacKenzie, C.L. �979!. Management forincreasing clam abundance. MarineFisheries Review 41�0!:10-22.

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Murawski, S,A. and F.M. Serchuk. �989!.Mechanized shellfish harvesting and itsmanagement: The offshore clam fishery ofthe eastern United States. pp. 479-506. In:J.F. Caddy ed.! Marine InvertebrateFisheries: Their Assessment and

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Peterson, C.H., P.B. Duncan, H.C.Summerson, and G.W. Safrit Jr. �983!.A mark-recapture test of annual periodicityof internal growth band deposition inshells of hard clains, Mercenariamercenaria. Fishery Bulktin of the USFish and Wi7dlife Service 81:765-779.

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Pratt, S.D., B.K. Martin, and S.B. Saila.�987!. Status of the hard clam, Mercenariamercenaria, in the Providence River andMount Hope Bay. Report no. NBP-88-08,Narragansett Bay Project, U.S.Environmental Protection Agency. 38pp.

Pratt, S.D., A.R. Ganz, and M.A. Rice.�992!. A Species Profile of the Quahog inRhode Island. Report no. RIU- T-92-001,Rhode Island Sea Grant, University of

Rhode Island, Narragansett. 117pp.Rhoads, D.C. and G, Panella. �970!. The

use of inolluscan shell growth patternsin ecology and paleoecology. Lethaia3: 143-161.

Rice, M.A. and J.A. Pechenik. �992!. Areview of the factors influencing thegrowth of the northern quahogMercenaria mercenaria Linnaeus,1758!. Journal of Shellfish Research 11:279-287.

Rice, M.A., C. Hickox, and I. Zehra.�989!. Effects of intensive fishingeffort on the population structure ofquahogs, Mercenaria mercenaria, inNarragansett Bay. Journal of'ShellfishResearch 8:345-354.

Royce, W.F. �984!. Introduction to thePractice of Fishery Science. AcademicPress, New York, 428pp.

Russell, H.J, Jr. �972!. Use of acommercial dredge to estimate hard-shellclam population by stratified randomsampling. Journal of the FisheriesResearch Board of Canada 29:1731-1735.

Saila, S.B. and T.A. Gaucher �966!.Estimation of the sampling distributionand numerical abundance of some

mollusks in a Rhode Island salt pond.Proceedings of the NationalShellfisheries Association 56:73-80.

Saila, S.B., J.M. Flowers, and M.T.Canario. �967!. Factors affecting therelative abundance of Mercenariamercenaria in the Providence River,Rhode Island. Proceedings of theNational Shellfisheri es Association57: 83-89.

Sisson, R.T. �977!, Hard clam resourceassessment study in upper NarragansettBay and the Providence River. RhodeIsland Division of Fish and WildlifeLeaflet no. 49.

Stickney, A.P. and L.D. Stringer. �957!. Astudy of the invertebrate bottom fauna ofGreeiiwich Bay, Rhode Island. Ecology38: 11 1-122.

Stringer, L.D. �959!. The populationabundance and effect of sediinent on thehard clam. In: Hurricane DamageControl, Narragansett Bay and Vicinity,Rhode Island and Massachusetts

Appendix E. A detailed report on fisheryresources. U.S. Fish and WildlifeService, 17pp.

V.S. EPA and R.I. Division of Fish andWildlife. �974!. State of Rhode IslandShellfish Atlas. 6 figs, and key.

Whetstone, J.M and A.G. Eversole �978!.Predation on hard clams, Mercenariamercenaria by inud crabs, Panopeusherbstii. Proceedings of the NationalShellfisheries Association 68:42-48.

Wood, L. and W.J. Hargis Jr. �971!.Transport of bivalve larvae in a tidalestuary. Proceedings of the EuropeanMarine Biology Symposia 4:29-44.

Questions and AnswersQ. {' Mr. John Finneran, shellfi sherman! Inthe Glude and Landers study, were diversused to determine what was left and whatwas broken on the bottom?A. Dr. Michael Rice, URI! No, the Gludeand Landers �953! study used a large grabsampler that brought up intact sediinents andquahogs after the dredging or bullrakingtreatments. Grab sampling can be usedeffectively for sampling because everythingis brought up � juveniles, adults, etc. Onepossible problem with grab mnpling is thatcare must be taken to avoid biases inbreakage estimates, because some breakagewill occur as the sampler hits bottom.Subsamples fram the center of the grabsample are representative of intactsediments. Grab sampling was used duringthe surveys of the 19%s and the Saila et al.�965! study. More recent studies have useddivers for sampling. Diving appears to bebest for sampling juveniles and determiningpopulation structure without gear bias.Indeed, the best way to calibrate samplinggear such as dredges, tongs, and rakes is bydiver sampling,

Q. Mr. George DeBlois, shellfisherman!I'ou mentioned the Kassner and Malouf l982! spawner transplant study in GreatSouth Bay. What was the time frame of thatstudy?A. Rice! Jeff Kassner, one of the peoplewho did that work is in the audience.Perhaps he could answer that for you .A. Mr. Jeffrey Kassner, Brookhaven,New York! What that study did was to lookat the spawning cycle of hard clams in theGreat South Bay that was performed over atwo-year spawning period. The underlyingprincipal behind the spawner transplant wasthat braodstock were brought in from morenorthern, colder waters and had a retardedgametogenic cycle. The idea was to exploitthe retarded cycle to extend the spawningperiod after the native stock had ceasedspawning. What we found is that the naturalspawning variability was so high thatbringing in clains did nat affect therecruitment. Additionally, bringing in 400 to500 bushels did not make much difference

when compared to the natural spawningstocks. Now the idea of spawner transplantsevolved into the idea of spawnersanctuaries. If you then know the likelyhydrographic larval dispersal patterns, yaucan strategicaily place your spawner stackfor settlement in preselected areas.

Q. Mr. Edgar Thompson! Are there anyrecent studies on the sects of pollution onquahogs? An example might be the effectsof heavy metals on quahogs. There are anumber of organizations such as Save TheBay that are committed to cleaning up theBay, and I want to know if there has beensome headway.A. Rice! There are a considerable numberof studies on this. In the first quahogconference we held in 1990, Ms. KatrinaKipp of the Environmental ProtectionAgency and the Narragansett Bay Projectgave a very excellent review of studies ofNarragansett Bay in which quahogs wereanalyzed for heavy metals, various organic

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pollutants, and pesticides. She outlined the riskassessment program in which the health risk topeople eating Narragansett Bay quahogs wascompared to other cornrnon health risks. Ingeneral, Narragansett Bay quahogs carried arather low-risk value to the consuming public.Sheldon Pratt from here at GSO had one studythat looked at quahog populations in theProvidence River, and the adults are alive andwell. One interesting study that came out of theNational Marine Fisheries Service shellfish lab inMilford, Connecticut showed that heavy metalpollutants are much more toxic to bivalve larvaethan they are to adults. It is possible thatshellfisheries in polluted areas might be damagedby reducing recnutment rather thart by theoutright killing of adults.

Q. Johnson! In a nutshell, do you believethat we are moving forward or backward.A. Rice! Well, I think we' re probablymoving forward, Improvements in upperNarragansett Bay water quality due to theirnprovernents by the Narragansett BayCommission to the combined sewer

overflow system are a positive step forward.Shellfish openings albeit not all the time! inthe conditional areas suggest someimprovement. Tlm next thing in line,however, is the nonpoint source pollutionproblem. This is a much more expensiveproblem, and technically a harder nut tocrack.

Q. Mr, David Borden, DEM! You havesummarized where there is knowledge andwhere there is need for more work Can yougive me some sense of priority on whatstudies you think are most important?A. Rice! As it so happens, we have a fullpanel discussion this afternoon, with yourqueslion as our topic So, please stay tuned.

IThis study was funded in part by Rhode Island Sea GrantMarine Advisory Service and Rhode Island CooperativeExtension. This is publication number 2779 of the RhodeIsland Agricultural Experitnent Station, College ofResource Development, University of Rhode Island.

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