Shellfish Committee ICES CM 1995/K:2

REPORT OF THE

STUDY GROUP ON SPATFALL AND RECRUITMENT IN BIVALVE STOCKS

This report is not to be quoted without prior consultation with theGeneral Secretary. The document is areport of an expert groupunder the auspices of the International Council for the Exploration ofthe Sea and does not necessarily represent the views ofthe Council.

International Council for the Exploration ofthe Sea

Conseil International pour l'Exploration de la Mer

Palregade 2-4 DK-1261 Copenhagen K Denmark

-------------

Table of Contents

Section Page

INTRODUCTION 1

2 MEMBERSHIP IN 1995 1

3 PROGRAM AND ACTIVITIES 23.1 Goals of the Study Group in 1996 2

3.1.1 Format and outline ofthe review 23.1.2 Hypothesis 1 23.1.3 Hypothesis 2 23. 1.4 Hypothesis 3 33.1.5 Hypothesis 4 3

4 PRINCIPAL ITEMS IN THE 1994 REPORT 34.1 Country Reports and Research Contributions 3

4.1.1 The NetherIands 34.1.1.1 Stocks, spatfall and recruitment ofmusseIs and cockles in 1994 34.1.1.2 Settlement ofmussels Mytilus edulis in the western Dutch Wadden Sea 3

4.1.2 Denmark 44.1.2.1 Stocks and recruitment ofbivalves in Danish waters 1993/1994 44.1.2.2 Cockles Cerastoderma edule 54.1.2.3 Spisula solida 54.1.2.4 Oysters Ostrea edulis 54.1.2.5 References 5

4.1.3 Spain 54.1.3.1 Introduction 54.1.3.2 Materials and methods 54.1.3.3 Results and discussion 54.1.3.4 References 6

4.1.4 Canada 64.1.5 Germany 6

5 REPORT FOR 199575.1 The Netherlands 7

5.1.1 Musseis Mytilus edulis 75.1.1.1 Cockles Cerastoderma edule 7

5.1.2 Spatfall and recruitment ofmussels in the Dutch Wadden Sea 75.2 United States 85.3 Canada 9

5.3.1 Growth and survival ofgiant scallop spat in collector bags 95.3.2 Intensity ofgiant scallop spat collection as an early index ofnatural recruitment. 95.3.3 Study ofspawning success as a function ofspawner density 95.3.4 Giant scallop spat collection: influence ofvarious structural components ofspat collectors 9

Figures 1-5 10

•

•

1 INTRODUCTION

The Study Group on Spatfall and Recruitment inBivalve Stocks was installed during the StatutoryMeeting of 1993 and was the follow-up of twoworkshops on spatfall and recruitment of musseis andcockles held in Yerseke, The Netherlands, in 1992 and1993. During the Statutory Meeting it was decided that,whereas the scope of the workshops had been restrictedto musseis and cockles (after aperiod of recruitmentfailure in 1990-1993), the scope of the study groupshould encompass all bivalve species, includingparticipation from the United States and Canada.During the Council Meeting in 1994, Dr C.H. Petersonwas appointed co-chairman.

Unfortunately, the copies ofthe 1994 report ofthe StudyGroup were lost before the meeting in St lohns. Thecountry reports of 1994 are now included in this report.

2 MEMBERSHIP IN 1995

Mr R. (Rengcr) Dijkcma (Co-Chairman)Netherlands Institute for Fisheries ResearchP.O. Box 774400 AB YersekeNETHERLANDSE-mail: renger@rivo.agro.nl

Dr C.H. (petc) Petcrson (Co-Chairman)Institute ofMarine SciencesUniversity ofNorth CarolinaP.O. Drawer 809Morehead City, NC 20550USAE-mail: cpetersiftemail.unc.edu

Or N. OankersIBNOLOPostbus 1671790 AD Den BurgTexelNETHERLANDS

Dr M. FrechetteInstitute Maurice-Lamontagne850 Route de la MerCP 1000Mont-lolie, Quebec G5H 3Z4CANADA

Mr S. RobinsonDepartment of Fisheries & OceansBiological StationSt. Andrews N.B. EOG 2XOCANADA

Mrs A. PulfrichInstitut für Meereskunde an der Universität KielDüsternbrooker Weg 2024105 KielGERIvfANY

Dr R. MeixnerBundesforschunganstalt für FischereiInstitut für SeefischereiPalmaille 922767 HamburgGERMANY

Or Th. NeudeckerBundesforschunganstalt für FischereiInstitut für SeefischereiPalmaille 922767 HamburgGERMANY

Mr M. RuthInstitut für I\feereskunde an der Universität KielOüsternbrooker Weg 2024105 KielGERIvfANY

Mr A. VillalbaCentro de Investigaciones MarinasAptdo 20836600 Villagarcia de ArosaPontevedraSPAIN

Or 1. FuentesCentro de Investigaciones MarinasAptdo 20836600 Villagarcia de Arosa PontevedraSPAIN

Or 1. MolaresCentro de Investigaciones MarinasAptdo 20836600 Villngnrcia dc ArosaSPAIN

Dr A. RödströmTjärnö Marine Biological LaboratoryPL27845200 StrömstadSWEDEN

Mr M. SeamanInstitut für Meereskunde an der Universität KielDüsternbrooker Weg 2024105 KielGERMANY

1

Dr P. Sand KristensenDanish Institute for Fisheries and Marine ResearchCharlottenlund Slot2920 CharlottenlundDENMARK

MrC. DugganDepartment ofthe Marine, Fisheries Research CentreAbbotstown, CastlcknockDublin 15IRELAND

Dr E. KensingtonDepartment of Fisheries and OceansP.0.B.550Halifax, N.S. B3J 2S7CANADA

Mr 1. KubeInstitut für OstseeforschungSeestrasse 1518119 WarnemündeGERMANY

DrH. LaiNortheast Fisheries Seience CenterNMFS/MOAAWoods Hole, MA 025·BUSA

Dr S.D. UttingMAFF Fisheries LaboratoryBenarth RoadConway Gwynedd LL328UBUNITED KINGDOM

Dr 1. WeinbergNortheast Fisheries Seience CenterNMFSINOAAWoods Hole, MA 02543USA

3 PROGRAl\I AND ACTIVITIES

In 1994 there were fjve contributions to the Study Groupreport from Canada, Spain, Gennany, the Netherlands,and Denmark, mainly country reports. In 1995, therewere only 3 contributions: from the United States,Canada, and the Netherlands. In order to deeide onfuture work for the Study Group, members consideredthe fo11owing proposal submitted by C.H. Peterson.

3.1 Goals orthe Slud)' Groll" in 1996

Thc Study Group on Spatfa11 and Recruitment inBivah'e Stocks proposes to the ShelICish Commiltee thatit work hard in the next year to produce a formal andcomprehensivc review of what is knmm about thebiological detenninants of recruitment of commereia11y

2

important bivalve stocks. This would involve thccompilation of a review of suffieient scopc andexcellence to be published in the peer-reviewedliterature, e.g., the Annual Reviews of Oceanographyand Afarine Biology.

To achieve this goal, the Study Group would need acommitment of active help by members of cach ICEScountry to assemble, reproduce, and delh'er to the co­chairmen copies of a11 relevant unpublished literatureand management agency reports relevant to this topic.Much valuable information is unavailable in thepublished literature, so the ICES format could bewonderfu11y effective in assembling such information toa110w a comprehensivc review by the Study Group. Thisreview will aid in formulating and modif)ingmanagement strategies on agIobaI seale.

3.1.1 Format amI outline orthe review

A review of thc status of each of several currentlypopular and 10gica11y viable hypotheses that relaterecruitment in bivalves to adult densities is proposed.Each of the hypotheses speeified below plus othersidentified by members of the Study Group would beevaluated for a number of ecologica11y andtaxonomica11y different groups of bivalves: (1) infaunalclams; (2) mussels; (3) sca11ops; and (4) oysters. Thehypotheses to be initia11y considered are as folIows.

3.1.2 IIYJlothcsis 1

On a large scale commensurate with the dispersal oftheir pelagic larvae, recruitment in bivalve mo11uscpopulations can become limited by reduction inabundance of sp~l\\ning stock. Such "recruitmentlimitation" must necessarily occur at some Iow level ofspawner density but the question of whether spa\merstock densities are ever reduced far enough to invokethis effect is not we11 answercd. The classic view held byshe11fisheries man:lgers is that adult she11fish produce somany eggs per capita that there is no relationshipbetwecn spawncr stock and recruitment. Thisassumption is coming under re-examination, and areview of the status of this hypothesis as it applies to thefour sep:lwte groups ofharvestable bivalves isjustified.

3.1.3 1I)"Jlothcsis 2

On a smaller spatial scale of an individual shellfish bed,recruitment is depressed at high adult density byneg:ltive adult larval interactions because adultsuspension-feeding bivalves consume invading larrae.This h)llothesis can be addressed indirectly byexamining the degree of overl:lp of spatial distributionsof scpar:lte year classes within a population or it can beaddressed more directly by evaluating whether larvaeare ingested and killed by ingestion.

•

. ,",

3.1.4 lI)"JlOthesis 3

On a loeal seale, reeruitment of bivalve larvae isenhaneed by the presenee of adults either beeause larvaeare indueed to settle by deteeting the presence of adultsor because adults provide or modify habitats in ways thatpromote postlarval survivaI. This hypothesis seems mostrelevant to oysters, which create the hard substrate uponwhieh larvae can and do settle, but it mayaIso applymore broadly because the presence of adults could serveas a reliable clue that the habitat is suitable formaturation and survival.

3.1.5 1I)"Jlothesis ..

The disturbance of the habitat associated with harvest ofadult bivalves or with harvest of other fisheries speciesdegrades the habitat and depresses recruitment successof the bivalves. This issue applies especially to bivalvesthat inhabit vegetated habitats because such vegetationis so readily harmed by bottom-disturbing harvestpraetices. Neverthelcss, even sedimentary structures canbe altered by harvest disturbance and harvest of adultscan also have direct negative efTects on juvenilebivalves.

To complete a thorough review of these and otherprocesses by which adults may influence recruitment inbivalves, the Study Group requests that the ShellfishCommittee grant permission to request submission ofnecessary literature from each ICES Member Countl')',transmitted by its representative on our Study Group.The chairmen will attempt an initial synthesis, followedby circulation to and review by the full Study Group.

debris, byssus, sand, sillt) was 51%. The mussei fi~~~~industry decided in its 1993 fishing seheme to refrainfrom fishing on the intertidal Oats and in the easternpart of the Wadden Sea. In total, 15000 tonnes of seedwere distributed over the musseI growers. The usualautumn fishel')' for musseI seed was cancelled in view ofthe poor stocks.

Spatfall of musseis, after a long (November-March), butnot very cold winter of 1993/1994, was considered to beprofuse in August 1993, although mortality is stillpossible before an autumn fishery. A stock survey isbcing planned for autumn 1994.

Cockles Cerastoderma edule

The regular survey of the adult stock in the spring of1993 (Dijkema, 1992; 1993) revealed in the Ooster­scheide .1 biomass of 33 000; in the Westerschclde10 000; and in the Wadden Sea 170000 tonnes ofcocklcs (fresh weight). This means that 3 800, 2 400 and17 000 tonnes, respectively, were in harvestable

2densities (> 50/m ). The survey in the spring of 1994gave significantly lower biomass figures: 32 000, 6000,and 92 000 tonnes wet weight total biomass in theOosterschelde, the WesterscheIde, and the Wadden Sea,respectively.

Cockle recruitment in 1992 was poor. The spatfall of1994, however, appears to be good to very good at firstsight (July 1994) in the Wadden Sea, the Oosterschelde,and the WesterscheIde, as weil as along the southernNorth Sea coast.

4.1.1 Thc Netherlands

4.1.1.1 Stocks, sllatfall and reeruitment of musseisand cockles in 1994

•..

4.1

PRINCIPAL ITEI\1S IN TUE 1994REPORT

CountQ' ReJl0r1s antI ResearchContributions

References

Kesteloo-Hendrikse, 1.1. 1993. Het kokkelbestand in deOosterschelde, de WesterscheIde, de Waddenzee ende Voordclta in 1993. Preliminary report. RIVO­DLO.

Kesteloo-Hendrikse, J.J. 1994. Het kokkelbestand in deOosterschelde, de WesterscheIde, de Waddenzee ende Voordelta in 1994. Preliminary report, RIVO­DLO.

l\1ussels Mytilus edulis

In the spring of 1993, an assessment was made of thesize of the stock of musseI seed and half-grown mussclsin the western part of the Wadden Sea. A preliminal')'survey showed that musseI seed beds of any significaneewere present only in this area. Storms in winter1993/1994 had decimated the musseI seed stock, formedby the good 1992 spatfall in the Wadden Sea. Of theentire musseI stock in this area, 95% appeared to be1992 recruits. In total 43000 (gross) tonnes of musseIseed were found in the area, of ",hich about 34 000would be fishable. The average percentage of tare (shell

Van Stralen, M.R., and Schol-Brand, C.M.. 1993. Hetmosselbestand in de Westelijke Waddenzee in hetvoorjaar van 1993. Preliminary report. RIVO-DLO.

4.1.1.2 Settlement of musscls Myti/us edulis in thcwestern Dutch Wadden Sea

by Cees de Vooys, DLO Institute for Forestry and NatureManagement (IBN-DLO)

Settlement of musseIs has been moderate to poor in thelast six years. In the years 1989 and 1990 settlement wasvery poor, it was better in 1991 and 1993, and appearspoor again in 1994. Most winters have been mild in this

3

period. It is weIl knmm that after a severe winter hugemusseI spatfalls occur often. In contrast, after very mild\\1nters with little or no frost, no spatfall at all has beenfound. Very severe or very mild winters, however,seIdom occur. Experience shows that after 'normal'winters spatfall is very variable. The factors that causethis variability are not known and have scarcely bccninvestigated. In the propagation of musseIs thrce stagcscan be distinguished.

Stage 1 begins \\hen gamctogenesis is finished and thegametcs are ripe. Spa\ming should be timed as much aspossible at the same time to guarantee maximalfertilizing of the eggs. After spawning a pcIagic larvastage occurs for one month. The 'trigger' for thespawning could be a temperature shock.

Stage 2. When the larvae are completely deveIopcd(pcdiveligcr stagc), thcy will attach thcmsclvcs onthrcad from substrates, on whieh they metamorphose.From their numbers, an assessment can be made on thesUf\lval ofthe laf\·ae.

Stage 3. After metamorphosis the small musscls detachand become pelagic again floating in the water columnwith a thread. When they find a suitable substrate theywill scttle.

In the westcrn Dutch Wadden Sca obsef\'ations havebccn made on the conccntration of laf\'ae in the watcrand the mctamorphosis of musseI laf\'ae over scvcralycars. Many of thcse sampIes still have to be countcdand analyscd. The spmming peaks of larvae can becorrelated with temperatures of the upper water layer toinvestigate if there exists a 'trigger' effect oftcmpcrature. The number of metamorphosing laf\'ae canbe compared betwecn years, and matched to the numberof laf\'ae found in the same ycar. On settlement and thepcriod between the scttlement and the spat stage hardlyany rescarch has been carried out (spat stage is dcfincdhere as young musseIs Icss than one year old, larger than1 cm occurring in autumn). Howcver, sUf\'ival in thispcriod could be an important factor in determining theamount of spat available for fishermcn and birds. Fromthe time of spawning, the following factors couldnegativcly influence the mass of spat:

(1) fel"eggs;

(2) bad quality of eggs (Le" little reSCf\'e food);

(3) fel" laf\'ae;

(4) too little food for laf\'ae;

(5) heavy predation of laf\'ae;

(6) lol" salinities at the time of settlement;

(7) heavy predation after settlement;

(8) effects of storms.

4

As for (1), the number of eggs will not soon be Iimiting.In 1992 in the west Dutch Waddcn Sea, an extreme lol"parent stock produced enough eggs to give a moderatespatfalI. The condition of the adult musseI willdetermine the quality of the cggs, and the number ofeggs and laf\'ae per musseI. It is desirable to match theoccurrence of the ph)10plankton \\1th that of the laf\'ae,to sec \\ hieh ph)10plankton species are important asfood for the laf\'ae, and its influence on laf\'al sUf\lvaI.Especially at the time of settling, larvae can be directlypredated by adult musseIs or other bivalves. When alarge settlement occurs this probably will not be aproblem, but it can be "hen relatively fel" laf\'ae arepresent with a large adult stock. In addition topredation, other factors mentioned "hich can influencesettlement and the sUf\'ival till the spat stage should bematched to the mortality and the amount of sUf\'ivingspat. Especially important for the western DutchWadden Sea are predation, salinity, storms, andsediment stability.

4.1.2 Dcnmark

4.1.2.1 Stocks amI rccruitmcnt of bh'ah'cs inDanish watcrs 1993/1994

by Per Sand-Kristensen, Danish Institute for Fisheriesand Marine Research, Charlottenlund

Sun'c,Y of Danish musscl stocks

In 1993, a comprehensive sUf\'ey on the musseI Mytilusedulis stocks in Limfjorden and the Danish Waddcn Scawas carried out (Hofman, 1993; Kristensen, 1994a).Stock densities in Limfjord varied between 0 and 416 g

2 , Iper m of total wet welg 1t.

The stock in Limfjord in 1993 consisted of up to 5different year dasses. The dominating year dass in theWadden Sea in 1993 was a ver)' strong 1987 cohort.

Rccruitmcnt

In 1993, the spatfall of musseIs has been poor in mostDanish waters.

The spatfall has only been good in one area in Limfjord.In April 1994, a comprehensive spatfall took place in alimited area in Lovns Bredning of Limfjord (Figures I

2and 2). Thousands of small musseIs per m (shelllength0,25-1.5 cm) had scttled on older musseIs, algae, andthe sand tubes of Korenii sp. (Kristensen, 1994b).

There is no information from 1993 or 1994 on thespatfall in the Little Belt and the other musseI fishingareas (Table 1).

•

Sun'e~' of musscl stocks

Table 1: Danish musseljishing areas and the spat/all in1993/1994.

deI af Nordsoen syd for Homs Rev. DFH-rapport ne.472.21. pp.

Kristensen, P.S. 1994b. Blamuslingebestander iVadehavet og fiskeriets effekt (1991-1993), DFHrapport nr. 476-94. 56 pp.

Kristensen, P.S. 1994c. B1amuslingebestander iLimljorden 1994. DFH rapport (preliminary report).31 pp.

SllatfallMussellishing areaLimljord

I

Generally poor, but cxellent inLO\ns

Little Bell UnknO\\TIWadden Sea PoorIsefiord UnknO\\TI

In May 1994 a comprehensive survey of the mussclstocks in the Kattegat area (ICES area IIIc (22) wasinitiated. The three most important fjords, i.e., Kolding,Vejle and Horsens fjords, will be investigated the firstyear (Figure 1). Other areas will be investigated duringthe follo\\ing years by standard survey techniques (VanVeen sampling (water depth <3 m) and swept areaanalyses (water depth > 3m).

4.1.3 Spain

Intcrannual Variability in Musscl Spat Abundance inthc Ria dc Arousa using Artificial Collcctors

by 1. Molares, J. Fuentes, and A. Villalba, Centro deInvestigaci6nes Marinas, Conselleria de Pesca,Marisqueo c Acuicultura, Xunta dc Galicia, Spain

4.1.2.2 Cocklcs Cerastoderma edule 4.1.3.1 Introduction

•

Rccruitmcnt

In 1993 the recruitment of cockles in the DanishWadden Sea was very poor. The musseI survey inLimfjord in April 1994 revca1ed large cockle stocks inthe westemmost parts of the fjord "hieh may becommercially exploitable, and which had settled in1991.

4.1.2.3 Spisula solida

Fishcry

In 1993 and 1994 around 1000 tonnes of Spisula solidahave been landed annually from the North Sea(Kristensen, 1994a).

Rccruitmcnt

There is no information on spatfall of Spisula solida in1993 or 1994.

4.1.2.4 O,)"sters Ostrea edulis

In 1993 around 100000 pieces of European flat oysters(0. edulis) were landed from Limfjord. The newcomprehensive stock was established around 1990.There is, at this moment, no knowledge about stock sizeor distribution of individuals in the stock.

Mussel cultivation in Galicia relies on the availability ofwild seed suitable for gro\\1h in rafts placed in protectedareas inside the Rias. Traditionally, two differentprocedures have been used by the musseI farmers toobtain seed for growing: collection ofyoung musseIs (1­2 cm long) from the intertidal exposed rocky shores andcollection of musseis directly settled on ropes hung fromthe culture rafts. In order to know the spatial andtemporal \'ariability of recruitment of this species in theRia de Arousa, a monitoring program of mussei spatabundancc on rock]' shores and collector ropes wasstarted in 1990. Data from 1990, 1991, and 1992 arereported here.

4.1.3.2 Materials and methods

Collectors used to monitor thc recruitment on ropes andtheir location in the Ria de Arousa are described inFuentes and Molares (1994). Collectors uscd to monitorthe rccruitment on rocky shore are pieces of jutematerial (18x12 cm in size) mounted on PVC plates bymeans of plastic frames. These collectors were attachedto the substratum by stainless steel bolts at 1.5 m abo\'emean lower low water (MLLW) in an exposed rockyshore in the middle ofthe Ria de Arousa (Figure 3). Onemonth after placing, collectors were removed and newones were put in place. The treatment of the sampIes isdescribed in Fuentes and Molares (1994).

4.1.2.5 Rcfcrcnccs4.1.3.3 ResuIts and discussion

Hofmann, E. 1993. B1amuslingebestanden i Limfjorden1993. DFH rapport nr. 465a. 78 pp.

Kristensen, P.S. 1994a. Spisula solidarrykskallettrugmusling. Fordkomst og udbredelse iden Dansks

Recruitment of Afytilus galloprovincialis in the Ria deArousa was restrieted to the March-September period;in this period, there was high intcrannual and spatialvariability. Recruitment of mussei was more abundant inrock)' share than in culture rans (Figure 4), and the ranin thc inner zone obtained the lowcst recruitment in the

5

Ria. Similar results were found by Caceres-Martinez el

al. (1993) in the Ria de Vigo.

Recommendations relatcd to the management of musseispat collection in the Ria de Arousa are as folIows:(I) rope collectors should be hung from the rafts inApril in order to make full use of the main peak ofrecruitment; (2) collectors should be placed in the outerand middle parts of the Ria; (3) the farmers shouldremove the mussei seed from intertidal beds fromJanual)' to March because in this period young musseishave suitable size to be tied onto ropes, and thus rockswill remain clean to accept new mussei settlement.

4.1.3.4 Rcfcrcnccs

Caceres-Martinez, J., Robledo, J.A.F., and Figueras, A.1993. Influencia de factores ambientales en lafijaci6n del mejill6n Mytilus galloprovincialis deGalicia. Actas IV Congreso Nac. Acuieult.: 377-382.

Fuentes, J., and Molares, J. 1994. Settlement of themussel Mytilus galloprovincialis on collectorssuspended from rafts in the Ria de Arousa (NW ofSpain): annual pattern and spatial variabilit)'.Aquaculture, 122: 55-62.

4.1.4 Canada

by Marcel Frcchette, Institut Mauriee-Lamontagne,Mont-Joli, Quebec

His field of research is maini)' in biomass-depcndence ofbivalves in general, particularly focusing on the bluemussel.\fytilus edulis and the giant scallop Placopecten11Iagellanicus. He is presently carl)'ing out a study on thecritieal spawner densit)' for successful fcrtilization innatural populations of the giant scallop. This workinvolves coupling a biological model of fcrtilization witha physieal model of the dispersal of gametes of bothsexes.

4.1.5 Gcrmany

by Maarten Ruth, Institut rur Meereskunde, UniversitätKiel

He reported on mussei recruitment in the Schleswicg­Holstein area of the Wadden Sea. There seems to be adifference between subtidal and intertidal recruitmcnt.Recruitment seems to show a distinct pattern in bothregions. The last more "global" recruitment occurred in1987. In the following years, the recruitment in theintertidal region was restricted to single tidal streamsystems. Within these systems, some beds showed"good" recruitment, resulting in expansion of the areacovered with musscIs. Settlement took place in thevicinity of existing beds, mainly in dense stocks ofZostera no/tii. From 1990 onwards, he observed bedsshowing this behaviour in different tidal stream systems.

6

On the other hand, there was no dear spatial or year byyear difference in numbers of recruits per biomass ofadults in the core zones of existing beds (areas coveredwith adults). From this point of "iew, the intertidalrecruitment could be c1assified as folIows:

1987: good1988: medium,1989: poor (hardly any results could be

found)1990: medium1991: medium1992: medium1993: medium

As the recruitment success in the intertidal region cannot be estimated before autumn 1994, there is noinformation on the influence ofthe moderate cold winter1993/1994 on the recruitment success in 1994 up to thepresent.

There are no existing long-term subtidal bcds in theSchieswieg-Hoistein region. Recruitment takes place atpredictable sites, but it is not predictable whether acertain site in a certain year is settled by spat or not.Some sites have a higher potential than others.Recruitment can take place within some wecks, as in1990, or "ithin several months, aS in 1993/1994. Forthis reason, the temporal occurrence of subtidalrecruitment could be defined as the date when fisheryfor spat started. No fishery for subtidal small spat meanspoor subtidal recruitment, fishel)' for only some daysmeans medium recruitment, and sumcient spat supplymeans good recruitment in the subtidal area. Followingthese categories, subtidal recruitment showed thefollowing temporal pattern:

1987: good1988: medium1989: poor1990: good (with one additional star!)1991: medium1992: poor1993: medium1994: good

One interesting observation can be added: The mostimportant spatfall area, called "Vortrapp Tief', showedsettlement of larvae from spring 1993 onwards, but noincrease in medium length of the O-group. This processcontinued untH May 1994, even in winter. From April1994 onwards, the cohort showed sudden rapid grO\\1h.The 1994 settlers reached the length of the 1993 settlerswithin some wecks. At the moment, there are about 150ha covered with 15 mm medium-length mussels, and thefishel)' will be able to stock most of the culture lots. Incontrast to 1990, the "Vortrapp Tief' seems to be theonl)' area with signifieant recruitment sueeess.

•

5 REPORT FOR 1995

•

•

5.1 Thc Ncthcrlalllis

by Renger Dijkema, Netherlands Institute for FisheriesResearch (RIVO-DLO)

5.1.1 l\Iusscls Mytilus edulis

Autumn Sun'c)' 1994

A mild winter 1993/1994 yie1ded a rieh spatfall ofmusseIs in the Waddcn Sca and, for the first time inmany years, spatfall also oeeurred in the Oosterschelde(SW Netherlands). A qualitative estimate of the stockwas made in September 1994. An aerial survey wasmade to localize seedbeds, and a number of beds andareas found not to eontain musseIs from thc air wassurveyed on foot in order to ealibrate these observations.All seedbanks appeared to be dearly visible from the air,and in areas whieh appeared "empty" from the air, nomusscls were found afterwards. In shallow areas withdear water, musseI banks eould be loealized on sightfrom small boats. In addition to this survey, fishing tripswere made with musseI dredging ships (towed dredge),whieh is the traditional method used by the musseIindustI)' to estimate the amounts of wild musseI seedprior to the seed fisheI)'. Dense banks were found in theintertidal area ofT the islands of Tersehelling andAmeland, gcnerally oeeurring in dosed mats with

2densities over 5 kg seed1m .

The "guesstimated" amounts of musseI seed, originatingfrom spatfall 1994, were (gross weight, about 40% oftare): 100000-200000 metric tonnes of seed in thesubtidal area, and 100000-150000 metric tonnes in theintertidal area. A fishing quota of 30 000 metric tonnesfor the Wadden Sea was fixed by the Dutch government.The stock of seed in the Oosterschelde (Vuilbaard area)was assessed using a Van Veen grab sampling and atowed sampling cage dredge. The estimated freshbiomass was 4200 metric tonnes, the seed densities ,,,ere

2up to 1-2 kg/m .

SIJring Sun'c)' 1995

In the subtidal areas an assessment was made wüh aspecially adapted hydraulic cockle dredge, using anarrowed cutting blade. Transeets were made throughareas with musseI seed, and the amount of seed caughtwas quantified. SubsampIes )ielded information on size,age, and specics eomposition of the cateh. The stocks inthe intertidal areas were assessed on foot. The peripheT)'of the banks was mapped using hand-held GPSequipment (Figure 5). In transects across the beds, coresampies were taken, making quantitative estimates ofbiomass, size distribution,and species compositionpossible.

Winter storms in winter 1994/1995 appeared to havedecimated thc musseI seed stocks in thc veI)' exposedand locally shallow Wadden sea, both in the intertidaland in thc subtidal areas. This could be ascribed to waveaction in the tidal area and current seouring in the tidalchannels. The subtidal stock was found to be 26 700metric tonnes net fresh weight, of whieh 23 400 tanneswere musseI seed, the remainder were half-gro\\ns. Inthe intertidal area, a stock of 4500 metric tonnes of seedand 400 tonnes of half-grown musseIs had sunived thewinter storms.

In the Oosterschelde, the area of "Vuilbaard" stillappeared to contain a fair stock of musseI secd whiehoriginated from previous spatfalI. This area has asheltered lie and in general the only cause of mortalityare starfishes Asterias rubens. The estimated musseiseed stock was 1800 metric tannes net fresh wcight.

5.1.1.1 Cocklcs Cerastoderma edule

Wild stocks of cockles, musscls Afytilus edulis and theBaItic tellin ,\facoma batlhica were assessed in theintertidal area of the Wadden Sea, the Oosterschelde,and the WesterscheIde, using the hand-held scoopingdeviee, described in earlier reports. Spatfall 1994appeared to have been good in the WesterscheIde,moderate to poor in the Oosterschelde, and good againin the Wadden Sea. As yet, no exact data on theamounts of reeruits and older cockles are available. Thetotal biomass of cockles was (proYisional data)approximately 30 000, 6000, and 60 000 metric tonnesfresh weight, respectively. Amounts ofAfacoma halthicawere 100, 1300, and 32 000 metric tonnes fresh weightin the Oosterschclde, the WesterscheIde (WesternScheldt), and the Wadden Sea, respectively.

5.1.2 S(Jatfall and rccruitmcnt of musscls in theDutch Waddcn Sca

by C. de Vooys, Institute for Forestry and NatureManagement (IBN-DLO), Texel

Settlement of musscls has been moderate or paar in theWadden Sea since 1988. It is well kno\m that often aftera severe winter (e.g., 1978/1979 and 1986/1987) hugemusscl spatfall occur. In contrast, after veI)' mildwinters with little or no frost, no spatfa11 at a11 wasfound. VeT)' severe or veI)' mild winters, however,scldom occur. Experience shows that in 'normal'winters spatfa11 is veI)' variable. The factars that causethis variability are not knmm and have scarcely beeninvestigated to date.

In the western Dutch Wadden Sea observations weremade on the eonecntration of lan'ae in the watcr and themctamorphosis of musseI larvae ovcr several years. Thcmajority of lan'ae and spat result from euItivatedmusscIs, whieh are rcared on plots along ehanncls at a

7

depth of3-10 m. Larvae are counted in sampies of 100 Iwater filtered through plankton gauze. The sampieswere taken from thejetty ofthe Netherlands Institute forSea Research in the Marsdiep, which has a depth of 10­20 m and connects the main basin of the western DutchWadden Sea with the North Sea. Metamorphosing, just­settlcd larvae were collected from pettieoat gauze thatwas hung from the jetty stretched on a frame. Both freelarvae and metamorphosing la1"\'ae were sampled twotimes per weck.

Spa\ming of musseIs generally is a mass phenomenon.The spawning could be triggered by a significant rise inwater temperature in a short time. Vugts andZimmerman (1975) demonstrated a relationshipbetween the tidal cycle and a rise in the temperature ofthe daily water-temperature cycle. At high tide the tidalflats are flooded, whieh means an increase in watersurface area by 30% for the western Duteh Wadden Sea.This causes a periodieal change in the surface area,through whieh solar radiation penetrates and heat isabsorbed. When high tide occurs at noon, the situationfor a warming up of the water is maximal, when hightide occurs at midnight, it is minimal. This mechanismcould provide a 'temperature shock' that could triggermass spawning of musseis. This hypothesis was tcstedby comparing the times of spa\ming over five years,measured as la1"\'ae peaks at the jetty, with the nearesttime when high tide occurs at noon. The differencesbetween these times, in six out of eight cases, show asignificant relationship.

When the times of occurrence of peaks in abundance ofla1"\'ae are compared over five years, a distinct patternappears. In three years, only one la1"\'al peak oceurs:twice in May and once in June. In the two other years,two peaks occur each year. In one year, peaks occur inApril and May, the other year in May and June. In fouryears, not only have peaks in abundance of la1"\'ae in thewater been obtained, but peaks in abundance ofmetamorphosing la1"\'ae on gauze as well.

When the position of the latter peaks over four years arecompared, at least one peak in June appears every year.In two out of the four years, two peaks ofmetamorphosing musscls are found in June, and in oneyear, one is also found in July. This pattern resemblesthe one found for la1"\'ae in the water. When thedifference in time between the peaks in abundance ofla1"\'ae in the water and those in metamorphosing la1"\'aeon gauze are compared, a difference of 24-37 days isfound, with a mean of 31 days. This is in agreement"ith the time of the pclagie la1"\'al period as reported inthc literaturc.

A preliminary eomparison between the peak hcight ofpelagic and metamorphosing la1"\'ac shows that thc ratiocan be very different in various years,

8

The investigations, of "hieh the first results are givenhere, have not been finished yet. To obtain a moredefinitive interpretation, the final results will becorrelated with meteorologieal data and the mass of spatobtained from estimates of fishelY research. In 1995investigations have started on newly formed intertidalmussei beds. After a good spatfall in 1994 thedevelopment of the juvenile musscls \,ill be followed.Emphasis will be laid on the factors which areresponsible for the sU1"\'ival of spat>1 cm. There is stillinsufficient knowlcdgc of the factors responsiblc for thesU1"\'ival of spat between settlement and the first winter.We hope to propose a project to the EU-FAIRprogramme in December 1995. Interested parties arerequested to contact us at the IBN. Telephone: +31­2220-69700. Telefax: +31-2220-19235,

5.2 United Statcs

by C.H. (Pete) Peterson, Institute of Marine Sciences,University ofNorth Carolina

There has been intense recent interest and researchactivity in the USA in several generie issues relative tothe recruitment of bivalve molluscs. Emphasis hascentered on: (1) the question of whether recruitmentlimitation plays any role in affeeting the population sizeand, thus, management of bivalve mollusc stocks;(2) the importance of maintaining and restoring habitatqualit)' for promoting recruitment of oysters; (3) theimplications of bottom-disturbing fishing practices onrecruitment of bivalve molluscs; and (4) applieation ofaquaculture technology to enhance recruitment ofdepressed stocks of commercially important bivalves.

Evaluation of the question of recruitment limitation as acontrol on adult population size has been an active areaof research interest among scientists studying reef fishpopulations in partieular but also among thoseinvestigating bivalve molluscs. Work b)' Summersonand Peterson on bay scallops has provided someevidence that for spccies with Iimited transport of la1"\'aeand distinct subpopulations onl)' weakly connected byla1"\'al emigration, such as some estuarine stocksincluding bay scallops, there is reason to recognize thesesubdivisions in management. Specificall)', managementunits should be defined by the reproductivesubpopulations. This recognition leads to the adoption ofspawner transplantation into isolated areas in whiehstocks have been grossly depressed as a means ofresource restoration. This transplantation of adult bayscallops seems responsible for restoration of ba)' sealloppopulations in a coastal lagoon in North Carolina andmay weIl perll1it restoration of this species along muchof the cast coast of the USA.

Therc is also intensc elTort in thc USA to evaluate the .role of maintaining oyster recf habitat as a means ofsustaining reeruitment, gro\\1h, and produetion ofoysters, The oyster creates a biogenie reef habitats,

•

•

•

•

which is sensitive to damage during harvest but whieh isnecessary for successful recruitment of subsequentgenerations of oysters. Much lcgislation exists to protectvegetated aquatic habitats such as seagrass andsaltmarsh, but oyster reefs enjo)' no such proteciion.Research efTorts in the Chesapeake Ba)' and in NorthCarolina's Pamlico Sound are activcly evaluating therole of reef habitat conditions in afTecting oysterrecruitment success. These studies are being conductedby Beral, Rothschild, Paynter, and Newell in theChesapeake Bay and by Peterson and Lenihan in thePamlico Sound.

An international concern about the impacts of bottom­disturbing fishing practices is evident also in the USA.One of the concerns about harvest practices involveswhether bottom habitat alteration reduces the quality ofthe habitat for settlement and sunival of lan'al shellfish.Various research programs nationwide are evaluatingthis question and assessing whether some alternativegears and methods might bettel' presen'c bottom habitatqualit)'.

The trcmendous and widespread success of artificialpropagation of many species of bivalve molluscsworldwide has now lead to the question of whether thistechnology can be applied to restoration andmanagement ofwild stocks ofbivalves. This approach ofselectivc breeding of disease-resistant strains of someshellfish stocks, cspecially oysters, is bcing activclyevaluated in the Chesapeake Bay region and in the mid­Atlantic states. Furthermore, the question of whetheraquaculture of native species results in a benefit ofproviding and releasing larvae to help populate publicbottom is being tested in some regions. On the otherhand, there is also concern over and research on thequestion of whether non-native stocks of aquaculturedbivalves can truly be prevented from spawning andtherefore prevented from invading the wild ecosystem.Finally, questions of the bioeconomic viability ofreseeding depleted shellfish beds with nursery raisedseed are being further investigated.

5.3 Canada

by Marcel Frechette, Institute Maurice.Lamontagne,Mont-Joli, Quebec

5.3.1 Growth and sun'ival of giant scallop sllat incollcctor ba~s

An expensivc step in scallop aquaculture and restockingoperations is intermediate culture in pearl nets. In anattempt to obtain scallops of suitable size for bottomsowing without using pearl nets, the fishermen in Iles­de-la-Madeleine, Quebec, have left naturally collectedspat to grow in collector bags for a "hole year, The goalof this study was to asscss whcthcr gro\\1h and survi\'alof spat in "intermediate culture" collectors are

-----------------------

competition-dependent, and if so, at whieh density levelthis would occur.SillCe initial density in bags cannot be controlledcxperimentally, competition is infcrred from therclationship between biomass and density in the bags.Without competition, this relationship is linear, withpositive slope. When competition is present, thebiomass-density relationship is cun'ed dO\\Uward,toward zero slope, 01' the relationship is negative. Thelatter case is interpreted as reflecting self-thinning.Preliminary analysis suggests that after one year in thecollector bags, spat grow1h and sun'ival are competition.independent for population densities lower than about500 individuals per bag.

5.3.2 Intensit~· of ~iant seallol1 spat collection asan early in()ex of natural recruitment

Intensity of scallop spatfall on artificial spat collectorshas been monitored sincc 1986 in Iles-de-la-Madeleine,Quebec, along with adult stock abundance, in order toassess whether spatfall would be a reliable early index offuture recruitment. In addition, drifting buoys andvertieal plankton net tows have been used to study lan'aldrift. Preliminary results suggest that the intensity ofspatfall is elosely linked to variations in wildbroodstock.

5.3.3 Stu()~' of spawning success as a function ofsllawner ()cnsit~,

A study on the critical spawner density for successfulfertilization of gametes in natural populations of thegiant scallop is undcr way. This work invol\'es couplinga biologieal model of fertilization with a physical modelof the dispersal of gametcs of both sexes.

5.3.4 Giant scallop spat collcction: innuence of,'arious structural components of spatcollcctors

The influence of various structural components of spatcollectors was studied in field experiments. Giantscallop spat were more abundant in collectors made withintermediate mesh size bags (0.4 X 0.5 cm) containinglow densitics of monofilament than in other types ofcollectors. Abundance did not vary significantly withspatial arrangement, diameter, 01' heterogeneity ofmonofilament.

9

Figure 1. The Danish mussel fishing areas. Limfjord mm ;Danish Wadden Sea ~ ; Little Belt (leES areaIIIc (22» ~ ; Iscfjord ~ .

10

•

•

LOVNS BREDNINGApril 1994

01i Lovns Br . 19HI Qpr.II;

SO

40

12:l 30on-Z-<-u 20Q"

• 10

0

7~ ,t~ lf.$ n.s Z7.$ 1U 37.S <U ".$ su S7.s &U &7.s n.s ".s a.s ".s ft.S .,~ 'ot.J

Length (mm)

:-50 I

• 40 ,.2:l 30on-U~-u 20Q"

Lovp.s Br. Qpr i99!

II'

I,,!

10

Length (mm)

Figure 2. The relative number and weight of musseIs in Lovns Bredning in Limfjord in April 1993 (Kristensen,1994c).

11

ßexposed 0 .0

R·lA· CE AROUSA

outer•

N

t51an

•

•

Figure 3. Map ofthe Ria de Arousa (NW Spain), showing the sites ofthe study.

12

....,

•

rall colleclors

inner zone

600

....G) 200 ..aE:lZ 100·

'"Gi 400·

'"'":lE 300·-o

o __....L. _

JfMAMJJA80NDJfMAMJJA80NDJfMAMJJA80ND80 81 82

inlertidal collectors

exposed rocky shore

600

....G) 200.aE:lZ 100·

'"Gi 400·

'"'":lE 300-o

ol.-__-.l_-L__.-.:::::...- .Jo, ...-.__-""'_

JfMAMJJA80HDJFMAMJJA80NDJfMAMJJA80ND80 81 82

600raU colleclors

600 .raft collectors

/-r~....

outer zone

~o ._.. __"1~.__~__--",-_JfMAMJJASONDJfMAMJJASONDJfMAMJJA80HD

80 81 82

'"Gi 400 .

'"'":lE 300·-o....G) 200 ..aE:lZ 100

middle zone

o .~.JfMAMJJASONDJfMAMJJASONDJfMAMJJASOND

80 91 82

...~ 200

E:lZ 100·

'"Gi 400·

'"'":lE 300·-o

­w Figure 4. Mean number ofmussels per 4 cm2 in collectors placed on rocky shore and on ropes suspended from rafts atthree sites in the Ria de Arosa.

..

•

•

..

••

DICHTHElD

o - 0.1 kglm'"2

g88n moss....,

•

0··

•o 00

•• 0• •••• ;,;p..,~.0-

:.~;..::..:ö~o •o • '00••00 • '

~. •• • •• -0-:. .....

• ••• Ca... . ..• 0 •• ·••o•

•

•

•.0

·.· .• •· .• ·0• •

••

•• •

0 0.1 • 0.5 kglm"2

0 0.5 • 1kg1m"2

0 1 • 2 kglm"2

0 2 - 5 kgI",..2

e meer dan 5 kgtm"2

!\ 8 ~ 00 Q

~~ - 10 0ID .- .. N 1lI

oft ... Nvi

20 30 4Q

o••-0....

·0· ,•

•

0.'0 0 :~.o 0 0

• ·0.00..r"\ • ·0 ••-:...J' •• o.~.o~.~~ -. :qQ.~.

~ • 8· 9 8 •• ö~:~.~ 0·. ':0.•••• •••••• --0.

••• 0-0-

~·Cll:Ioo_ •••

o • • .~~:~.•••. ..

••••....o

•CI· ••. ........,.'1'

53;15.00

0

80

8 0

ci oft 010

~Q... ... 10

-100 10

40

10

20

30

so

70

60

80

90

100 T--------------=;;~::_:r_:r)----_;.:=::::===_-------_bestandsopname I110SSeIzaad 1995

Figure 5. Distribution ofthe density ofthe mussel stock (seed + adults) in the western part ofthe Dutch Wadden Sea,spring survey by RIVO-DLO in 1995.

14