This paper not to be cited without prior reference to the author

International Council forthe Exploration of the Sea

C.M. 1984/F:36Mariculture CommitteeRef.: Shellfish Committee

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OVERWINTERING OF OYSTERS (CRASSOSTREA GIGAS THUNBERG) IN GERMANY===========================================~========== ==========

byTh. Neudecker

Bundesforschungsanstalt für FischereiInstitut für Küsten- und Binnenfischerei

Laboratorium Langballigau

ABSTRACT

The overwintering of oysters has been the most hazardous obstacle in thedevelopment of a new German oyster culture. Trials have shown that mortalitiesof various oyster-groups overwintered at different sites, in different systemsand ecological different conditions ranged from 100 to 0 %. The resultsindicatethat winter-mortalities are avoidable,if overwintering occurs insystems that provide following conditions:

water temperature ranging from -0,5 to 20 C, no ice, no organic or inorganicparticles in the water. These conditions may be achieved in an upwellingsystem sheltered ashore with recirculating sea water which should be exchangedweekly. Stocking densities may be as high as 200 kg of oyster spat (10 g)per m3 of the system-volume.

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1. INTRODUCTION

The over~interingof oysters has always been a problem. Already Moebius (1877)noteda so called"frost-disease" facing an intensive mortality in local oysterpopulations (Ostrea,edulis, L.) during severe winters.'He realized low tempe­

ratures t reduced salini,ty and suspended particles as causes for these losses.Another reason was the mechanical action of driFting ice piling up in theshallow waters of the Wadden Sea.

All this is still valid as it was a hund red years ago and similar experienceshave been made in recent years with thePacific oyster (Crassostrea~ Thun.):

If overwintered in the sea t losses of oysters and equipment were not avoidable twhen low temperatures caused formation of ice. Mortalities were noted in

4It particular in the Baltic Sea in springt when rising water temperatures approachedthe SOCihreshold following ice-winters. This was the case in 1978/79 (almost100% mortalities) and 1981/82 (approx. 50% mortalities) though the oystersseemed to be healthy and had a high meat content.

Mild winters without ice resulted in comparatively low mortalities. This was

also the case when small samples of oysters were kept in tanks of various sizesby privatepeople along the North Sea coast as well as in the Langballigau laboratory.

As already Meixner (1974) had made the same experience t additional trials became

desirable to develop appropriate methods of overwintering oysters safely and todetect suitable sites on the German coast for this purpose. The first series of

trials were conducted in 1982/83.

• 2. MATERIAL AND METHODS

The overwintering-sites chosen are listed in table 1. They were:At the Baltic Sea: the Langballigau laboratorYt giving overwintering facilities inthe summer grow~out system in the Flensburg Fjord as well as in an especiallyprepared upwelling system sheltered ashore.

At the North 'Sea: some sea-water storage ponds belonging to a mussel-processingplant at Emmelsbüll which were supplied by newly pumped sea-water every other

day; a privately owned growing site and the harbour on the, island of Amrum;

one eel-tank which was supplied by heated effluent-water of the power-plantat Wilhelmshaven und finally the harbour and a small upwelling-system with a

direct sea-water line near the west tip of the Island of Wangerooge.

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Oysters (Crassostrea gigas Tllun.) of different sizes starting from 0.08 9 spatto half-grown ones of 7.1 9 mean weight were cOllnted and weighed from 15 to 17November 1982. In addition one mesh-frame (7.5 cm square-mesh) was used for the

re-attachment of oysters. Their individual weight had been recorded and they wereprovided for Wilhelmshaven.

Besides the selected groups the total amount of oysters cultured were splitup as controls and placed at the sites of Langbqlligau (Baltic) and Emmelsbüll(North Sea). They were recorded by weight only.

A schematic summary of all sites, overwintering systems and weight-groups isgiven in table 1, which may be used as a reference table. It contains therunning trial numbers of table 2 giving all corresponding data.

The four smaller groups of oysters were from Scottish origin and imported aslarvae in July 1982, while the remaining two weight-groups were oysters thathad been reproduced in Germany itself in 1981.

•As the size of the oysters was different, different systems for overwinteringwere necessary as well. The small oysters were placed in stocks of wooden andplastic trays described by Neudecker (1981). The mesh size was 2 mm for thesmall oysters, while the larger ones were kept cn a 4 mm-lining. The 7.1 g-oysterrequired no more special mesh-lining in the plastic trays, which were stocked in5 and brought to the overwintering-sites.

In the Baltic Sea near Langballigau the stocks of oyster-trays were kept incontainers similar to those described by Meixner (1976) in approximately 4 mdepth. Another group of oysters was located in an upwelling system that ~

continually received small quantities of recently pumped sea water and main-

tained ambient sea water temperature. Fig. 1 shows this set-up which wassheltered inside the station building.

Stocks of oyster-trays were also used in the sea water storage tanks at Emmels­büll (fig.1), in the harbour of Amrum und Wangerooge as well as in the eel-tanksof Wilhelmshaven. Identical systems of wooden trays were used in the upwelling

systems. While the upwelling system in Wilhelmshaven, was placed inside an eel­

tank, the one on Wangerooge was standing outside against the wall of a building.

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Japanese lantern nets (9 mm, 5-layer) were another system that was used thevery frist time for overwintering oysters on long-lines in the Baltic.

For the purpose of recording individual growth rates, which.were exspected due. . . 0

to the high water temperatures in the heated effluent water (approx. 7 C aboveambient water temperature), a reattachment-frame similar to the system describedby Neudeckei (1982) .was used in the eel-tank of Wilhelmshaven as well.

While the oysters in the governmental station of Langballigau were subjectto almost daily control, those at the other sites were nearly left by them-

I

selves for the whole experimental period.

The experimental period went from mid November until the end of April orMai respectively, because the most critical time at the beginning of springactivity hadto be included. The end of each trial is noted in the'last columnof table 2.

3. RESULTS

The winter 1982/83 was considered as a normalone without extreme frostyperiods. Therefore no harshy test-conditions occured which had been hoped foreNevertheless table 2 shows considerable differences in the overwintering resultsof the various sites and systems. They become most obvious if one looks atthe percentage loss and the percentage growth increment of the oysters whichare listed up in table3 and table 4 respectively for better comparison.

Both extremes, 0 and 100 %, were observed. 0 %was recorded in the upwellingsystem at Langballigau e.g. and a 100 %loss was suffered in the same systemat Wilhelmshaven~ The latter one has doubtlessly to be attributed to an extremlyhigh silt load of the effluent water of the power plant, which caused an siltingup of the oyster trays and the whole upwelling system within few days. Theopposite conditions existed in the Langballigau upwelling system.

Low mortalities were also observed in the stocks of trays placed in the eel tank,where no siltation occured due to the activity of the eels, which loved tohide themselves inside the oyster trays. Therefore this system ranked second.

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As good as the trays in the eel-tank were alse those trays placed in the sea waterstorage tank at Emmelsbüll~ As shown in table 2, line 23, countings of oystershad not been done at the beginning of the experiment. Therefore only the rate of

dead shells to live animals can be used for comparison.

Possible losses by slipping through the mesh-walls of the trays may be considered

low, as the mean weight of the oysters did not alter very much. At this sitesiltation was a problem as well, because silt was continuously resuspended bythe pumping of new sea water. Only few oysters were unable to maintain somechannels for filtering water under these conditions and died off. If cleaningfrom silt would have been possible, losses would probably have been lower. Thesame is valid for the trial numbers 18 and 20 b in table 2 while those traysbeneath (trial numbers 19, 20a, 21 and 22 d) had to endure less siltation.Smal.

oysters (number 18 in table 2) were obviously more endagered.

This was also the case in Amrum. This site is ranking fith having a mean per­centage loss of 11.61 %. Table 3 shows clearly the incrase of percentage lossfrom larger to small oysters. The main reason for losses seems to have beenthe siltation as well at this site.

The next following rank (6) in table 3 is for the lantern nets in the WesternBaltic. High losses in the oyster group of 1.41 and 2.13 9 mean weight werenot caused by biological reasons but by the inadequacy of the mesh-size of9 mm for these size-groups of oysters. Many oysters of the smaller group were

found in the other one, which gave reason to pool the data of both groups. On

the other hand quite a number were lost in the sea which is being proved by

the relatively low rate of dead shells (2 %) and the comparatively low mortali~esof 0.5 % and 3.0 % in the 4.99 and 7.10 g-groups respectively.

The results of the oysters from Wangerooge are ranking seventh. The upwelling­system provided could not be used for technical and personal reasons, as thesea-waterline was not working. Therefore all oyster groups were held in theharbour of Wangerooge. Details are not known from this site, but it may beconcluded from experience that siltation as well as wave beat or transport

caused death or loss in the sea. Proof for the latter assumption gives thedifference of values for percentage loss (c..n%) and percentage dead shells

(LS %) in table 2, line 42 to 46.

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The high mortalities of oystcrs reattached on the frame which was floating inthe eel tank are not traceable. In spite of the high silt load of the water,

siltation was technically impossible on this frame. It may be possible that thesilt load itself was lethal for small oysters. But a negative effect of the eelscannot be excludedas well in thiS particular case. Only 11 out o'f 156 oysterswere fallen off, 7 of them in the 0.36 g-group, which was due to carelessnessin handling at an intermediate cantral.

The poorest results besides the 100% loss in the upwelling system at Wilhelms­haven were recorded for the overwintering of oysters in containers in theBaltic Sea at Langballigau. As the containers were standing on the sea bed.starfish had access to the trays and caused considerable martalities. whichmay be proved by the higher proportion cf dead shells of oysters within the2. and 3. weight-group (line 2 and 3 in'table 2) which nearly amounts to the

total,loss observed in these oyster groups.

Summarizing the results in terms of losses or mortalities. it may be statedthat the oysters are very much endangered in the natural environment. Theyare subject to predation by starfish and siltation. In off-bottom overwintering.in systems that give little chance to siltation as the Japanese lantern nets e.g .•the situation is very much improved especially for small and susceptible oysters.Basins sheltered behind a dike (e.g. Emmelsbüll) seem to be much safer than anysystem in the sea or in a harbour. The optimal overwintering system seems to be

given by an upwelling-system ashore.The resultsobtained by the trials in Langballigau demonstrate this clearly,where water without silt gave very good overwintering results with neglectablelosses even within the smallest oysters.

As the weight of all test groups of oysters was recorded 1 growth during theexperimental period can be estimated. These data. listed in table 2, have beensummarized in table 4.

Oysters may show some growth'during winter in our waters, as reported by Meixner, (1974).

The highest growthrates were recorded at the site of Wilhelmshaven, where

increased temperatures of about 7 to 90 C above ambient sea water temperature wererecorded.

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The total growth increment under these conditions ranged from 44.71% to229.83% depending on oyster size. All other sites were subject to naturalwater temperatures which are given in tab1e 5.

The fourth ranking group of oysters in tab1e 4 are the oysters of Emme1sbül1.Considering the ear1y contro1 data on April21st, very good temperature andfood conditions may be conc1uded in the sea-water storage ponds.

The third ranking oysters kept in 1antern nets in the Flensburg Fjord until

18th of May were given four more weeks of grewing peried and outgrew the1atter group only to a sma11 extent. Moreover it may not be over100ked that the10ss of sma11er oysters, as previously noted, gives a higher mean weight of

the remaining oysters which is not attributable to growth. The difference in ~the percentage gain of total weight (AG%) and percentage increasp ofmean weight (~g %) may be a hint for this, considering the low proportionof dead shells (LS%) as well.

Even the fifth ranking oysters of Amrum that achieved 16.72% of growth unti121st ef April demonstrate positive effects for growth during spring alongNorth Sea sites. This confirms other investigations that reca11ed the sameconclusion (Neudecker, 1980).

The comparatively 10w growth rate of those oysters kept in containers in theBa1tic is due to the high morta1ity of the bigger oysters out of the 0.36 g_

group. The loss in total weight of these oysters of 4.53% reduced the meanva1ue of growth considerab1y.

The most 1ike1y cause for comparative1y10w growth rates in these oysters atWangerooge may be the fact that they were held much higher above low watermark than any of the other groups.

The last ranking oysters in terms of growth where those in the upwellingsystem in Langba11igau.

Two causes make this easi1y understandab1e: One is the slower rising watertemperature as shown in table 5, the second is the low partic1e content

ef the water in the upwelling system, giving little feod to the oysters.

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4. SUMMARY AND CONCLUSIONS

Up to six size-groups of oysters (Crassostrea gigas Thun.) have been over~

wintered at 5 sitesalong the German coast applying stacks of plastic trayshanging from floats or placed in containers on the sea bed as well as Japanese

lantern nets as used in summer grow-out systems. Additionally seawater storagetanks sheltered behind a dike, eel tanks supplied with heated effluent waterof a power plant and especially prepared upwel1ing systems were availab1e for

testing.

The oysters were counted and weighed at the beginning and at the end of theexperimental period and the final proportion of dead shells was determinedas well .

The resu1ting data concerning percentage loss and growth have been compared

per size group, system and site tested giving the best results for the up-we11ing-system at .Langba11igau with 0.51 % 10ss of oysters. Highest growth

. rates were achieved in the power plant effluent at Wilhelmshaven with a meanof 107.4% from 15th of November to 27th of April 1983.

As the winter wasrelatively mild and without ice, the results cannot standfor extreme ice-winters. Nevertheless conclusions may be drawn from experiencegiving some direction for an optimal overwintering unit for oysters:

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water without particles does not give growth but results in highestsurvival rates of almost 100 %

- a small heating unit may help to overcome extreme frosty periods and seemsto be profitable on the long run

- the use of heated effluent water must not be done without food as starvationwould be the consequence

- the reduction of particle content of the heated effluent waters of Wilhelms­

haven could result in an excellent overwintering site also giving good growthduring the winter season

- sea water storage tanks without resuspending silt would improve the overwinteringconditions in Emmelsbüll ...

- the comparatively good results of the sites harbour Amrum, harbour Wangeroogeand Flensburg Fjord were only due to the mild winter conditions

- the overwintering unit at Langballigau station seemsto be at present the safest

place for oysters despite of the reduced salinity of the Baltic and could onlybe surpassed by a newly constructed system at the North Sea coast especiallynear a power plant

- without an overwintering unit there will be no consistent development of anoyster culture along the German coast.

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Neudecker, Th.1981

Neudecker, Th.1982

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Überwinterung von Pazifischen Austern'Crassostrea gigasan der deutschen Ostseeküste.Arch. Fisch.Wiss. 25, 1/2: 47-52

Culture of Pacific Oysters (Crassostrea gigas) inContainers in German Coastal Waters.FAO FIR:AQ/Conf/76/E.28

Die Auster und die Austernwirtschaft.Verlag v. Wiegandt, Hempel und Parey, Berlin, 126 S.

The Performance of the Condition Index of Oysters(Crassostrea gigas) during spring 1980 at two SelectedSites of the German Coast.ICES, C.M. 1980/F:12

Experimentaloffshore nursery-growing of Crassostreagigas Thunberg in Germany.EMS Special PublicationNo. 7: 197-210

New oyster on-growing containers for German Maricultureoperations.AquaculturalEngineering 1: 193-204

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s<ll

4-l .w0 CI)

S Po ~<ll S ~ S <ll S-O CI)

.w :::J 0 0 Po CI) <ll 0 <llCI) Po ~ l-t '.-l l-t. U .w CI) 0~ ~ 4-l 4-l Po <ll III .w 0 .wCI) bO c:: <ll .w Po O~

c:: tU c:: ~ c:: Po CI) CI) ..0 U ~.

4-l • .-l .w l-t 0 l-t '.-l. ~ 00 .w :::J ...-l :::J Po 0 'l-t . -o,.c ...-l

III bO .w 4-l .w öl) '<ll ' <ll .w 4-l~ ~ c:: <ll c:: <ll ~ c:: 4-l .w .w •.-l c::0 :::J • .-l l-i '.-l l-t 0 '.-l 0 III III ~ • .-l

...-l U l-t Po ~ ...-l ~ l-t4-l l-t <ll l-t l-t S l-t 4-l ...-l l-t 0 CI) CI) l-tl-t • .-l 4-l Q) Q) :l Q) l-t Q) <ll c:: 4-l :>-...-1 <ll<ll U 4-l .w .w Po .w <ll ~ ~ <ll l-t 1Il...-l .w:> <ll :::J III III III :> Po III Po <ll l-t III III0 l-t ..0 ~ ~ ~ 0 :::J ...-l 0 Po.w ~ ~

Fig. 1: Design of an upwellirlg system for the safe overwintering of oystersor other' rrolluscs ashore

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Fig. 2: I:esign of the seawuter storage ponds bchind a dike with "1ong-lines"1= seawater intake an:) draining pipe; 2= varying water surface;3= anchorage pole; 4=,rope; 5= float; 6= lantem net or stack of trays;7= mud and si1t; 8= PVC-lining of storage pond

Table 1: Design of field trials for the overwintering of spat and half grown Crassostreagigasduring winter 1982/83. The numbers give the corresponding line/trial number in table 2

Site overwinterung- No. ovster qroups - mean weiqht in qramsystems 0.08 0.36 1.41 2.13 4.99 7.10

container in sea 1 1 2 ___~n~ __ 13 14------------------- -------- -------- --------- -------- ----------u Langba11 igau 1antern nets in 2 4 4 5 6.,... sea - -+-' ------------------- -------- -------- --------- --------- -------- ----------~ ttl upwe 11 ings ashore 3 7/17 8/17 9 10 11 12ttlOJCl:l V')

Emnelsbüll stacks of trays 4 18 19 20 21 22in tanks -------------------- ---- -------- -------- --------- --------- -------- ----------stacks of trays 5 23in tanks - - - - -

Amrum stacks of trays 6 24 25 26 27 28 29in harbourWilhelmshaven upwe11ings ashore 7 30 31 32 33 - -

------------------- ---- -------- -------- --------- --------- -------- ----------Ittl stacks of trays tOJin tanks 8 - - - 34 - 35

V')

..t::-----~-------------

---- -------- -------- --------- --------- -------- ---------- .+-' reattachment frame 9 36 37 38 39 40 41~

0z

Wangerooge upwell ings ashore 10 - 42 43 - - -------------------- ---- -------- -------- --------- --------- -------- ----------stacks cif trays 11 44 45 46in harbour - - -

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Tab. 2:

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Data concerning overwinterinq trials at different sites applying different systems accordingto trial numbers in table 1.n = number of oystersl G - total weight of oyster groupl 9 = mean weight of oysters:A : change: LS : dead shells: ~ : percentages based on n1for n, on n2for LS, on G1for G,9 2 fvr ~.

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site and I - duration:system I oyster "1 GI 91 "2 G2 92 LS A" AG A9 A" LS AG A9 15,11. 82a=.to I I % % % %group I untiltable 1 I I

a I 0.08 200 14,6 0.073 385 42.1 0.109 5 15 ' 8.7 0.026 -3.75 1.30 26.05 30.54f b , 0.08 200 18.8 O,09~

27.~.83

c\

0.08 200 20,0 O,10C 370 46.9 0.127 5 30 5.1 ,0.033 -7.50 1,35 24.07 34.39d

I0.08 200 17,8 0,089

L i 0.08 800 71.2 0.089 755 89 0,118 10 45 17.8 0.029 -5.63 1.32 25.00 32,58

2 i 0.36 300 95,2 0,317 114 34,S 0.303 163 186 '-60,7 -0,014 -62.00 142.98 -63,76 -4.53 27.5.83

3 i 1.41 250 344.0 1,376 104 160.9 1.547 144 146 -183.1 0,171 -58.40 140,39 -53.23 12.43 27.5.83I

Il,15CI 1.41 250 287.4

4 362 697.4 1.927 10 138 11,8 0,556 -27.60 2.76 1.72 40.52 17.5.832.13 250 398,2 1.593

5 4.99 200 974,6 4.87 199 1077.7 5.42 0 1 103.1 0.54 0.50 0.00 10.58 11.14 17.5.83

a 7.1 100 783.3 7.83 97 841.0 8,67 3 3 57.7 0.84 3.00 3,09 7,37 10,73 17.5.83

- · 10000 - 634 10962 17.29 73 - 962 - . 11.51 9.62 -- - 10389 - 846 11261 13.31 45 - 872 - - 5.32 8.39 -- - 10000 - 798 9939 12.45 89 - -61 - - 11,15 -0,61 -

6 b - · , 10000 · 763 10069 13,20 82 - 6~ . - 10.75 0.69 - 18.5.83

- - 10000 · 929 10736 11.56 49 - -736 - . 5.27 7.36 -- · 15000 - 1339 15252 11,39 53 - 252 - - 1,68 1.68 -

, - - 15000 · 1277 14972 11.72 70 - -28 - - 5.48 -0.19 - I

I - · 10943 - 1360 9754 7.12 118 - -1189 - - 8.68 -10,87 -- - 13022 · 1217 4700 3.86 1930 • -8322 - - 158.59 -63.91 -

Lb . - 104354 - 9163 97645 10,66 2509 • -6709 - - 27.38 -6.43 -a 0,08 200 19,4 0,097 199 20.6 0.104 0 1 1.2 0.007 -0.50 0.00 6.19 7.22

7 b 0.08 200 18.4 0.092 198 19.7 0.100 0 2 1.3 0,008 -1.00 0.00 7.07 8.70, 22.4.83c 0.C8 200 16.3 0.082 195 18,2 0.093 1 5 1.9 0.011 -2.50 0.50 11,66 13,41

d 0,08 200 20,8 0.104 199 20.3 0,102 0 1 -0.5 -0.002 -0.50 0.00 -2.40 -1.92

L 0,08 800 74.9 0.094 791 78.8 0.100 1 9 3,9 0.006 -1,13 0.13 5.21 5.38

8 0.36 300 93.8 0.313 302 101,8 0,337 1 +2 8.0 0.024 0,67 0.33 8.53 7,67 27.5.83

9 1.41 250 285.8 1.143 248 312.9 1.262 2 2 27,1 0.119 -0.80 0.80 9,48 10.38 22.4.83

I 10 2,13 250 532.8 2,13 248 569.1 2.30 2 2 36.3 0.164 -0.80 0.80 6.81 7.68 22.4.83I

11 I 4.99 100 486.8 4.87 99 541.0 5.47 1 1 54.2 0,60 1.00 1.00 11.13 12,32 22.4.83i

12 I 7.1 70 539.9 7.71 70 613.1 8.76 0 0 73.2 1.05 0,00 0.00 13.56 13.62 22.4.83

1 - - - 3670 3348 0,91 604 - · - - 16.46 - -2 · . - 3148 3144 1.00 - · · - - . - -3 - . - 3491 2950 0,85 396 - · - - 11,34 - -4 - - · 2075 3281 1.58 564 · - - - 27,18 - - 9•• 11. bzw.,5 - - - 2428 3587 1,48 379 - - - - 15.61 - - 16.5.83

136 - - · 2225 3464 1.56 441 · - - - 19.82 - .7 · - - 1418 3530 2.49 44 - · - - 3,10 - -8 - - · 2389 6229 2.61 123 - - - - 5.15 - -9 - - · 2371 5985 2.52 228 - · - . 9.62 - -

10 - 6000 - 1175 4674 3.98 391 - -1326 - - 33.28 22.10 -11 - 6000 - 1239 5228 4.22 233 - -772 - - 18.81 12.87 -12 · 6000 · 957 4089 4,27 491 - ·1911 . - 51.31 31,85 -

2658649509 1,86 3894 . .L 1 1 1 23438 - - 3894 - - - - 16.61 -22,27 - i

Tahle 2: Continuation

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site and duration:system oyster n1 Cl 91 n2 G2 92 lS . t:.n t:.G t:.9 t:.n lS t:.G t:.9 15.11.82% % % "acc. to group ,.

untiltable 1

1 - 8000 - 376 9268 24,65 17 - 1268 - - 4,52 +15.85 -2 - 8000 - 359 9070 25,27 22 - 1070 - - 6,13 +13,36 -3 - 8000 - 366 9117 24,91 19 - 1117 - - 5,19 +13,96 -4 . 5688 - 254 6398 25,19 19 - 710 - -' 7,48 +12,48 - 18. und

14 5 - 8000 - 474 8344 17,63 47 - 344 - - 9,92 +4,3 - 19.5.83: 6 - 8000 - 480 8523 17.76 43 - 523 - - 8,96 +6,59 -

7 - 8000 - 478 8758 18,32 29 - 758 - - 6,07 +9,47 -8 - 8076 - 450 8514 17,38 31 - 438 . - 6,33 +5,42 -9 . 7880 - 729 8315 11,41 54 - 435 - - 7,41 +5,52

r - 69644 - 4006 76307 19,05 281 - 6663 . - - 7,01 9,57

1 - 9000 - 5826 8774 1,51 77 - -226 - - 1,32 -2,51 -2 - 9000 - 3548 8902 2,51 58 - -98 - - 1,63 -1,09 -3 - 8028 - 3536 7857 2,22 78 - -171 - - 2,21 -2,13 -4 a - 6000 - 2444 5820 2,38 40 - -180 - - 1,64 -3,0 -4 b - 3582 :. 962 3541 3,68 26 - -41 - - 2.70 -1,14 -

15 5 - 15000 - 914 16396 17,94 57 - +1396 - - 6,24 +9,31 -6 - 15000 - 1363 15715 11,53 76 - +715 - - 5,58 +4,77 -7 a - 5000 - 462 5320 11,52 26 - +320 - - 5,63 +6,4 -7 b - 9935 - 1216 10081 8,29 62 - +146 - - 5,10 +1,47 -8 - 15000 - 1878 15576 8,29 91 - +576 - - 4,85 +3,84 -9 a - 2500 - 296 2451 8,28 13 - -49 - - 4,39 -2,45 -9 b - 11362 - 1866 11591 6,21 98 - +229 - - 5,25 +2,06 -r 109407 - 24311 112024 4,61 702 - .... 2617 .. - - 2,89 2,39 -1 - 22500 - 2845 24362 8,56 90 - +1862 - - 3,16 +8,28 -2 - 22500 - 2833 24251 8,56 63 - +1751 - - 2,22 +7,78 -3 - 22500 - 2940 25167 8,56 67 - +2667 - - 2,28 +11,85 -

16 4 - 22500 - 3936 24522 6,23 81 - +2022 - - 2,06 +8,99 -5 - 22500 - 3932 24499 6,23 97 - +1999 - - 2,47 -8.88 -6 - 22500 - 4228 23463 5,55 146 - +963 - - 3,45 +4,28 -7 - 22500 - 4327 24015 5,55 96 - -1515 - - 2,22 +6,73 -r 157500 - 25041 170279 6,80 640 - 12779 - - .. 2,56 8,11 -1 - 1000 - 1074 0.46 - - +74 - - - +7,4 -2 - 1000 - 6300 1036 0,44 - - +35 - - - +3,6 -3 - 597 - 710 0,45 - - +13 - - - +1,87 -

17 4 - 592 - 6600 597 0,09 - - +5 - - - +0,84 - 3.5.835 - 193 - 4573 209 0,05 - - +16 - - - +8,29 -6 - 511 - 16692 439 1,03 - - -72 - - "hoch" -14.09 -7 - - - - - - - - - - - - - -r 3993 - 134165 4065 0,12 - - 72 - - - 1,80 --

18 0,35 300 92,8 0.309 194 79,8 0,411 0 -106 -13,0 0,102 35,33 - -14,01 32,98 21.4.83

19 1,41 250 311,0 1,244 250 390,4 1,562 0 0 79,4 0,318 0,00 - 25,53 25.53

20 a 2,13 250 516.3 2,065 243 586.7 2,414 3 -7 70,4 0,349 -2,80 1,23 13,64 16,91b 2,13 250 492,7 1,971 188 443,9 2,361 4 -62 -48,8 0.390 -.?4,80 2,13 -9,91 19.81

L: 2,13 500 1009 2,018 431 1030,5 2,3912 7 -69 21,6 0,3732 -13,80 1,62 2,14 18,49

Tab1e 2: Continuation

-15-

•

•

site and.0.9

d'.:::atior::system oyster "I" GI 91 "2 G2 92 lS .0." .o.G .o.g .0."" lS .o.G

15.11.82acc. to group ~ ~ ~ ~ur.ti1tab1e 1

21 4,99 100 536,1 5,361 100 643,6 6,436 0 0 107,5 1,075 0,00 0,00 20,05 20,05

22 7,10 70 495,0 "" 7,071 69 601,3 8,115 1 1 105,3 1,643 ·1,43 1,43 21,48 23,24

18-22 2: 1220 2443,9 2,00 1044 2745,7 2,630 8 176 201,8 0,627 -14,43 0,77 12,35 31,29

1 - 8000 - 960 9127 9,51 79 - 1127 - - . 8,23 14,09 -2 - 8000 - 1032 9280 8,99 35 - 1280 - - 3,39 16,00 -3 - 8000 - 1259 9183 7,29 58 - 1183 - - 4,61 14,79 -4 - 8000 - 1260 9339 7,41 55 - 1339 - - 4,37 16,74 -

23 5 - 8000 - 1310 9489 7,24 51 - 1489 - - 3,89 18,61 - 20.~.83

6 - 8000 - 1185 8814 7,44 68 - 814 - - 5,74 10,18 - !

7 - 8000 - 1387 9310 6,71 54 - 1310 - - 3,89 16,38 -8 - 8000 - 1405 8894 6,33 62 - 894 - - 4,41 11,18 -9 - 8420 - 1085 9782 9,02 42 - 1362 - - 3,87 16,18 -

10 - 8306 - 1410 9275 6,58 51 - 969 - - 3,62 11,67 -2: - 80726 - 12293 92493 7,52" .555. - 11767 - - .4,52 14,58 -

18·23 2: - 83169,9 - 13337 95238,7 ". 563 - 12068,8. - - 4,22 14,51 -a 0,08 200 18,9 0,095 342 41,1 0,120 2 -58 5,0 0,030 -14,50 0,59 13,85 33,16

24 b 0,08 200 17,2 0,086 21.4.83c 0,08 200 17,9 0,09 244 27,S 0,113 2 ·156 -8,1 0,024 -39,00 0,82 -22,75 26,64d o 08 200 17,7 o 089

2: 0,08 800 71,7 0,09 586 68,6. 0,117 .. 4 .-214. -3,1 .0,027 -26,75 0,68 -4,32 3:>,07

25 0,36 300 93,7 0,312 222 78,9 0,355 1 -78 -14,8 0,043 -26,00 0,45 -15,80 13,79

26 1,41 250 333,6 1,330 233 .355,7 1,527 5 -17 22,1 0,192 -6,80 2,15 6,62 14,40

27 2,13 250 504,7 2,019 244 550,6 2,257" 5 -6 45,9 0,238 -2,40 2,05 9,10 11,78

28 4,99 100 488,5 4,885 99 552,7 5,583 1 -1 64,2 0,698 -1,00 1,01 13,14 14,29

29 7,10 70 510,7 7,300 . 66 558,6 8,464 1 -4 47,9 1,168 -5,71 1,52 9,38 16,01

24-29 .2: 1770 2002,9 1,13 1450 2165,1 1,49 17 -320 162,2 0,362 -18,08 1,17 8,10 31,95I

30 0,08 200 15,3 0,077 - - - - - - - 100,0 - - . 2i.-'.S3200 17,2 0,086 - - - - - - - 100,0 - - -200 15,8 0,079 - - - - - - - 100,0 - - -200 14,8 0,074 - - - - - - - 100,0 - - -

31 0,36 600 185,2 0,309 - - - - - - - 100,0 - - -32 1,41 250 331,1 1,324 - - - - - - - 100,0 - - -33 2,13 250 532,8 2,131 - - - - - - - 100,0 - - -34 2,13 250 504,9 2,020 241 954,2 3,96 1 -9 .. 449,3 1,94 -3,60 0,42 88,99 96,01

35 7,10 70 552,3 7,890 67 823,7 12,29 0 -3 271,4 4,40 .4,29 0,00 49,14 55,82

Table :: Continuation

-16-

site and duration:system oyster "1 GI 91 "2 G2 92 LS A" AG Ag An LS AG Ag

15.11.82a=. to group % % % % untiltable 1

36 0,08 26 2,72 0,1046 10 3,45 0,3450 10 16 0,73 0,2404 61,54 100,0026,84 229,83 27.4.1983

37 0,36 26 9,39 0,3612 14 14,37 1,0264 4 12 4,98 0,6652 46,15 28,57 53,04 184,16

38 1,14 26 31,41 1,2081 13 25,76 1,9815 5 13 -5,65 0,7734 38,46 50,00 -17,99 64,02

39 2,13 26 61,25 2,3558 17 68,35 4,0206 6 9 7,10 1,6648 34,62 35,29 11,59 70,67

40 4,99 26 123,02 4,7315 25 178,7 7,1480 0 1 55,68 2,4165 3,85 (0,00) 45,26 51,07

41 7,10 26 209,99 8,0765 23 268,81 11,6874 2 3 58,82 3,6109 11,54 8,70 28,01 44,71

36-41 L: 156 437,78 2,8063 102 559,44 5,4847 27 54 121,66 2,6784 34,61 26,47 27,79 95,44

42 0,36 300 104,6 0,3487 172 62,2 0,3616 3 -128 -42,40 0,0130 -42,67 1,74 -40,54 3,72 27.4.1983

43 1,41 250 341,5 1,3660 240 368,0 1,5333 7 -10 26,50 0,1673 -4,00 2,92 7,76 12,25a 2;13 250 530,5 2,1220 235 542,6 2,3089 15 -15 12,10 0,1869 -6,00 6,38 2,28 8,81b 2,13 250 476,0 1,9040 168 354,6 2,1107 2 -82 -121,40 0,2067 -32,80 1,19 -25,50 10,86

44

L 2,13 500 1006,S 2,0130 403 897,2 2,2263 17 -97 -109,30 0,2133 -10,86 4,22 -19,40 10,60a 4,99 100 486,0 4,8600 95 533,4 5,6147 0 -5 +47,40 0,7547 -5,00 0,00 9,75 15,53

45 b 4,99 100 528,1 5,2810 78 483,4 6,1974 2 -22 -44,70 0,9164 -22,00 2,56 -8,46 17,35

L 4,99 200 1014,1 5,0705 173 1016,8 5,8775 2 -27 2,70 0,8070 -13,50 1,00 0,27 15,92

46 7,1 70 544,5 7,7786 59 522,0 8,8475 4 -11 22,50 1,0689 -15,71 6,78 4,13 13,74

42-46 r 1320 3011,2 2,2812 1047 2866,2 2,7375 33 -273 -145,00 0,4563 -20,68 3,15 -4,82 20,00

•

Table 3: Overwintering losses 1982/83 at different sites using different systems. The percentage loss 'is based on the number ofoysters at the beginning of the experimental period.+ proportion of dead shells. as numbers of oysters were not determined. Values in parenthesis include broken oysters

at final control as survivors

Site overwinterungs- oyster groups- mean weight in gram system-resultssystem No. 0.08 0.36 1.41 2.13 4.99 7.10 s rankn x

Itl Langballigau container in sea 1 5.63 '62.00 5fl.40 1 16.61+ ,16.61+ 7.01 6' 27.71 25.61 9Q1 ------------------- ------- --._------------ ----------------- --------- -------- -------- -------VI lantern nets in sea 2 - - 27.60 27.60 0.50 3.00 4 14.68 14.96 6u -------.-------.--- ---. ------- ---------------- ------.-- .---.-- --------- ---- ------- -------- -------....~ upwell ing ashore 3 1.13 -0.67 0.80 0.80 1.00 0.00 6 0.51 0.70 1r-Itla:l

Emmelsbüll stacks of trays 4 35.33 0.00 "13.80 0.00 1.43 5 10.11 15.24 4in tanks ----.--------------- ---- ------. ----------------- --------- ------- --------- ---- ------. --------- .------stacks of trays 5 4.22+ 1 4.22 3in tanks - - - - - -

Amrum stacks of trays 6 27.75 26.00 6.80 2.40 1.00 5.71 6 11.61 12.02 5in harbourWilhelmshaven upwellings ashore 7 100.00 100.00 100.00 100.00 - - 4 100.00 0.00 10

------------------- ---. ------- -------- -------- --.----- -------- --------- ---. ------. --.------ -------Itl stacks of trays 8 - - - 3.60 - 4.29 2 3.95 0.49 2Q1 in tanksVI -------.----------- ---- ------- -------- --.---- -------- -------- --------- ---- -_.---- ------.-- -------.s: reattachment frame 9 61.54 46.15 38.46 34.62 3.85 11.54 6 32.69 21.59 8....s.. (50.00) (42.31) (l9.23) (23.08) (O.OO) ·(8.70) 6 (23.89) (19.21)0z

upwe 11 ings ashore 10 - - - - - - - - -Wangerooge ------------------- ---. --.---- ----------------- ----------.---.-. .----.--. -----~-- ..---- --------- .-.---..stacks of trays 11 - 42.67 4.00 10.86 13.50 15.71 5 17.35 14.82 7in harbour

" "

Mean of groups exa;pt tota1 losses n 4 6 7 8 7 9including parenthesis values fromWi lhelmshaven x 21.13 34.61 16.69 12.34 4.66 5.56

s 22.49 20.95 21.04 9.85 7.17 4.65

Tab1e 4: Mean individual growth increment in percent of oysters overwintered at different sites using different systemsduring winter 1982/83

overwintering No. l..-QY.ster groups- mean we;ght in gr-;\m svstem-resultsSite system 0.08 0.36 1.41 2.13 4.99 7.10

n x s rank

'" Langba11 igau container in sea 1 32.58 __ :1:.~~_ 12.43 - - - 3 _!~:.~2_ _!§:.~§-- 6C.J -VI .----------------- --------- ------- ------- -------- --------u lantern nets in sec 2 - - 40.52 40.52 11.14 10.73 4 25.73 17.08 3.~ .----------------- ---- -------- . . ------- ------- ------- -------- -------- ---- ----_.-. --------. ----~.- upwelling ashore 3 6.38 7.67 10.38 7.68 12.32 13.62 6 9.68 2.90 8'"co

Emme1sbüll stqcks of trays 4 - 32.98 25.53 18.49 20.05 23.24 5 24.06 5.69 4in tanks ----.-------.-.----.-- ---- --------- -------- ------- ------- .------- -------- ---- ------- --------stacks of trays 5 -in tanks - - - - - - - - -

Arnrum stacks of trays 6 30.07 13.79 14.40 11.78 14.29 16.01 6 16.72 6.68 5in

'" \.oIiHie1mshaven upwell ing ashore 7 - - - - - - - - - -C.J ------------------ ---- .-------- .------- ------- ------- -----.-- -------- ---- -------- ---.---- ----VI

.s:: stacks of trays 8 - 96.01 55.82 2 75.92 28.42 2..... in tanks - - -s.. -------- ----0 .----------------- ---- .----.--- .------- ------- ------- .------- -------- ---- -------z reattachment frame 9 229.83 184.16 64.02 70.67 51.07 44.71 6 107.41 79.01 1

Amrum ~e~~11i~g~_~~~2~~_ 10 - - - - - - - - ----------. _._------ ------- ------- -------- -------- ---- ------- -------- -.--stacks of trays 11 3.72 12.25 10.60 15.92 13.74 5 11.25 4.64 7in harbour ..

I

/·lean of groups n 4 6 7 7 6 7x 74.72 39.63 25.65 36.54 20.80 25.41s 104.08 71.92 20.03 34.55 15.15 17.71

.....co-

•

- 19 -

Table 5: Water temperatures during winter 1982/83 at two sites+ Data obtained by Deutscher Wetterdienst

month Langballigau List +Baltic Sea North Sea-x s n x s n

November 9.2 1.41 4 7.8 - 30

Dezember 6.8 0.71 2 4.3 - 31

Januar 4.7 0.21 3 4.5 - 31

Februar 1.2 0.07 2 1.6 - 28

März 3.23 1.18 4 4.1 - 31

April 5.93 1.67 4 7.5 - 30

Mai 10.77 1. 70 3 11.4 - 31