Claus 1983 Aquacultural-Engineering

7/28/2019 Claus 1983 Aquacultural-Engineering

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A q u a c u l t u r a l E n g i n e e r in g 2 (1983) 13-26

O n sh o r e N u r se r y Re a r in g o f B iv a lv e Mo l lu sc s inBe lg iu m

Christine Claus, H enk M aeckelberghe and N iels de PauwLaboratory for Mariculture, State University of Ghent, J. Plateaustraat 22,

B-9000 Ghent, Belgium

A B S T R A C TF r o m O c t o b e r t h r ou g h A p r i l t h e g r o w t h o f sp a t o f Ostrea edulis, Crasso-strea gigasa n d Venerupis semidecussata i n an onshore n ur se ry was r ecorde da s a f u n c t i o n o f w a t e r t e m p e r a t u r e a n d f o o d le ve l.

The r e su l t s o f t h i s s t udy i nd i ca t e t ha t e ven ve ry eu t roph ic wa te r , sucha s t h a t o f t h e S l u i ce D o c k a t O s t e n d , B e l g iu m , d o e s n o t p r o v i d e e n o u g hmicroa lgae t o su s ta in g row th o f b i va l ve spa t dur ing w in t e r , i r r e spec ti v e o fwhe ther t he wa te r i s hea t ed o r no t . I t i s nece s sary t o supp ly add i t i ona lf o o d ( c u l t u r e d l iv e alg ae ). F o o d s h o r ta g e f o r t h r e e m o n t h s a t h i g h e rt emp era tur e i ndu ced a st r e ss s i t ua t i on r e su l t i ng in h igh mor ta l i t y ra t es ,e v e n a f t e r t ra n s fe r ri n g a l l th e s p a t i n t o a n o p t i m a l c o m b i n a t i o n o f t e m -pera ture an d f o o d ava i lab i li ty .

INTRODUCTIONNursery rearing of bivalve molluscs, as the intermediate step betweenthe controlled produc tion of larvae in commercial hatcheries and thegrow-out of juveniles in the wild, is a practice in mollusc farming whichis receiving more and more atte ntion . The goal of mollusc nurseries is toraise cultchless spat of a few millimeters to a size of 1-2 cm, in a mini-mum of time, at the highest possible density, and with a minimum ofcosts and risks. 13Aquacu l tu ra l Eng ineer ing 0144-8609/83/0002-0013/$03.00- Applied SciencePublishers Ltd, England 1983. Printed in Great Britain


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14 C . C l a u s , H . M a e c k e l b e r g h e , N . D e P a u wA recent review of the methods used in nursery culturing is given by

Claus (1981). All of these nursery systems result in diffe rent growthrates for the various species. The majority of systems rely on theproductivity of the natural environment, and are utilizing unheatednatural seawater as sole food source for the bivalve spat. Subsequentlygrowth of the bivalves is limited to the warmer seasons of the year.Heating of the water, or moving the nursery plant to a subtropical ortropical climate can only partially offer a solution for the insufficientproduction during the winter, since the major limiting factor for optimalgrowth is food availability which under these circumstances is often stillvery low (Malouf and Breese, 1978 ; Claus e t a l . , 1981 ; Malouf, 1981 ).

Subsequently several controlled culturing systems have beendeveloped in which the bivalves receive supplemental feeding, mainlyconsisting of live microalgae. Recent experiments on nurse ry rearing ofbivalve spat by means of induced blooms have been described by Lucas(1976), Mann and Ryther (1977), Riva and Lelong (1978, 1981),Mercer (1981a,b), Mann and Taylor (1981), De Pauw e t a l . (1983),reflecting the latest evolution in the matter. A general review is given byDe Pauw ( 1981 ).

Based on results of previous studies on inducing mixed phyto-plank ton blooms, a semi-industrial nursery pilot-plant has been designedand was recently built at the border of the Sluice Dock in Ostend,Belgium. The nursery consists of two parts: a series of four outdooralgal tanks and an indoor nursery proper based on the upwelling tech-nology. The ultimate goal of our studies is to determine the cost-benefit of two types of'nursery operations: the first is based on storingduring winter, at low temperature, of readily graded juveniles whichhave been grown in autumn when the natural food supply was stillsufficient; the second consists in growing spat during winter on an algaldiet or inert food in heated seawater by using f.ex. thermal effluent s ofa power plant.

MATERIALS AND METHODSDescription of the nursery system (Fig. 1)Seawater is pumped into two constant head devices (800 liter) fromwhich the water is distributed by gravity into four rearing tanks (775


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16 C. Claus, H. Maeckelberghe, N. De Pauwliter). For two rearing tanks the sea water flows through an intermediateheating tank (775 liter). Heating occurs indirectly by means of anelectric boiler and two radiators.The algal suspension is pumped into an algal storage tank (120 liter)from which it runs by gravity to the rearing tanks at a constant flowrate. The outflowing seawater runs through a recycling reservoir (300liter) from which it can eventually be recycled through the cultures.The outflowing heated seawater is used to preheat the inflowing freshseawater.

The spat are stocked in up flow cylinders with a mesh bo ttom (2 mm).The whole system is conceived in such a way that the influence of thevariation of many parameters such as temperature, flow rate, stockingdensi ty, etc., can be assessed.Set up of experiment IT e s t s p e c i e s

flat oyster O s t r e a e d u l i s L. (3-4 ram)Japanese oyster C r a s s o s t r ea g i g a s Thunberg (3-4 ram)Manila clam V e n e r u p i s s e m i d e c u s s a t a Adams and Reeve (3-4 mm)

The oysters were provided by Seasalter Shellfish Ltd, Whitstable, UK,the clams by Satmar, Barfleur, France.S e t u pThe four rearing tanks (numbered 1-4) received the following culturingregimes:

tank 1 : unheat ed seawater, no additional supply of algaetank 2: unheated seawater, additional supply of algaetank 3: heated seawater, no additional supply of algaetank 4: heated seawater, additional supply of algae

In each tank an equal batch of the three test species was exposed to theexperimenta l conditions.T e s t c o n d i t i o n s

acclimatization period :stocking density:

1 week37 ind./cm 2 (30 000 ind./cylinder)


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Onshore nursery rearing o f bivalve molluscs in Belgium 17f l o w r a te :t e m p e r a t u r e :

f o o d l e v e l :d u r a t i o n :

a p p r o x i m a t e l y 1 0 l i te r / m i n / c y l in d e rh e a t e d w a t e r , a p p r o x i m a t e l y 1 5 C; u n h e a t e dw a t e r , f o l l o w i n g t h e t e m p e r a t u r e o f S l u ic eD o c k w a t e r1 0 0 0 0 - 5 0 0 0 0 c e l l s/ m l4 m o n t h s , f r o m 2 8 O c t o b e r 1 9 8 0 to 6 M a r c h1 9 8 1

Growth parametersS i e v in g o f t h e g r o w i n g s p a t l e a d s t o s p l i tt i n g o f th e s h e l l fi s h p o p u l a t i o ni n t o s iz e c la s se s . G r a d i n g w a s p e r f o r m e d w i t h 3 - 4 w e e k s i n te r v a l s b ys ie v in g w i t h 5 m m a n d 8 m m s q u a r e m e s h s i ev e s. I n t h is w a y , t h r e e s i zec la s se s w e r e o b t a i n e d : a n i m a l s s m a l le r t h a n 5 m m m e s h , b e t w e e n 5 a n d8 m m m e s h , a n d l ar ge r th a n 8 m m m e s h . S u b s e q u e n t l y t h e t o t a ln u m b e r o f i n d iv i d u a ls in e a c h c a t e g o r y w a s r e c o r d e d . S p a t o f t h e l a rg e s ts iz e cl as s w e r e r e m o v e d f r o m t h e n u r s e r y a n d t r a n s f e r r e d t o t h e S l u i c eD o c k . T h e m e a n i n d i v i d u a l s h e ll le n g t h a n d li ve w e i g h t o f e a c h s iz e c la s sw a s m e a s u r e d a f t e r e a c h g r a di n g o p e r a t i o n .S e t u p o f e x p e r i m e n t I II n c o n n e c t i o n w i t h e x p e r i m e n t I , a s e c o n d e x p e r i m e n t w a s c a r ri e d o u tw i t h t h e s a m e t e s t a n i m a l s f ro m 5 M a r c h t h r o u g h 1 3 A p r i l 1 9 8 1 . T h eo y s t e r s a n d c l a m s g r o w n in h e a t e d s e a w a t e r w i t h a d d i t i o n a l s u p p l y o fli ve a lg a e , w e r e n o t i n v o l v e d i n t h i s e x p e r i m e n t s i n c e m o s t o f t h e s ew e r e t o o l a rg e a n d w e r e a l r e a d y t r a n s f e r re d f r o m t h e n u r s e r y i n t o t h eS l u ic e D o c k . A n a t t e m p t w a s m a d e t o h a s t e n t h e g r o w t h o f a ll r e m a i n -i ng s p a t b y e x p o s i n g t h e s m a l l b i v a lv e s t o o p t i m a l g r o w i n g c o n d i t i o n s( h e a t e d s e a w a t e r + a d d i ti o n a l s u p p l y o f al ga e ). T h e t e s t c o n d i t i o n s a n dg r o w t h p a r a m e t e r s w e r e s im i la r a s f o r e x p e r i m e n t I .

R E S U L T SP h y s i c o - c h e m i c a l p a r a m e t e r s ( e x p e r i m e n t s I + I I)T h e t e m p e r a t u r e o f t h e r u n n i n g s e a w a t e r i n t a n k s 1 a n d 2 f l u c t u a t e df r o m 1 2 t o 1 8 C . T h e h e a t in g s y s t e m w a s o u t o f o r d e r d u r i ng th e f i rs tw e e k o f D e c e m b e r 1 9 8 0 . C o n s e q u e n t l y , f o r a f e w d a y s t h e t e m p e r a -


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18 C. Claus, H. Maeckelberghe, N. De Pauwture dropped to 3C. The mean water temperature in tanks 3 and 4was approximtely 5C during winter, with a minimum of--IC duringDecember 1980.The mean salinity was about 25%o, with minor fluctuations to 24and 26%0. The dissolved oxygen concentration in the rearing tanksvaried from 6 to 8.5 ppm 02. Ammoniaca l nitrogen concentrationnever exceeded 2 ppm. The mean pH value was 8.9 from January tillMarch and 8-4 during November and December.Biological parametersSupp ly o f algae fr om the outdoor algal tankThe dominant species in the induced algal blooms was the diatomSkeletonema costatum during the whole course of both experiments.During a short period in February 1981 the algal growth in the outdoortanks was insufficient. Subsequently in the nursery the minimal level of10000 cells/ml could not be met for a couple of weeks. This foodshortage seriously affected the growth of the bivalves during this timeperiod.Growth and mortalityThe data for initial and final individual length and weight, relativeincrease, percentage mortal ity and percentage of shell deforma tion, forbot h expe riments are given respectively in Tables 1 and 2.Experiment I (Table 1)As expected, spat held in heated water with additional supply of algaegrew fairly well, whilst spat in cold water or without sufficient foodshowed very poor growth, and in several cases even an importantmortality occurred.Crassos trea gigasIn unheated water the growth of the oysters was negligible; however nomortal ity was recorded. Heating of the water without addition of algalsuspension, resulted in but a limited growth and a slightly increasedmortal ity rate. Gr owth was only considerable when the oysters werefed and exposed to an increased temperature. Figure 2A shows the


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Onshore nursery rearing of bivalve molluscs in Belgium 19T A B L E 1

G r o w t h a n d M o r t a li ty o f S p a t o f Ostrea edulis, Crassostrea gigas a n d Venerupissemidecussata i n U n f a v o r a b l e G r o w i n g C o n d i t io n s ( E x p e r i m e n t I ) ( L = M e a nL e n g t h i n r a m ; W = M e a n W e i g ht in m g ; M = M o r t a li t y in % )

Unheated water Heated water Unheated water(no supply of (no supply of (with additional

algae) algae) supply of algae)

2 8 O c t o b e r 1 9 8 0

5 March 1981

Re la t i ve i nc r ea se

2 8 O c t o b e r 1 9 8 0

6 March 1981Re la t i ve i nc r ea se

2 9 O c t o b e r 1 9 8 0

5 March 1981

Re la t i ve i nc r ea se

Ostrea edulis' L 4 .45 + 0"93 4 .45 + 0 .93 4 -45 + 0 .93W 11 "9 11-9 I 1.9L 4-8 4 + 1.15 5.66 -+ 1.42 4.81 -+ 1-18W 14.4 19-1 15.2M 2-2% 15-3% 1-2%IL 8 .8% 27 .2% 8 .1%W 2 1 -0 % 6 0 . 5 % 2 7 . 7 %

Crassostrea gigas~ L 4 . 9 8 + 1 - 0 3 4 . 98 _ + 1 .0 3 4 . 98 _ + 1 .0 3

17 '9 17 "9 17 -9I ~ 5.06 + 1-21 5-62 -+ 1 "26 5.42 -+ 1-68

24-1 36-5 36"00 -0% 5 ,5% 0 .0%~ L 1"6% 12"9% 8"8%/w 34"6% 103.9% 101"1%

Venerupis semidecussata'L 4 .43 -+ 0 .45 4-43 + 0 .45 4-43 + 0-45W 3 7 . 6 3 7 , 6 3 7 - 6

L 5-15 + 0.5 9 5-36 -+ 0.55 6.0 2 +- 0.7 7W 3 7 . 9 4 5 . 4 6 7 . 3M 1 .3% 35-4% 1 .5%' L 16 .5% 21 -0% 35 -9%W 0 .8% 20 .7% 79 -0%

m e a n i n d i v i d u a l l iv e w e i g h t s o f t h e v a r i o u s b a t c h e s in th e n u r s e r y a st h e y w e r e s p l i t b y g ra d i ng . T h e s p e c i m e n s r e t a in e d b y t h e 8 m m m e s hs ie v e w e r e t r a n s f e r r e d t o s u s p e n d e d t r a y s i n t h e S lu i ce D o c k . F i g u r e 2 Bi l lu s t r a t es t h e n u m b e r o f i n d i v i d u a l s i n e a c h s iz e c l as s . I n t e r e s t i n g t on o t e is t h e f a c t t h a t s o m e i n d i v id u a l s a m o n g t h e s l o w e s t g r o w e r s ( s iz e


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20 C. Claus, H. Maeckelberghe, N. De Pa uwTABLE 2Growth and Morta l i ty o f Spa t o f Os trea edulis, Crassostrea gigas and Venerupis

semidecussata after i ts Transfer into Favorable Growing Conditions, Namely Heat-ing o f the W ater and Supply o f Live Food (Exper imen t I I ) (L = mean leng th in m m;W = Mean W eight in rag; M = M ortal ity in %;D = De form ation Rate in %; RelativeIncrease in %)

Origin o f the Unheated water Hea ted water Unheated waterspa t f rom (no supp ly o f {no supp ly o f (w i th addi tional

experim ent I a lgae) a lgae) supply o f a lgae)

5 March 1981

8 April 1981Relative increase

6 Janu ary 1981

13 April 1981Relative increase

Ostrea edulis{ ~ 4 .84+-1.15 5 .66+-1.42 4 .8 1+ 1.1 814.4 19.1 15.22-2% 15 -3% 1-2%L 6-5 6 -+ 1-72 6.52 +- 1-69 6.49 +- 1.49W 40-0 45.5 43.5M 12.3% 28-1% 19.4%~ L 35.5% 15.2% 34.9%/w 177-8% 138.2% 186.2%

Crassostrea gigas{ ~ 5.06+-1.2124.1 5-62+-1.266-5 5-42+1-686.0

0.0% 5.5% 0.0%I ~ 6 . 3 5 + 2 . 4 9 7 -0 1+ -3 .1 4 8 -3 5 + -2 - 8 356-6 90-1 121.13-2% 10.3% 2.3%

~ L 25.5% 24.7% 54-1%134-6% 146-8% 236"4%

Venerupis sernidecussata{ ~ 5 -16+0-59 5"36+0-55 6 .02+-0.775 M arch 1981 37-9 45.4 67.3

1.3% 35.4% 1-5%L 6.21 +- 1.16 5.73 -+0-85 7-66+- 1-04W 74-4 56.8 143-38 Ap ril 1981 M 21-5% 30.9% 1-5%D 1-6% 0-0% 19-4%

Relative increase { ~ 20-3% 6-9% 27-2%96-3% 25.1% 112-9%


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Onshore nursery rearing o f bivalve molluscs in Belgium 21

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F i g . 2 . ( A ) G r o w t h o f s p a t o f Cr assostrea gi gas i n f a v o r a b l e g r o w i n g c o n d i t i o n s .I n c r ea s e i n m e a n i n d iv i d u a l l iv e w e i g h t o f t h e v a r i o u s b a t c h e s a s t h e y w e r e s p l it te d

b y g r a d i n g . ( B ) T o t a l n u m b e r o f i n d i v i d u a l s i n e a c h s i z e c l a s s .


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22 C. Claus, H. Maeckelberghe, N. De P auwclass >5 mm) began to grow much faster after the first sieving and evenreached the largest size class (>8 mm) when the batch was graded asecond time.O s t r e a e d u l i sThe results obtained with O s t r e a e d u l i s are very similar to thoseobtained with C. g igas . However, it must be emphasized that the flatoyster is a much more delicate organism than the Japanese oyster. Inall combinations, but especially in the heated and unfed condition, themortal ity rates are more important. In favorable conditions the growthrate of O. e d u l i s is always inferior to that of C. gigas (Figs 3A and 3B).V e n e r u p i s s e m i d e c u s s a t aApart from obvious analogies with the results of oysters, attention mustbe drawn to the very high mortality that occurred when the clams werekept in heated water without addition of food. On the other hand, atlow water temperatures, the small clams still assimilated a certainamount of the live algae which were added to the water. This resultedin a slow but substantial growth, and a lower mortality rate as well. Inheated water with additional supply of algae 1I. s e m i d e c u s s a t a grewmore slowly than oysters (Fig. 4).Experiment II (Table 2)The growth and mortality rates in this experiment revealed that thepast history of the batches has a prolonged effect on the performancesof the juvenile bivalves. Even after transfer of the spat to bet ter growingconditions for one month, growth was still poor and mortality was highif the spat had been previously kept in heated water without supply offood, except for C. gigas where the mortality rate was acceptable andgrowth fairly good. Although previous storing of the spat in cold waterwithout any supply of algae resulted in low mortality rates afterchanging the test conditions, growth was obviously better if the testanimals had been fed during the previous test period. This was mostdistinct for C. gigas and V . s e m i d e c u s s a t a .

At the end of the experiment a relatively high percentage of theclams showed various shell deformations. In the test combinationwith unheated seawater and with additional supply of algae this abnor-


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Onshore nursery rearing of bivalve molluscs in Belgium 23

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F i g . 3 . ( A ) G r o w t h o f s p a t o f Ostrea edulis i n f a v o r a b l e g r o w i n g c o n d i t i o n s .I n c r e a se i n m e a n i n d i v i d u a l li v e w e i g h t o f t h e v a r i o u s b a t c h e s a s t h e y w e r e s p l i t t e d

b y g r a d i n g . ( B ) T o t a l n u m b e r o f i n d i v i d u a l s i n e a c h s iz e cl a ss .


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24 C. Claus , H . Maecke lberghe , N. De Pauw

F i g . 4 .

3 0 0 .&

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unheated water with supply of algae heated water without supply of algae~t unheated water without suppl y of algae heated water with supply of algae

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G r o w t h o f sp a t o f Venerupis semidecussata in d i f ferent exper imentalconditions; increase in mean individual live weight.

m a l i t y c o u l d b e o b s e r v e d i n 2 0 % o f t h e p o p u l a t i o n . I n t h e te s t c o m -b i n a t i o n w i t h h e a t e d s e a w a t e r a n d s u p p l y o f a lg a e t h e d e f o r m a t i o n r a t ew a s a l s o 2 0 % . T a k i n g i n t o a c c o u n t t h e r e s p e c t i v e m o r t a l i t y r a t e s i n t h eo t h e r t e s t c o m b i n a t i o n s o n e c o u l d a s s u m e t h a t t h e m i s s h a p e n in di -v i ua ls d i d n o t s u r v iv e t h e m o r e u n f a v o r a b l e c o n d i t i o n s . H o w e v e r ,t h e r e a r e m o r e a r g u m e n t s f o r b e l i e vi n g t h a t t h e a p p e a r a n c e o f s he lld e f o r m a t i o n i s s i m p l y l i n k e d t o g r o w t h i ts e lf .

C O N C L U S I O N SW h e n f a v o r a b l e g r o w t h c o n d i t i o n s c a n b e e s t a b l is h e d , th e d e s c r i b e dn u r s e r y s y s t e m is v e r y s a t i s f a c t o r y f o r r e a ri n g b i v a lv e m o l l u s c s p a t . F a s tg r o w e r s c a n l ea v e t h e n u r s e r y a f t e r 2 m o n t h s . S l o w e r g r o w e r s s h o u l db e c u l l e d a f t e r f iv e m o n t h s .


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Onshore nursery rearing of bivalve molluscs in Belgium 25The demand for cultchless seed from the nursery is for more than

75% confined to early spring. Subsequently a commercial nursery hasto take one of the following options for its operation:

(1) inducing fast growth of the spat during winter by heating thewater and supplying live algal food, in order to have the shellfishseed ready for the spring sales;

(2) growing the spat during summer-aut umn and overwintering thereadily graded spat at low temperature;

(3) inte rmediate to (1) and (2): cold storage of small spat wi thaddition of a small quantity of live food. This procedure hasproven to enhance the growth in early spring when conditionsare again favorable for growth.

The final choice between these alternatives must be made with regardto the function of cost-benefit analysis for each commercial nurseryoperation.

It is emphasized that successful overwintering of small spat wouldallow the nursery to work in summer conditions when the operationsare most efficient (Hidu e t a l . , 1981). The present study contributes tothe development of an optimal overwintering procedure with accept-able risks for the nursery operator. The preliminary results are en-couraging and corroborate the findings of Hidu e t a l . (1981), that itmay be possible to hold large numbers of small bivalves with slightlytempered water temperatures and low-level feeding.

Although nursery rearing of burrowing bivalves such as clams appearsto be feasible, attention should be paid to the shell deformations occur-ring during the growth of clams in the nursery. Further research on thismatter is necessary to solve this problem which is important for theeconomics of the venture .

REFERENCESClaus, C. (1981). Trends in nursery rearing of bivalve molluscs. I n : N u r s e r y C u l tu r -

i ng o f B i va l ve M ol l usc s , eds C. Claus, N. De Pauw and E. Jaspers, EMS SpecialPublication No. 7, European Mariculture Society, Bredene, Belgium, pp. 1-33.Claus, C., Van Holderbeke, L., Maeckelberghe,H. & Persoone, G. (1981). Nurseryculturing of bivalve spat in heated seawater. In: A q u a c u l t u r e i n H e a t e d E f f lu e n t s


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26 C. Claus, H. Maeckelberghe, N. D e Pauwand Recirculat ion Sys tem s, vol. 2, ed . K . Tiew s, H een em an n Verlagsgesellschaftmb H, Be r l in , pp . 465 -80 .

De Pauw, N . (1981) . Use and p rod uc t ion o f mic roa lgae as food fo r nu rse ry b iva lves .In : Nu rser y Culturing o f Bivalve Molluscs, eds C. Claus, N. De Pauw and E.Jaspers , EMS Spec ia l P ubl ica t ion No. 7 , Eu ropea n M aricul ture Soc ie ty , Bredene ,Be lg ium, pp . 3 5 -69 .

De Pauw, N. , Verboven, J . & Claus. C . (1983) . Large sca le microa lgae product ionfor nursery rear ing of marine b iva lves . Aquacultural Engineering, this issue,p . 27 .

H idu , H . , Chap man , S . R . & Dean , D . (1981) . O ys te r mar i cu l tu re in subborea l(Ma ine , USA) wa te r s : Cu l t ch le ss se t ting and n u rse ry cu l tu re o f European andAm er ican oys te r s . J . Shellfish Res., 1 (1 ) , 57 -67 .

Lucas, A. (1976) . A new type of nursery for rear ing b iva lve post la rvae . Const ruc-t ion , equ ipmen t and p re l imina ry re su l t s . In : Pro c. lOth Europ. Sy mp . MarineBiol., vol. 1, eds G . Persoo ne and E. Jasp ers, Universa Press, W ettere n, Belgium,p p . 2 5 7 - 6 9 .

M a louf , R . E . (1981) . Use o f hea ted e f f luen t s fo r the nu rse ry cu l tu re o f b ivalvemol lusc s : i t s p rob lems and po ten t i a l . In : Nu rsery Culturing of Bivalve Molluscs,eds C. Claus, N. D e P auw and E. Jaspers , EMS Special P ubl ica t ion No. 7 , Euro-pean M ar icu l tu re So c ie ty , B redene , Be lg ium, pp . 171-88 .

M alouf , R . E. & Breese , N. P . (1978) . In tensive cul ture o f the Paci fic oyste rsCrassostrea gig as Thunbe rg in hea ted e f f luen t s . Sea G rant Progra m Publ. No.ORESU-T-78-O03, Oregon Sta te Universi ty , Corval li s.

M ann, R . & R yth er , J . H. (19 77) . G row th of s ix spec ies of bivalve mo l luscs in awas te r ecyc l ing aquacu l tu re sys t em . Aquacu l ture , 1 1 , 2 3 1 - 4 5 .

M ann , R . & Tay lo r , R . E . J r. (1981) . G row th o f the bay sca llop Argopecten i rra-dians i n a was te r ecyc l ing aquacu l tu re sy s t em. Aquacu l ture , 2 4 , 4 5 - 5 2 .M ercer , J . P . (198 1a) . Nu rsery cu l ture of m ol luscs in I re land. Progress and prob-

l ems . In : Nursery Culturing of Bivalve Molluscs, eds C. Claus, N. De Pauw andE. Jaspers , EMS Spec ia l Publ ica t ion No. 7 , European Maricul ture Soc ie ty ,Breden e , Belg ium, pp . 189-9 5.

Merce r, J . P . (1981b) . Low cos t spa t p roduc t ion un i t s . In : Proc. 12th Shell f ishConf . , She l lf i sh Assoc . o f Grea t B r i t ain , Lon don , pp . 6 9 -78 .Riva , A. & Lelong, P . (1978) . A l ime nta t io n e t c ro issance de b iva lves f i l treurs en

bassin am6nag6. Publ. Sci . Tech. CNEXO, Actes Colloq., 7 , 4 1 5 - 3 6 .Riva , A. & Le long, P . (1981) . G row th o f juveni le b iva lve mo l luscs assoc ia ted wi th

c o n t i n u o u s c u l t u re s o f n a t u r a l m a r i n e p h y t o p l a n k t o n . I n : Nursery Cul turing ofBivalve Molluscs, eds C. Claus, N. De Pauw and E. Jaspers, EMS Special Publica-t ion N o . 7 , E uropean Mar icu l tu re Soc ie ty , B redene , Be lg ium, pp . 253-68 .

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Claus 1983 Aquacultural-Engineering

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