Aquat. Lhil/g Resour., 1991,4,265-274

Growth and Illortality of the European in the Bay of Arcachon (France)

René Robert (1), Michel Borel (1" Yves Pichot (2) and Gilles 'l'rut (1)

(1) IFIlE/'v/RH, Q]mi du Commandant Sillwuetle, :'1."1120 Arcachon, France. (2) IFREAfER, Chemin de Jlaglielonf, 34250 Palaral-lel-Fltll.r, France.

Reccin:d May 3, 1991; acceptcc! Septembcr 20, 1991.

Robcrt R., M. Borel, Y. Pichot, G. Trut. Aquat. Uring Resour., 1991,4,265-274.

Abstract A study of the growth and mortality of the European oyster Os/rea edulis, including the effeets of j\1arteilia refringens and Bonamia ostreae, was earried out in three oceanic areas in thc Bay of Arcachon from May 1989 to Fehruary 1991. One-year-old spa t, free [rom diseasc were used for the trials. Six months after beginning the trials, Marteilia refrinKens had severely infested the cultures. Although Bonamia ostreae was found during the first yeur, the rate of infestation was low. In the second year Bonamiasis increased and high mortality (> 60%) occurred in Ostrea edulis populations. Although thc growth of the was good, their condition index was generally poor. Moreover, shell perforations by Polydora sp. wcre ohserved. ln spring 1990, a low glycogen storage was noted which may have been rclated to the Marteilia refl'inf?ens infestation although no rclationship with the carbohydrate content and mortality was detected. The influence of different rearing procedures on Ostrea edulis survival and development in four commercial batchcs of oysters, free of disease, was al50 studied. The growth of Ostrea edulis in single eulture was netter than in cultures mixed with Kigas. However, infestation by Marteilia refringens and Bonamia ostreae in the laid cultures was quitc similar to that of thc cxperimental cultures (bags). Due to the persislencc of these two parasites, the culture of Ostrea edulis on a commercial scale is not advisable in the Bay of Arcachon.

Keywords : Ostrea edulis, pathology, mortalily, growth, reproduction, gross biochcmical composition, bay of Arcachon.

Croissance et mortalité de l'huître plate Ostrea cdulis dans le Bassin d'Arcachon (France).

Résumé Une étude de la croissance, de la mortalité ct des taux d'infestation parasitaire, par Marteilia refringens ct Bonamia ostreae, de l'huître plate Ostrea edulis, a été réalisée dans le bassin d'Arcachon de mai 1989 à février 1991, à partir de naissain âgé d'un an ct indemne de parasitc à l'origine. Dans les trois secteurs concernés, une importante infestation par Marteilia refrinKens était notée six mois après la mise en culture. Bien qu'au cours de la première année d'élevage, la présence de Bonamia ostreae ait été décelée, son degré d'infestation est resté faiblc. Au cours de la deuxième année, ce parasite affectait LO à 15 % de la population. Au-delà de la première année, de fortes mortalités d'Ostrea edulis, supérieures à 60 %, ont été enregistrées. Si une bonne croissance a été relevée, les huîtres ont généralement présenté un index de condition médiocre. De plus, dans la zone la plus performante sur le plan de la croissance, le Banc, la qualité de la coquille a été sévèrement affectée par Polydora sp. Au printemps 1990, une faible accumulation des réserves de glycogène était observée, celle-ci pouvant être liée à l'infestation des mollusques par Marteilia refringens. Par contre, il n'a pas été possible d'associer les mortalités observées à un épuisement préalable de ces réserves. D'autre part, le suivi de quatre élevages professionnels, cxempts de parasite à l'origine, a permis de déterminer l'influence de la pratique culturale sur Je développement d'Ostrea edulis. Comparées à des cultures mixtes (Crassostrea gigas+ Ostrea edulis) de meilleures croissances étaient enregistrées dans !cs cultures

Aquar. Living Resour. 91/04 265 10 $ 3.00ilC> IFREMER-Gaulhier-Villars 266 R. Robert et al.

monospécifiqucs. Par contre, la pratique culturale semble n'avoir que peu d'efTet sur la propagation de ces parasitoses, les taux d'infestation par Marteilla refringens et Bonam/a oSlreae dans ce type de culture (au sol) étant aussi importants que dans les cultures expérimentales (poche). En raison de la persistance de ces deux parasites, l'élevage d'Ostrea edulis, à une échelle commercialc, n'cst pas recommandé dans le bassin d'Arcachon.

Mots-clés: OSlrea edulis, pathologie. mortalité, croissance. reproduction, composition biochimique, hassin d'Arcachon.

INTRODUCTION Marteilia njringens development (Grizel, 1985). Con­ scquently, to locate the most suitable area for the As a rcsult of cpizootic diseases, thc annual produc­ cultivation of the European oyster in the bay, one­ tion of the European oyster, Ostrea edulis, in the bay year-old Ostrea edulis spat (30 mm shell 1ength or Arcachon has droppcd rrom 20000 t in 1900 LO approximatc\y 3 g), free of disease, purchased from 2 tonnes in 1990 (Deltrcil pers. corn.). Thc fîrst dis­ Morbihan (Brittany-France) were reared in bags, in ease, whosc origin is still unknown, appcarcd in 1920. three oceanie areas of the bay of Arcachon, from A dramatic fall in flat oystcr production occurred May 1989 to February 1991. These cultures will be and the stock was exhausted in 1922. Ten years later, called experimental cultures. To determine the influ­ its culture was reinitiated in the bay, but competition ence of the rearing proced ure on Ostrea edu/is survival with the Portuguese oyster (Crassostrea angulata) and development, one-year-old 3 g spat (30 mm), also regularly introduced Înto the bay during the previous purchased from Morbihan, were laid on sites locatcd ten years, limited its extension. Because of Crassostrea in an oceanic arca of the bay of Arcachon. Thcy were angulata spread, only 10'% of the appropriate sites reared in single or mixed cultures with Crassostrea were available for Ostrea edulis culture and its pro­ gigas, al dirrcrcnt densities, from May 1989 to Decem­ duction never exceeded l 500 tonnes. ber 1990. These cultures will be called commercial cultures. Since 1970, a new discase, eaused by Marteilia refringens, has affected ()strea edulis. From 1970 to 1976 the disease increased in the bay (His et al., 1976), Experimental cultures after which the parasitosis seemed to decline (Tigé et al., 1979). From 1980, the disease reappeared and Mortalily, growlh and condition index most of the commercial oyster cultures failed (Deltrcil Experimental cultures were placed at the density of pers. corn.). 200 to 500 individuals. m" 2, in oyster he Id on In 1980, Bonamia ostreae, was found in Brittany 30cm high trestles, in the intertidal zone, al three (Comps et al., 1980), and was noticed in the bay of oceanic areas of the bay of Arcachon, "Ferret", Arcachon. As a result, the commercial culture of the "Banc" and "Courbey" (fig. 1). At eaeh site, two flat oyster in the bay was stopped. kinds of surveys were carried out over 21 months. Nevertheless, since 1987, the number of Ostrea In the first, mortality, shell length and total weight edulis larvac in the bay has increased and two wild increment were determined monthly from whole oyster stocks were discovered in the main channel of populations of 100 oysters per bag (200 indi­ the bay. In spite of substitution of this species by the viduals. m 2). In the second survey, samples of 30 cupped oysters, the French market's demand of flat individuals were taken each month from other popu­ oysters greatly exceeds supply. ln answer to the Arca­ lations placed in 6 oyster bags per site at a density chon oyster farmers' request, the present study was of 500 individualS.m - 2 (simple random sampling carried out to assess the potential for the renewal or without replacement). Length, total weight, dry meat Ostrea edulis culture in the bay. weight, dry shell weight and condition index (accord­ ing to Medcoff and Needler, 1941) were recorded. Differences between stations were assessed using one­ way analysis of variance (ANOYA) for the survey 1 MATERIALS AND METHODS and t-tests for the survey 2. Gametof!,enesis and shell infestation Previous studies have shown the importance of local cnvironmental conditions on the growth of the During the second year, fatness of the molluscs, Japanese oyster Crassoslrea gigas (Maurer, pers. gamctogenic development and degree of infestation com.) and on the Manila Ruditapes philippina­ or the shells by Polydora sp. were also recorded rum in the bay of Arcachon (Robert et al., 1991). monthly from the second survey. Three arbitrary sta­ Furthermore, high salinÎties have negative effects on ges were used to deseribe the change in fatness: glairy

A'fuat. Living Resour. Ostrea edulis in the bay of Arcachon 267

1 o 1 2 3 4 km N

OCEAN

BASIN OF ARCACHON

CF Cap Ferret B Banc C Courbey --.- ...... •* ...... ~

Figure 1. - Situation of the experimental and commercial cultures of Ostrea edulis in the bay of Arcachon. to thin, thin to fatty and fatty to very fatty. The Histopatho!ogy gametogenic development was studied by means of microscopie observations after piercing the gonad. Infestation by Marteilia refringens and Bonamia Four stages according to Marteil (1960) were used to ostreae was dctermined quarterly from histolo.!,tÎcal describe the reproductive cycle of Ostrea edu/is: stage observations of 20 to 30 individuals sampled from 0-1, no or rare traces of sexuality present; stage 2, each area from the second survey. For light micro­ oocytes or spermatozoa present but few in number; scopy, animais were fixed in Davidson's solution and stage 3, numerous and active spermatozoa or free embedded in paraffin. The sections were stained with oocytes; stage 4, spawning. Direct observations of the Masson's trichromic (Pichot, 1984). Bonamia ostreae number of flat larvae in the plankton were also made. was also detected from squash preparations made of the heart which were stained with methylene blue­ Biochemistry eosine (RAL 555). From Banc only, the gross biochemical composi­ tion was also recorded monthly using vacuum-dried pooled tissue (30 individuals sampled from the second Commercial cultures survey) as recommended by Giese (1966). For each component, 3 to 5 grammes of homogenized tissue were used and aU measurements were performed in Four commercial cultures were surveyed over a duplicate. The methods of Lowry etai., (1951), 19 -month period. Two batches were reared at low Dubois et al., (1956), Bligh and Dyer (1959) and density of 50 Ostrea edulis. m - 2 and two others were Marsh and Weinstein (1966) were used to determine mixed with Crassostrea gigas at a density of 200 the protein, carbohydrate and lipid levels respectively. individuals. m - 2 (50 flat oysters. m - 2 and 150 cupped 2 The biochemical results were expressed as percentage oysters.m- ). Samples of 30 animais were randomly of ash-free dry weight. The proportion of ash was taken quarterly, and length, total weight, condition estimated after incinerating tissue homogenates in a index, prevalence of Marteiliasis and Bonamiasis were muille furnace at 500·C for 24hrs. measured using the techniques described previously. Vol. 4. nO 4·1991 268 R. Robert et al.

Hydrobiology 100 Seawater temperature, salinity and chlorophyll a :;j were monitored weekly at midday on water surface ~ 60 at the stations of Banc and Courbey. To minimize P::: ::> advection problems, sampling was made along with U) 60 the ineoming tide, starting 1 hrs. before high or low ...0 tide. Temperature and salinity were measured with a rz:I cCl portahle T-S Meter (YSI model 33). For chlorophyll a ....« 40 determination, 100 ml of seawater was filtered on Z 0-0 Banc rz:I U t..-t. Courbey GF/C membranes. Filters were kept in an icc hox Cl:: 20 rz:I a until lluoremeter analysis (Turner model 112), accord­ P.. ing to Lorcnzen (1967). 0 60

70 RESULTS S 3 60 Experimental cultures :r:.... cCl 50 Z Surrey 1 ~ ~ 40 Mortality. - In aIl areas, high mortality (60 to rz:I 80%) was recorded at the end of the trials (fifi. 2 a). ::::!l 30 Severe mortalities began in February 1990, ten months after the beginning of the trials. In spring 20 and summer 1990, the mean lengths of live and dead 60 oysters were similar, indicating that mortality was not selective but covered a large size range. The highest 3 50 .... mortality rates occurred in summer 1990: 15% in the :r: Courbey area, 20% in the Ferret and 23% in the cCl 40 i:J Banc. iii: 30 Growth. - From July to Septemher 1989, growth ~ rates sharply increased, th en declined until the foIlow­ :r:0 ing summer (fifi.2b). From May 1989 to February iii: 20 1991, flat oysters grew from 30 to 50-60 mm. Increases ~ rz:I C in total weight were steadier in aIl areas hut develop­ ::::!l 10 ment was better at the Banc site (fig. 2 c; ta hie 1). At --.--...... ,. ---T --,----r----r------, the end of the trials, the mean total weights ranged M A N F M A N F from 30 to 40 g and a high dispersion of these values was noted with a coefficient of variation of 52%. 1989 1990 1991

Surrey 2 Figure 2. - Perccntagc of survival (a). shell length (h) and live Growth. - As shown by Newman-Keuls tests in weight incrcment (c) of Ostrea edulis. in experimental cultures, in the survey 1, Ferret was an intermediate growing area, Banc. rerret and Courbcy. thrcc occanic arcas of the bay of and consequently, resuJts will only concern Banc and Arcachon, from May 1999 to Fcbruary 1991 (monthly means calculatcd from whole populations). Courbey in this survey 2. On the other hand, to make fifiure 3 clearer, data will be represented quarterly. As previously shown, flat oysters grew faster in the Banc found in the plankton from June (beginning of plank­ area at p = 0.05 (fifi. 3 a). Compared to total weight, ton sampling) to September (Jlfi. 4 c). increases in dry shell weight and dry meat weight Fatness, condition index and shell infestation. - followed similar patterns (fifi.\'. 3 h, c). In February In spring and summer 1990, only 40% of oysters were 1991, mean dry meat weight ranged from 0.45 to fatty (Jlfi. 5 a) and condition index was generally poor 0.65 g and a high dispersion of these values was with values of 40 to 80 (fifi.5h). In February 1991, recorded with a coefficient of variation of 65%. individual measurements of this index showed Gametogenesis. - In 1990, one or two spawning important dispersion with a coefficient of variation periods occurred in each area in April-May and June­ of 48%. In both stations, early infestation by Poly­ August (figs. 4 a, h). However, in both stations most dora sp. was noted but was higher in Banc (fig. 5 c). of the oysters (>60%) generally exhihited spent Proximate biochemical composition. - In Banc, gonads characteristic of the stage 0-1. Larvae were ash content showed !ittle change over the study period

Aguat. Living Rc,our. 2~)

Ta:;:~ l. - Erfect of site on wcighl of Oll/'I!U edulis. One-way analyses of variance werc made in May 1990 and in February 1991 (1- p: power of the test; U signifieant al p = 0.001; * significanl al /1= 0.05).

Source 1 year culture 2 years culture of variatîon Degree of Sum F 1-)3 Degree of Sum F 1-)3 frecdom squarcd (ratio) p (%) freedom squarcd (ratio) p (%) Factor (sîle) 2 570.18 5.57 ** 88 2 1005.27 3.73 * 77 Resîdual (error) 204 102.32 69 269.56 Total 206 106.g6 71 290.29

80, le) ~--o...... 9- ----<>-- -- -é ...... 1"-, /' 1 ""l / ..... \ 50 ~ D-C Eane / 3 E) \ / E-< li. -0. Courhey \ :z:: / .0 ~ l...... _'" D t= (J "t~: ti 30 /' 0 û :il C~ !-< 0 0..." 43 !-< 20 3 a Zl ...,t~ 10

0 0 ~~t ii, t -1 80 100 '"'- , r -e--e-----o I5 """'-~ / Cl , E-< tJ ,,- 1 -t'l '0, 1 r~ 4J '.t " t= Cl ::J l n ZJ tJ f·3 n b ~) n () 0.. ~ r, U tJ ~ 10 n b ~ l:;?: /~'ô, ., • ~. 0 0 fIIl, Iii, Bi 10:0 CO

....-f__ ) U '"1 ...... r:::l rl f ~ '1 Cl ..Cl 1 1 :J ~~; Cl f:: E-< .~ :J À [J 1 /" r .... CJj C Cl l t/',J 0 ç~ f:î r i'1 U .L --- El ::; [:l c ~I~ ~ ", g /. \ [1 c ! '\1 0 ': j .. J U A- Il r u A 1< F J il 1:1 J S U leJ 1:::0 101 1000

Fi.;::::re 3. - Increase in live weîghl, shell weighl and dry mcal Fi.;:::.e 4. - Gametogenesis in Oslrea edulis, in experimcntal cul­ weight of Os/reu edulis, in experîmcntal cultures, in Dane and tures, in Cane (a), and Courbey (b) and number of flat oysler Courbey (bay of Arcachon), From May 1989 to February 1991 larvae în the channel of Piquey, West part of the bay of Arcachon, (quarterly mcans calculatcd from samples of n = 30). (c) From January 1990 to December 1990,

VoL 4, n· 4 - 19'11 2il> R. RG~2rt et d.

60 l e. a 0- -0 Banc 11 ::t: .0, (1).., A-Â Courber ~~ t:>..rn t) JO oiïJ "':."'~ ~~ ~3:j() ~A ...:....:1 î:îi5

EÇj 60 Z U'" 0 "m :1 u ~f1 40 (l '-' ; l' rj ~i.: n:!- f:l ~ •

d

J.I A N F lJ A N F A N F lJ N F 19:::a lSGO 19a1 lS'60 lS::Il

Figure 5. Index of condition (a), fatness (h) and degree of Figl:re 6. Diagram of the change of ash in Ihe dry meal (li) and infestation of the shells by Polydora sp. (c) of Ostrea edulis, in the change of carbohydralc (h), lipid (c) and protcin (d) in ash­ experimcntal cultures, in Banc and Courhey (hay of Arcachon), free dry meat of o.\/rca edulis, in experimental cultures, in Banc from May 1989 to Fcbruary 1991. (bay of Arcachon), from May 19i:i9 to Fcbruary 1991.

(fig. 6a) and the average ash content of the dry meat ail samples examined was 11.30±0.54%. Over the in ail samples examined was 19.42± 1.90%. Carbohy­ experimental period protein contents fluctuated errati­ drate contents increased from 3.2% in July 19&9 to cally, ranging from 27.7 to 49% (fig. 6 d). 15.7% in November 1989 (Jig. 6b). A low glucid sto­ rage was noted in spring 1990 with values fluctuating Prevalencc of Marteilia and Bonamia. In Octo­ from 11.2 to 16.9%. Glucid contents decreased from ber 1989, six months after the beginning of the trials, 16.6% in May 1990 to 8.8% in July 1990 and then infestation by Marteilia refrillgens was high. In both recovcred to earlier leve15. Glycogen represented at areas, 50 to 60% of oysters were affected, particularly least 85% of total carbohydrate and followed the with numerous young plasmodia in the stomach epi­ same pattern (fig.6h). From July 19&9 to February thelium (fi?,. 7 a). From September 1989 to September 1991, neither lipid storage nor depletion were obser­ 1990, infestation sharply decreased from 60 to 10% ved (fig. 6 c). The values ranged from 9.2 to 12.7% in Courbey, but Marteilia infestation was still high in and the average lipid content of organic matter in jane.

Aqual. Living Resour. 271

Infestation by Bonamia ostreae was low during the diffcrences became Jess apparent. At thc end or the firsl 16 months «5%) thcn increased. with values survey, the mean lengths of 62 to 65 mm and the rising to 10-15% (fig. 7 b). The degrcc of infestation mean total weights of 57 to 58 g were recorded in by both protozoans seemed to be rclatcd to the 1055 single cultures. Respective values were 55 to 59 mm of flesh, beeausc the change in ratio of dry Ilcsh and 44 to 48 g in mixed cultures. Moreover, during weight/dry shel1 weight decreased with lime (fig. 7 c). the tirst year, better condition index were noted when Ostrea edulis was grown alone (fig. 8 h).

• -. s:.r.;:. eu1tur. 1 :1 1) - 1) Slngl. oull1.l.Nl 2 D-O L:::~e :3 Q ~ ...;.: .. , . " • U2"d oultun 1 0-0 Courbey '-" .,1;1 "1.. ... Llud eult'lln la ~ , 0 N "1 r~i " ~ .. a ~;::.

b

11 " Il ~ :': 0 i3 fi ..,,~ u b .

100 '":

c

,. , ,l'" ' ..

d . " ' c

A F A F 1.1 A F 1.1 A N le:o le:}1 ItJl

Fi;,;:::re 7. Degree of infestation by Ma/"leilia n1dngens (a), Fig::ire n. - Incn:ase in live wcight (a), change in condition Eonamia ostreae (b) and change in ratio of dry tlesh weight/dry index (h) and degrcc of infestation by Marteilia refrinKens (c) and shell weight (c) in Ostrea edulis, in expcrimental cultures, in Danc /Jonamia oslreae (d) in 4 commercial cultures, single and mixcd, and Courhey (bay of Arcachon), from May 1989 to February 1991 of Ostrea edulis. laid on the ground, in Danc area (bay of Arca­ chon), from May 1939 to Fcbruary 1991.

During the first year, Ostrea edu/is had a signifi­ Infestation by Marteilia refringens was high in one cantly higher growth rate when grown alone than mixed culture, with values higher than 30%, and quite when mixed with Crassostrea gigas (fig. 8 a), th en low in the second mixed culture with values generally

Vol. 4, nO 4 - IS11 272 R. Robert et al.

lower than 20%. In single cultures, infestation rates ranged from 30 ta 60% and followed similar patterns to experimental cultures (fif? 8 c). Jn ail cultures, 30 Bonamiasis prevalence was low (fig. 8 d). 0-0 Banc 25 Hydrobiology 20 t'zl Il: Fluctuations in scawater temperature, sali nit y and ~ chlarophyll a are represented in ji!!,ure 9 and show ~ 16 Iittle differcnces between both stations. Winter 1989 t'zl was mikl with water temperature exceeding 9°C. With Si 10 sali nit y values generally greater than 32°/00' the ycar ~ of 1990 was especially dry. In 1989, unusual summer 5 and autumnal phytoplankton blooms of high Înten­ sities were recorded. In contrast, blooms of low inten­ 40 si tics were noted in spring 1990.

35 ...-. DISCUSSION AND CONCLUSION ~

30 ln the bay of Arcachon, the failure of Ostrea edulis ~

culture is clearly Iinked with the persistence of ~(Il l' Marteiliasis and Bonamiasis. 25 1 The effects of Bonamia os/reae iflcreased after 16 monlhs on-growing but this could not be cIearly shown in this study because the period of testing was not long enough. In contrast, the effects of Marteilia ;::;- rcjringens on Os/rea edulis have been reported. 1 Although the degree of infestation was higher in Arca­ .... 15 chon, the pattern of Martei/ia rl'[ringl'ns disease is ~ quitc similar to those already described in Brittany (Grizel, 1985). While in Brittany mortality occurred in the autumn of the second year, severe mortalities were recorded in the spring in Arcachon which may result in an earlier and higher increase of seawater temperature. For similarly reared disease-free oysters, natural mortality of 5 to 15% has been recorded in other regions (Walne, 1961; MarteiI, 1979; Paq uotte A N F A N F and Moriceau, 1987), and consequently, more than 50% morta1ity may be attributed ta Martl'ilia refrin­ 1989 1990 1991 f?ens infestation. Similar resulls have been reported in Spain by Figueras (1991). Figure 9. Fluctuation in seawater tcmpcraturc (a), salinity (h) Amongst the different clinical signs proposed by and chlorophyl! a (c) in Banc and Courbey areas (bay of Arca­ Grizel and Tigé (1973) a low mollusc fatness was also chon), from May 1989 to February 1991. observed. Because the 1055 of oystcr flcsh was related to thc degree of infestation, Marteiliasis induces or exaccrbates the poor condition of the molluscs. these gonadal deve10pment stages (Marteil, 1960; Wil­ Infestation by Marteilia refrùl!!,ens seemed to have son and Simons, 1985). Similar observations have negative effects on the gametogenesis of OSlrea edulis. becn reported with Bonamia as/reae (Van Banning, A low numbcr of individuals reached stage 3 and 1990). only 25% of oysters were ripe. Ncvertheless, the delay The evolution of the proximate biochemical compo­ between two consecutive observations was too long sition confirms a disturbance of Os/rea edulis gameto­ for a more detailed reproductive cycle study. To coun­ genic deveJopment. Jn nonnal conditions, the accu­ terbalance this inconvenience, the total number of mulation and storage of carbohydrate in spring pre­ oysters belonging to the subsequent stages 2 and 3 cedes go nad development (Eble, 1969; Walne, 1970; will be taken into account. From this, only 35% of Holland and Hannant, 1974, 1976). These authors oysters were estimated to be in active stages. In Brit­ found low perccntages of carbohydrate in winter, with tany before Marteiliasis sprcad, or today in regions values ranging from 5 to 10% of dry flesh wcight, an free of disease, 60 to 80% of oysters generally reached increase in spring and a maximum ratc in June with

Aquat. Living Resour. Ostrea eduli,~ in the bay of Arcachon 273 values of 20 to 30%. In the bay of Arcachon, the The losses caused by Bonamia ostreae must not be lowest values werc also found in winter, ranging from minimized, however the swift and high infestation by 11 to 13.5% but the highest value recorded in May Marteilia rejringens alone explains the failure 1990 was only 14%. This poor carbohydrate storage of commercial Os/rea edulis culture in the hay of during spring 1990 (3%) may result from \ow sexual Arcachon. activity in Ostrea edulis. Moreover the lipids showed For short-term purposes, two alternatives may be little variation over the 2 year period. proposed to Arcachon oyster farmers. The tirst one No relationship was found between previous glyco­ is to attempt to restore the bay by de-stocking the gen depletion and Os/rea edulis mortality. Such a area by either stopping commercial cultures or hy correlation has been reported on Crassostrea gigas by dredging and destroying the infected oyster beds. It Maurer et al. (1986) and on Rudi/apes philippinarum is weB known that a sm ail infected stock is sufficient by Goulletquer (1989), with glycogen values Jower to infect newly laid oysters (Tigé et al., 1984; Van than 1 % of dry !lesh weight. In the present work 5.6% Banning, 1987, 1988). was the lowest value recordcd. Mortalitics, which are not coarsc but progressive, and sampling of infected The second solution is to continue such cultures oysters mixcd with uninfected ones may explain this but by minimizing the incidence of Marteiliasis and result. Bonamiasis in relaid stocks. For such a management, the culture strategy should be to rotate oyster beds A dec1ine in infected oyster growth rate has been and to employ only short-term culture periods (one shown by Grizel and Tigé (1973). In the present study year). Medium-sized juveni1es (30 mm length) orig­ this pattern is not consistent. Mean total weights of inating from regions free of disease must be planted lOto 20 g and 30 to 60 g were reeorded at the end of in the oceanic areas of the bay in March-April at the first and second year, respectively. Such growth low densities of 50 individuals. m - 2 to improve their rates have he en monitored in regions free of disease growth. In the following spring, when oysters exhibit (Walne, 1958; Walne and Mann, 1975; Askew, 1978). the best condition index, the oysters should he har­ Differences in growth rates have been found between vested and the unwanted ones destroyed on land away the thrce stations selected here, with the best growth from seawater. in Banc. There were Iiule differences in cnvironmental conditions hetween the stations. The differences in For a medium-term solution the introduction and growth reeorded here may be related to differenccs acclimatization of non-indigenous flat oyster species, in immersion times. The experimenta1 site located in resistant to disease, may be another alternative to Banc was at the lowest intertidal level, resulting in a improve !lat oyster production. At present, four non­ greater immersion time during which oysters fed indigenous species have been tested on French coasts; more. Differences in growth were also observed when Ostrea chilensis (Grizel et al., 1983), Os/rea dense/a­ oysters were reared in single or mixed cultures. When mellosa (Le Borgne and Le Pennee, 1983), Os/rea grown with Crassostrea gigas, the development of angasi (Bougrier el al., 1986) and OSlrea puelchana Os/rea edulis is poorer because of differences in total (Pascual etai., 1991). The sensitivity of these exotic density, as proposed by Le Bec et al. (1991) or inter­ species to Marteilia and BonamÎa or with respect to specifie competition, as proposed by Briggs (1978). cold temperature has preventcd any sueeess to date.

Acknowledgements The authors are grateful to Michel Comps and RhÎan Robert for their crîtical reading of the manuscript.

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