Multidisciplinary Scientific Conference on Sustainable Coastal Zone Management Rights and Duties in the Coastal Zone, June 2003, Stockholm

Transcription

1 Private adaptation to ecological constraints in a rightsbased management system: the strategies of mussel growers in the Mont-Saint-Michel Bay (France) Rémi Mongruel, Aurore Davaine, Olivier Thébaud 1 Multidisciplinary Scientific Conference on Sustainable Coastal Zone Management Rights and Duties in the Coastal Zone, June 2003, Stockholm ABSTRACT Over the past forty years, mussel farming in the bay of Mont-Saint-Michel has developed into an activity representing an important part of the French production of mussels. Allocation of marine areas to this activity is administered via a system of individual concessions on the public domain. An empirical investigation concerning the economics of mussel farming in the bay was carried out in 2002 : the survey showed the existence of markedly different strategies of producers in terms of locations choices for their activity. The objective of the paper is to present an analysis of some of the reasons for these location choices. In the first part of the paper, a brief presentation of the economics of mussel farming in the bay is provided, along with a description of the location choices made by producers. Three major constraints are identified as important determinants of these choices, i.e.: (i) distance from port; (ii) variable productivity of concessions depending on their location; and (iii) variable exposition of concessions to the predation of mussels by wild species. The second part of the paper discusses the institutional arrangements that have allowed producers to adapt to these constraints. First, collective rules concerning the density of mussels on individual concessions were developed in response to a production crisis. Second, producers obtained from the Administration that mussel farming be partially transferred to a more productive part of the bay. Third, there has been a trend towards allowing greater flexibility in the exchange of concessions between producers. In conclusion, the paper stresses that understanding the actual impacts of the concession system on the dynamics of mussel farming in the bay of Mont Saint-Michel requires that the entire regime regulating access to the coastal ecosystem be taken into account. This includes not only the concession system, but also the informal market that has evolved for these private use-rights, as well as the high degree of coordination that exists between private producers, and between producers and the Administration. KEYWORDS Mussel-farming, rights-based management, localisation choices, ecological constraints, economic analysis 1 Contact author : Ifremer, Maritime Economics Service. BP 80, Plouzané, France. Olivier.Thebaud@ifremer.fr 1

2 The objective of this paper is to present an analysis of some of the reasons that explain the localisation choices strategies followed by mussels producers. This analysis is based on an empirical investigation concerning the economics of mussel farming in the Mont-Saint- Michel Bay, carried out in The analysis focuses on the complex ecological interactions which lead to spatially heterogeneous constraints on mussel breeding in the Mont-Saint- Michel Bay, and on the functioning of the administered system for the attribution of individual concessions. INTRODUCTION : A BRIEF HISTORY OF MUSSEL FARMING DEVELOPMENT IN THE MONT-SAINT-MICHEL BAY Mussel farming is a recent activity in Mont-Saint-Michel Bay, since it started only in 1954, with the creation of two small plots of concessions localised on both sides of biez du Viviersur-mer. Nevertheless, its expansion has been extremely rapid, as evidenced by the evolution of the length of mussel beds lines conceded: starting from 12 km in 1954, it reached 78 km in 1957 and 128 km as early as Despite the first production crises due to the Mytilicola intestinalis parasite that occur first in 1962 and then in 1965, the expansion keeps going: the length of mussel beds lines grows up to 166 km in 1963, then to 180 km in 1966 and eventually reaches 197 km in This early period of development corresponds to the creation of two mussel breeding zones in Cherrueix from 1958 and in Saint-Benoît-des-Ondes from 1963 (see figure 1) [Gasquet 1996, Le Mao & Gerla 1998]. Concerning farming methods, the arrival of Charentais in 1957 allows to solve the problems of spat harnessing met in the bay, thanks to the introduction of spat born in the Noirmoutier area [Frangoudes 1999]. Besides, output differences appear following a decreasing east/west gradient. This observation leads to the idea of transferring some of the least productive lines to a new production zone located more in the east, that will be created in 1975 north of the Banc des Hermelles. A second transfer to the zone takes place in 1983, just one year before its new enlargement [Gasquet 1996]. After this restructuring, a new equilibrium is reached with an annual production regularly locating above tonnes at the end of the 1980s. However, the gradient of increasing productivity from west to east nourishes the intention of mussel breeders to have re-examined the status of the insalubrious classified zone. A study conduced by the Ifremer-CSRU laboratory of Saint-Malo during the years confirms the insalubrity of the area and its grading in C quality [Gerla & Le Mao 1990]. In 1994, the Mont-Saint-Michel town, which then already receives 2,5 million of tourists a year, at last equips of a refining station out: the question of the healthiness of the zone located east of the Banc des Hermelles can again be raised. A new study is ordered in 1997 to Ifremer by the mussel breeders of the bay union [Frangoudes 1999]. The area then gets a provisory classification in B category, which allows the breeding of shellfishes provided they stay in a quality A water before their commercialisation. In parallel, conflicts appear between shellfish farmers, following the stakes presented by this new possibility of spatial extension of the exploitation. But an agreement is eventually reached, which defines the modalities of the transfer: mussel breeders will transfer a part of their concessions into the new zone starting from summer 2002, while oyster-farmers will get back the concessions deserted in the area of Saint-Benoît-des-Ondes [Davaine 2002a]. 2

3 Since its creation in1954 and until the end of the 1980s, the mussel breeding field of the Mont-Saint-Michel Bay has thus known a continuous development as far as exploited areas are concerned. However, this extension was accompanied by a decrease in the density of mussel beds lines implantation: since 1986, the total length of mussel beds lines became stabilized at 271 km, for an annual production also stabilized around tonnes. Mussel beds breeding is one of the four techniques used in France [Daniel 2000]. The mussel breeding sector in the Mont-Saint-Michel Bay represents on its own 15 to 16% of the national mussels production, and 25% of the production of mussels grown on mussel beds. Figure 1. Localisation of shellfish farming in the Mont-Saint-Michel Bay. 3

4 1. MATERIALS AND METHODS The study of the structure and functioning of mussel breeding enterprises of the Mont- Saint-Michel Bay is based on an economic survey realised in June 2002 and initiated by Ifremer s Maritime Economic Service (SEM) and by the Centre for Sea Law and Economics (CEDEM) of the University of West Brittany (UBO). This survey focused on two sets of themes: (i) production, investment and sources of revenues and (ii) interactions between enterprises on the one side, and on the other side natural surroundings, other sectors of activity and institutional environment. It took the form of direct talks with mussel breeders. Since the analysis deals with the strategies of mussel breeders, the enterprise was chosen as production unit upon which to base the reasoning. For practical reasons, mussel breeders were contacted in a random manner in the big land installations centres (Le Vivier-sur-Mer Port). Given the difficulty to locate all the affected population, no sampling nor a priori stratification of the target group was made. The representativeness of the portion of population interviewed should therefore be controlled after the event. The rate of sampling for each of the characteristics of the mussel breeding sector will then have to be compared to a reference rate: since the reasoning is based on the enterprise, the selected reference rate is the percentage of surveyed enterprises (see table 1). Table 1. Representativeness of the surveyed sample compared to the total mussel breeding population in the Mont-Saint-Michel Bay. TOTAL* Survey % S/T Number of mussel breeding enterprises % Exclusive mussel breeders % Mixed enterprises % Producers cooperatives Length of the mussel beds lines (km) 271,00 103,35 38 % Produced tonnage % Exploitation societies % Agreed establishments (expedition & purification) % Mussel breeders using amphibian boats % * Source : [DDAM 1999 & 2001]. Whereas the rate of surveyed enterprises is 27%, the surveyed sample represents 38% of the total for both the length of mussel beds lines and the produced tonnage. This means big and medium-sized firms are over-represented to the detriment of small enterprises. This bias linked with the dimension of the enterprises results automatically in over-representing firms with formal legal status and high degree of mechanization: the sample totalises 40% of the enterprises constituted as exploitation societies and 39% of the enterprises using an amphibian ship. On the other hand, the sample distribution between exclusive mussel breeders and mixed enterprises is satisfactory. At last, the representativeness of the sample in terms of geographic cover of mussel breeding zones must also be examined. In this case, the reference is given by the percentage of the total number of pales covered by the sample (see table 2). The sampling rate is approximately equivalent in the three areas. 4

5 Table 2. Representativeness of the sample in terms of geographic covering of the three mussel breeding implantation zones. TOTAL* Survey % S/T Total number of pales % Number of pales in the St-Benoît-des-Ondes zone % Number of pales in the Cherrueix zone % Number of pales in the Hermelles zone % * Source : [DDAM 1999 & 2001]. 2. THE ECONOMICS OF MUSSEL BREEDING IN THE MONT-SAINT-MICHEL BAY Economic structure of mussel breeding farms Given the history of mussel breeding implantation in the bay, a geographic criterion taking into account the diversification of implantation areas for each enterprise appears the most relevant of all to characterize the structure of the enterprises. This criterion permits to clearly distinguish three major categories of enterprises (see table 3 and figure 2): -the enterprises whose implantations are exclusively distributed among the Saint- Benoît-des-Ondes and the Hermelles zones (category 1); -the enterprises implanted almost exclusively in the central zone of Cherrueix (category 3); -the enterprises whose implantations are equally distributed among the three zones (category 2). Table 3. Typology of mussel breeding enterprises of the Mont-Saint-Michel Bay according to the geographic distribution of the concessions. Category 1 Category 2 Category 3 (%) Moy. ET CV Min max Moy. ET CV min max Moy. ET CV min max STB 58% 20% 35% 23% 86% 41% 15% 36% 24% 68% 10% 9% 89% 0% 25% C 0% 0% 0% 0% 35% 17% 49% 17% 69% 84% 11% 13% 69% 100% H 42% 20% 49% 14% 77% 24% 13% 55% 0% 40% 6% 4% 59% 0% 13% The modalities of the successive extensions of mussel breeding implantations from the central zone of Cherrueix on probably played an important role in the current land structure of the enterprises. For instance, the Saint-Benoît-des-Ondes zone being from the very beginning recognized as the last productive one, the holders of concessions in this zone were given a priority range for attribution when the Hermelles zone was created. The existence of this 5

6 agreement explains the characteristics of category 1 enterprises, whose concessions are nearly equally distributed among these two zones. But historical factors do not explain entirely the land structure of mussel breeding enterprises in the Mont-Saint-Michel Bay: as exchanges of concessions were authorized after the initial attributions, adaptation strategies could be adopted and led to these particularly marked differences among localisation choices. Figure 2. Validity test (Tukey boxes) of the typology distinguishing three categories of enterprises according the geographic distribution of the concessions, expressed in length of mussel beds lines hold in each zone (length in metres). Category of enterprises Figure 3. Length (m) of exploited mussel beds lines per category of enterprises Category of enterprises 6

7 The length of mussel beds lines per category of enterprises makes clearly the category 3 stand out as the one including the biggest industries, with an average mussel beds lines length of meters (see figure 3). On the other hand, no marked distinction can be drawn between the firs two categories in terms of size, even though the average mussel beds lines length is slightly smaller for category 1 (3 230 m) than for category 2 (3 530 m). The tonnage of mussels produced per category of enterprises confirms the category 3 specificity: it is constituted of enterprises whose production never falls under 100 tonnes of mussels a year. In average, the production level of category 3 enterprises (270 tonnes) highly exceeds the level of the two other categories (130 tonnes). Economic performances of each mussel breeding farms category Based on the turnover generated by the sole mussel breeding and related activities, a serial of economic performance indicators were calculated: -the apparent labour productivity, corresponding to the annual turnover obtained by the employees (seasonal workers included); -the labour productivity per hour, corresponding to the turnover obtained during worked hours; -the added value, given by the difference between turnover and intermediate consumptions; -the rate of added value, given by the ratio of added value to turnover, and which describes the wealth created by the enterprise ; -the part gross exploitation surplus (GES), representing the added value minus employees remuneration, social security contributions and taxes 2 ; -the part profitability rate, given by the ratio of GES to turnover; -the part net result, given by GES minus provisions for depreciation, worked out according to two methods : first, linear depreciation, and second, depreciation proportional to the reverse order of the years. The indicators elaborated in this manner allow to draw from each category a typical profile of enterprise, with the following characteristics. Category 1 enterprises. They present average levels of turnover, intermediate consumptions and taxes, a low level of social security contributions and average levels of productivity and depreciations. These characteristics lead to average gross exploitation surplus and net results, but with the best profitability rate. Category 1 enterprises have in recent years invested in material and have few employees. Category 2 enterprises. They present low levels of turnover, intermediate consumptions, taxes and depreciations, and an average level of social security contributions. These characteristics imply relatively modest levels of productivity, gross exploitation surplus and net results. Their 2 Only a part gross exploitation surplus is calculated here since the pieces of information gathered through the survey as far as labour remuneration (company manager included) and social security contribution are incomplete ; and though these data were partly reconstituted, the total labour cost cannot be taken into account. 7

8 investments in material are anterior to those of category 1 enterprises. The number of their employees is higher: this can be linked to the distribution of their concessions among the three zones, by contrast with categories 1 and 3 enterprises, whose activities are concentrated in one or two areas. Category 3 enterprises. They present the highest levels for all indicators, excluding the profitability rate which is the lowest of the 3 categories. Category 3 enterprises present levels of turnover and added value much higher to those of categories 1 and 2, with levels of social security contributions and depreciation also much more important, which does not stop them obtaining the highest levels of gross exploitation surplus and results. Category 3 enterprises are the biggest of the sample, with the largest means of production, inherited from important and recent investments. The typology of the enterprises based on the geographic distribution of the concessions seems to be supported by the analysis of economic performance indicators: each category of enterprises actually presents operating characteristics that can be attributed to peculiar structures and practises. These results fully justify the decision to continue the analysis of mussel breeders strategies from their geographic implantation choices. These elements of analysis of the strategies adopted by each category of enterprises to take advantage of the differential productivity of the exploitation zones will further be completed by some considerations regarding adaptation strategies to other types of interaction. 3. DETERMINANTS OF PRODUCERS LOCALISATION CHOICES The Mont-Saint-Michel Bay, which presents a very high degree of biological diversity, is both the centre of many human activities and the target of many protection measures. This is the reason why mussel breeding can be involved in three kinds of interactions : with the natural ecosystem, with the other human activities and with the institutional regulations. Most interactions experienced by mussel breeding produce similar impacts wherever the concessions are: this is namely the case for impacts produced by other human activities, as well as by institutional environment. On the other hand, interactions with the natural ecosystem lead to spatially heterogeneous constraints. The variations of water quality in different points of the bay, mainly due to pollutions carried along by rivers and streams, directly influence the definition of the extension limits of the mussel breeding zone. The question of trophic competition around the primary resources of the bay can be stated in the same terms as the question of the productivity gradient around which mussel breeders compete with one another: mussel breeders compete spatially with oyster-farmers in the western part of the bay, where productivity is the lowest, whereas this competition disappears in the eastern part, where productivity is precisely the highest. As far as adaptative answers to the problem of competition around primary trophic capacity are concerned, the gradient of decreasing east-west productivity will thus be the reference criterion to explain the choices of mussel breeders. At last, the predation of mussels by wild birds frequenting the Mont-Saint- Michel Bay protected zones occurs with highly decreasing intensity from the eastern part of the mussel breeding zone, where it is particularly high near Banc des Hermelles, habitat for important colonies of black scoters (Macreuse noire), to the western part. 8

9 Preliminary comprehensive framework of the localisation choices under ecological constraints Two major ecological constraints thus seem to explain the mussel breeders localisation choices in the Mont-Saint-Michel Bay : the decreasing productivity gradient from east to west and the exposure to birds predation, also decreasing from east to west. In first analyse, the economic impacts of these constraints can be qualitatively expressed (see table 4). Table 4. Qualitative appraisal of the economic impacts of ecological constraints in the three mussel breeding implantation zones. Saint-Benoît-des-Ondes Cherrueix Hermelles Productivity of concessions Exposition to predation The analysis of mussel breeding economics in the Mont-Saint-Michel Bay shows that big enterprises (category 3) historically preferentially established in the Cherrueix zone, which was the most productive area until the creation of the Hermelles zone of production. The less productive zone of Saint-Benoît-des-Ondes was also leaved to small producers. As a compensation, small producers got concessions in the Hermelles zone at the time of its creation. As the implantation in the Hermelles benefited in priority to holders of concessions in the least productive zone (Saint-Benoît-des-Ondes), it led to a kind of redeployment of small or medium-sized enterprises of category 1 and of already geographically diversified enterprises of category 2. Nevertheless, as we saw, the modalities of the successive extensions of the mussel breeding area do not fully explain the differences in land structure among enterprises. These differences also result from operations of concessions buying back. Repurchases seem to be mainly carried out by big enterprises (category 3) following a geographical concentration objective, and by enterprises whose geographic implantations are diversified (category 2). The concessions localisation choices of these two categories of enterprises attest the range of possible ways of adapting to the site s ecological constraints. For category 2 enterprises, exposure to predation is perceived as a fluctuating disadvantage: they consider that predation does not always have the same level of negative impact on harvesting. Thus, diversification of implantations is fully justified. If predation is intense, economic losses recorded on the Hermelles zone concessions are absorbed by the stability of production brought by the other zones of concessions, especially the Cherrueix zone. When predation is on the contrary relatively weak, mussel breeders really benefit from the very higher productivity gains of the Hermelles zone. For category 3 enterprises, the choice of concentrating the concessions in the Cherrueix zone amounts to consider that, in the long run, the higher productivity of the Hermelles zone does not allow to compensate the negative impacts of predation. This strategy consists in limiting the risks by concentrating the exploitation in a zone of medium but secure productivity: it is logically followed by the enterprises that carry out the major investments, and thus spread their activities on the most remote time horizon. 9

10 A quantitative estimation of the economic losses due to predation The phenomenon of decreasing productivity gradient from east to west is known since the beginning the mussel breeding in the Mont-Saint-Michel Bay, but it has only recently been suggested that important losses in tonnage could be due to predation by wild birds. The survey realised in 2002 allowed a first tentative of quantification of economic impacts of predation. From the statements of mussel breeders about the tonnage losses they impute to predation, the ideal tonnage (without predation) and the ideal turnover of the three categories of enterprises were appraised. Two methods were then used to appraise the theoretical economic performances of the enterprises in the absence of predation. According to the first method, exploitation charges remain the same, although the enterprise had to process a more important tonnage: this method underestimates production costs in the absence of predation, which amounts to overestimate the economic losses due to predation. According to the second method, exploitation charges are expressed proportionally to turnover, from the results observed in the face of predation, then recalculated on this basis using the ideal turnover: as in reality production costs grow less than proportionally to turnover, this method overestimates the production costs in the absence of predation, which amounts to underestimate the economic losses due to predation. The combination of these two calculation methods leads eventually to an estimation interval of the economic losses due to wild birds predation. Figure 4. Observed values and estimation intervals of the losses due to predation for turnover (TO), added valued (AV) and part gross exploitation surplus (GES). Value ( ) Category 1 Category 2 Category Losses (min) Losses (Max) Observed TO 1 AV 1 GES 1 TO 2 AV 2 GES 2 TO 3 AV 3 GES 3 10

11 The average results by category of enterprises according to the estimation intervals obtained by combination of the two evaluation methods are presented in figure 4. These results show that in terms of added value, category 3 enterprises undergo the more important losses (from to ), ahead of category 1 enterprises (fro to ) then to category 2 ones (from to ). This is explained by the activity volume of category 3 enterprises, that undergo important tonnage losses in the absolute, but rather weak relative to their global level of activity. Consequently, the tonnage losses of category 3 enterprises do not influence much their gross exploitation surplus. As for gross exploitation surpluses, category 1 enterprises undergo the more important losses, around to The losses of gross exploitation surpluses of categories 2 and 3 enterprises are at about the same order: from to for category 2 enterprises, and from to for category 3 enterprises. By contrast with category 1 enterprises, categories 2 and 3 manage to contain the impacts of predation on their gross exploitation surplus and thus probably also on their net result. It is worth noting that gross exploitation surplus is a far more determining economic performance criterion in producers strategic choices than turnover or added value. We already brought to the fore that land structures of categories 2 and 3 enterprises attest the elaboration of two differentiated strategies, but both aiming at optimising the advantages and constraints due the heterogeneity of ecological constraints met in the Mont-Saint-Michel Bay. The results of the estimation of predation s economic impacts show that these strategies allow the enterprises to limit the impacts of predation on their exploitation result. On the contrary, the enterprises of category 1, which do not seem to have adopted voluntary structural strategies, face surplus losses whose amounts could be up to twice higher. 4. DISCUSSION: INSTITUTIONAL ARRANGEMENTS Economic factors are not the only determining factors of mussel breeders localisation choices. Two more elements must be taken into account to analyse the adaptative answers of mussel breeders to the heterogeneity of ecological constraints in the Mont-Saint-Michel Bay: (i) the process of coordination among actors, and (ii) the influence of the people in the business on the way concessions are managed and attributed by the Administration. Producers coordination : regulation of technical practices After the starting of exploitation in 1954, the good results obtained in terms of mussels growth arouse new concessions extension requests to public authorities. The government thus completes in 1965 the definition of the implantation area for mussel breeding exploitations, already limited in the west by the presence of oyster beds: the part localised east of Banc des Hermelles is declared insalubrious zone (C quality), since the Coueston, Sée et Sélune waters reduce in excess the salinity and bring pollutions [Gasquet 1996, Le Mao & Gerla 1998, Frangoudes 1999]. In parallel, from the first extension in 1957, an estate plan was established, which fixes notably the minimal spacing out of posts and mussel beds lines [Frangoudes 1999, Fontenelle et al. 1998, Fontenelle 2000]. 11

12 The early 1970s is marked by a new outburst of infestation by Mytilicola intestinalis. The mussels mortality rate reaches 50% in the Cherrueix zone and 80 % in the Saint-Benoît-des- Ondes zone [Frangoudes 1999]. The people in the business reacts creating a professional union, advocating to thin the concessions by removing double lines and trays, which would reduce the density of stocking [Gasquet 1996, Fontenelle 2000]. At the beginning of the 1980s, the density of posts seems still excessive, due to a lack of application of the rules. Meanwhile, the extra-pressure on the surroundings increases the risks of production crises and outburst of Mytilicola intestinalis parasite [Le Mao & Gerla 1998]. This conjunction of factors emerges violently in : though it exceeded tonnes in the early 1980s, the production of mussels falls around tonnes in 1984 [Gasquet 1996, Frangoudes 1999]. Mussel breeders then make an agreement to reduce the density of stocking and increase the exploitation surface. This strategy concretises by a new transfer to the Hermelles as soon as 1984 and especially by the reaffirmation in 1985 of the rules on implantation density and on plots size. The following thinning affects 76% of the mussel beds lines, ends up in a 40% decrease of the number of posts, and is partly compensated by the creation of two new rows of bed lines northern of the Saint-Benoît-des-Ondes et Cherrueix zones [Gasquet 1996, Frangoudes 1999, Fontenelle 2000]. In 1989, dryness thwarts spat harnessing, hindering the product estimates for the following year. Its decrease is all the more marked that mussel beds are heavily harmed by the 1990 tempest [Gasquet 1996]. This conjunction of climatic factors thus causes at the beginning of the 1990s a slowing down of the production around tonnes, but the tonnes mark is passed again in 1994, attesting the relative stability of the new equilibrium reached in terms of biomass supported by the environment. The year 1990 also sees the launching of the «moule de bouchot de la baie du Mont-Saint-Michel» registered mark, whereas a procedure, that did not end up yet, is initiated to obtain an AOC acknowledgment [Gasquet 1996, Fontenelle 2000]. These initiatives give concrete expression to the will of mussel breeders to enhance the value of their products and reveal that a new strategic orientation has been adopted, less focused on produced volumes. Introduction of flexibility in the concessions transfer mechanisms : the creation of a quasi rights market system Instituted by a 1852 decree, the Public Maritime Domain (PMD) can be defined as a space belonging to the State, a priori inalienable, on which only economic activities subject to prior authorization can take place. Shellfish farming concessions present thus the following characteristics: they are temporary, individual and above all non-transferable. Colonisation of the French littoral by marine farming activities, punctuated by technical progress and comparatively high death rate periods linked with epizooties, has nevertheless induced a rarefaction of available space and, consequently, the development of informal systems of allocation and exchanges of concessions, giving rise to mainly occult financial transactions. The will of making these transactions transparent, in order to facilitate a public and concerted management of marine farming development, led in the 1980s to a major reform. For mussel breeding as for all marine farming activities, the occupation of the Public Maritime Domain is from now on administered through a system of attribution of concessions, fixed by a 1983 decree modified in 1987, and working as follows [JORF 1983, JORF 1987]: 12

13 -the concession is granted to persons who can prove the required professional ability, and for a domain fee due to the State; -the term of the concession cannot exceed 35 years; -the concession can be transferred to a third party before expiration in the frame of a substitution, for an indemnity based on the value of the improvements carried out. The successive 1983 and 1987 decrees made official the setting up of a quasi-market for shellfish farming concessions, whose running is surrounded by three institutional creations: first, the marine farming commission supervises the attribution of concessions; second, the structures scheme defines the use of the space; third, compensatory indemnities are paid when substitutions of concessions take place between shellfish farmers. Mussel breeding professionals have of course played a major role in these legal evolutions and largely favoured the institution of this quasi-rights market system on mussel breeding land. This mechanism introduces in effect some flexibility in space occupation modalities. Further research should be conducted in order to understand the functioning of the quasirights market system on mussel breeding land. CONCLUSION Space occupation is a determining factor in shellfish farmers strategies, since the range of hold concessions conditions the growth and perpetuation of the activity. Two criteria of optimisation can then be applied to shellfish farmers, depending on whether they position in the perspective of current running of the exploitation, or in the perspective of patrimony transmitting. In the first case, the optimisation applies to the global productivity level of the enterprise, according to the constraints and advantages presented by the various localisations. In the second case, the optimisation applies to the total value of the land patrimony, according to the complementarity of the appropriated goods. Mussel breeders have searched for the flexibility of concessions exchanges in order to implement adaptative answers to the heterogeneous ecological constraints met in the Mont- Saint-Michel bay. These answers have been to a large extent guided by economic considerations: by making choices of preferential localisation or of diversification of their concessions distribution, mussel breeders have tried to optimise both their exploitation results and the value of their land patrimony. 13

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