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International audience; Mullet populations are abundant in littoral waters throughout the world and play a significant role in organic matter fluxes. Mullets are opportunistic feeders: adults have frequently been shown to feed on primary producers (e. g. fresh or detrital plant material, microphytobenthos) but they may also feed on meiofauna. The population structure and stomach contents of mullets that colonize salt marsh creeks in Aiguillon Bay (French Atlantic coast) were studied to determine if they use salt marshes as a feeding ground in spring. Stable isotope analyses were carried out on mullets sampled to assess their diet during their spring migration. The mullet population was primarily composed of young-of-the-year (G0), 1-year old (G1) of both Liza ramada and Liza aurata species and 3-year-old or older (G3+) L. ramada individuals. G0 and G3+ population densities increased during the spring period: catch per unit effort (CPUE) increased from 0.22 to 1.49 ind.min-1 for the G0 age group; but stomach content analyses revealed that only G1 and G3+ feed in the salt marsh. Isotopic signatures of G1 (spring: δ13C: -14.8‰, δ15N: 14.1‰) and G3+ mullets (spring: δ13C: -16.9‰, δ15N: 13.8‰) indicate that mullet growth is supported largely by primary consumers, such as benthic meiofauna or small macrofauna. Mullets are thus positioned at a much higher trophic level than true primary consumers.

Coastal fish populations settle more or less frequently, at various stages, estuarine systems. Tidal marshes are one of the components of these mosaics of habitats. The first part of this work describes fish assemblages through the utilisation of these systems by fishes analysed in order to propose a revised functional classification of ecological guilds. T h i s new classification is used to compare fish communities of four estuarine systems in the western coast of France: the Mont Saint- Michel Bay, the Saint-Brieuc Bay, the Loire estuary and the Aiguillon Bay. The second part aims at characterizing the nursery function as defined by Beck et al. (2001). Acombinative approach based on gut content analysis, and stable isotope analysis shows that Yo Y s e a bass (Dicentrarchus labrax) that are caught in tidal marshes feed actively in situ and that the salt marsh preys contribute significantly to the first year food resources. An examination of the microstructure of the Yo Y sea bass otoliths using SEM shows characteristic marks, which are interpreted as fingerprints of the passage in the salt marsh. The microstructural analysis showed that sea bass recruits in estuarine areas after variable length of time (33 days to 126 days) spent in marine waters during migration from spawning areas to estuaries. After entry into estuarine complexes, growth rate increases. Microchemical analysis of the whole otolith using ICPMS technology revealed characteristic signatures of each of the studied estuarine systems. This suggests that the chemical composition of trace elements in the otoliths can be used as a chemical tag of fish. After a synthesis of this combinative approach, a reflection is committed concerning the efficiency of these approach in order to advise managers. The same though is engaged to evaluat the sustainabilityof these studies that mix strong field investments and theoretical ecology (guilds, life history traits, etc.)., Les populations de poissons côtiers colonisent de manière plus ou moins fréquente et à diverses écophases les écosystèmes estuariens. Les marais salés constituent un des nombreux éléments de ces mosaïques d'écosystèmes. La première partie de ce travail consiste en une approche descriptive de la composition des assemblages piscicoles au travers de l'analyse des guildes écologiques, et débouche sur la définition et la validation de guildes colonisatrices. Elles sont définies puis critiquées au travers de l'étude des populations de poissons présentes dans quatre systèmes estuariens de la façade ouest de la France : baie du Mont Saint-Michel, baie de Saint-Brieuc, estuaire de la Loire et baie de l'Aiguillon. La seconde partie s'intéresse à la caractérisation de la fonction de nourricerie de ces écosystèmes chez les juvéniles de bars européens (Dicentrarchus labrax) au travers de la définition proposée par Beck et al. (2001). Une approche combinée, basée sur des études de contenus stomacaux et sur l'analyse des rapports isotopiques des muscles de juvéniles capturés dans les marais salés, montre que les poissons s'alimentent activement dans ces systèmes et que les proies consommées in situ contribuent de manière significative à la première année de croissance. Une étude des microstructures des otolithes par MEB révèle la présence de marques spéciales qui peuvent être attribuées au passage dans les marais salés. L'analyse microstructurale montre que le recrutement des jeunes bars est étalé dans le temps avec des migrations marines plus ou moins longues (de 33 à 126 jours). Une fois installés dans les systèmes estuariens, leur croissance s'accélère. Une analyse microchimique des otolithes par ICPMS révèle des compositions caractéristiques de chacune des nourriceries estuariennes. Ceci suggère que les compositions en éléments traces peuvent être utilisées comme signature chimique discriminante chez les poissons. Enfin, après un bilan de cette approche combinée, une réflexion est engagée concernant l'efficacité de ces analyses dans le conseil aux gestionnaires. Une réflexion similaire est menée concernant la pérennité et la reproductibilité de telles études qui mélangent de forts investissements de terrain et de l'écologie théorique (guildes, traits d'histoire de vie,...).

An analysis of the fish community of Aiguillon bay (France) and Mont Saint-Michel bay was performed in order to identify the fish settler guild structure. Some species settled in salt marshes only in their first years when they were young-of-the-year or juveniles ("yoy salt marsh settlers" and "juvenile salt marsh settlers"). Other fish species were "resident salt marsh settlers" like gobies (Pomatoschistus spp.) that conduct all their life cycle in this ecosystem. Another settler guild is composed of species that show opportunistic settler behaviour. These species forage in the salt marsh at either young or adult stages ("opportunistic salt marsh settlers"). Management for bird conservation leads to modifications in vegetation structure. The halophytic community is mowed every year during spring and autumn to create favourable habitats for Anatidae and wader conservation. This management seems to have adverse effects on the nursery role for fish populations such as Seabass. A comparison between the nursery role of Aiguillon bay and Mont Saint-Michel Bay showed significant differences. This latter bay sustains preferentially young-of-the-year of pelagic species. The Aiguillon bay sustains many YoY species (Seabass, European anchovy and Clupeidae YoY). The key nursery role often described seems to be disturbed in Mont Saint-Michel bay. Low juvenile abundances were observed. Sheep grazing impact and invasion by the grass Elytrigia spp. seems to have modified the initial nursery function of these coastal zones.