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Etude De L'impact De L'algue Envahissante Caulerpa Taxifolia Sur

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Etude De L'impact De L'algue Envahissante Caulerpa Taxifolia Sur Etude de l’impact de l’algue envahissante Caulerpa taxifolia sur les peuplements de poissons. Etude approfondie sur une espèce « modèle » : Symphodus ocellatus (Labridae) Nous avons étudié les mécanismes d’impact de l’algue envahissante Caulerpa taxifolia (Vahl) C. Agardh sur les poissons en Méditerranée, en particulier à l’aide d’une étude approfondie sur une espèce « modèle » : Symphodus ocellatus (Forsskål). Un suivi des peuplements de poissons ainsi que des expérimentations in situ ont été réalisés au niveau du département des Alpes-Maritimes, sur des milieux colonisés par l’algue et des milieux témoins. Caulerpa taxifolia qui forme des prairies de frondes très denses et hautes, de la fin de l’été jusqu’au début de l’hiver, représente une gêne physique pour l’accès à la nourriture benthique et aux microhabitats. Le suivi de deux récifs artificiels crées in situ a montré que l’envasement et l’obstruction de cavités par C. taxifolia diminuent la capacité d’accueil d’un milieu rocheux pour la faune ichtyologique. La raréfaction d’abris convenables mène à une augmentation de la mortalité des petits individus par prédation. L’étude de contenus digestifs ainsi que des acides gras corporels chez S. ocellatus montre un changement de régime alimentaire sur les milieux envahis (augmentation de la consommation de plancton au détriment des proies benthiques). En aquarium, nous avons montré que l’accessibilité aux proies benthiques diminue avec la densification de l’algue. Nous avons observé in situ que C. taxifolia pose également le problème de l’accès et de la détection des proies benthiques pour les rougets de roche Mullus surmuletus. Sur les milieux colonisés, le bilan énergétique global se traduit par des déficits de 21% dans le contenu lipidique corporel chez les jeunes S. ocellatus et de 59% dans les lipides de stockage du foie ainsi que par un déficit pondéral au niveau du foie et des ovaires chez les femelles reproductrices. Cependant, la qualité des œufs n’est pas affectée. Par ailleurs, in situ, les toxines de C. taxifolia n’ont pas d’influence sur le succès de développement embryonnaire de S. ocellatus. Mots clés : Caulerpa taxifolia, toxines, complexité structurale, Symphodus ocellatus, Mullus surmuletus, Méditerranée, microhabitat, prédation, nutrition, croissance, lipides corporels, reproduction, œuf. The impact of the invasive alga Caulerpa taxifolia on fish assemblage. The study of a "model" fish species: Symphodus ocellatus (Labridae) We studied the impact of the invasive Caulerpa taxifolia (Vahl) C. Agardh, on fish in the Mediterranean and particularly on a fish "model" species: Symphodus ocellatus (Forsskål). A fish monitoring and experiments have been led in sites located in the Alpes-Maritimes region, on sites colonized by C. taxifolia and on uncolonized sites. C. taxifolia forms a very dense and high meadow from the end of summer to the beginning of winter, altering microhabitats and presenting a physical barrier to benthic prey access. In situ surveys of two artificial reefs showed that the silting and obstruction of holes by C. taxifolia decreases the accessibility of the substratum to shelter fish. This decrease in shelter leads to an increase in young fish predation. Gut and body fatty acid analyses of S. ocellatus showed a diet switch on C. taxifolia-colonized sites, i.e. increase in plankton- and decrease in benthic- foraging. These analyses were further supported by correlating an increase in C. taxifolia frond density with a decrease in benthic prey accessibility in the aquarium. In situ observations also corroborated our findings at C. taxifolia sites (i.e. decrease of prey accessibility) for the red mullet (Mullus surmuletus). At colonized sites, the body lipid content in young S. ocellatus was 21% less and the liver storage lipid content was 59% less in reproductive females. In addition, liver and ovary weights were also diminished in females from the C. taxifolia meadow. However, analyses of eggs yielded no discernible differences, and in situ C. taxifolia toxins have no observable influence on embryogenesis. Key words: Caulerpa taxifolia, toxin, structural complexity, Symphodus ocellatus, Mullus surmuletus, Mediterranean, microhabitat, predation, nutrition, growth, body lipids, reproduction, egg. 1 REMERCIEMENTS Je tiens en premier lieu à remercier le Professeur Alexandre Meinesz pour avoir soutenu mon projet de thèse et m’avoir accueillie pendant ces trois ans au sein du Laboratoire Environnement Marin Littoral. Je remercie le Professeur Patrice Francour pour son encadrement. Merci aux Docteurs Mireille Harmelin-Vivien et Pierre Lejeune de m’avoir fait l’honneur d’accepter la tâche de rapporteurs, ainsi qu’au Docteur Enrique Macpherson d’avoir accepté de faire parti de mon jury. Merci au Docteur Patrick Coquillard pour ses corrections et ses conseils dans la rédaction de mon mémoire mais également pour son aide en statistiques. Merci au Docteur Thierry Thibaut pour sa correction minutieuse, mais également de m’avoir accompagnée en plongée. Merci au Docteur Jean De Vaugelas pour ses nombreux conseils, sa disponibilité et son sourire permanent… d’être l’homme de toutes les situations. Merci au Docteur Audrey Robert pour son aide en statistiques, à Dany Chiaverinni, Laetitia Trastour et au Docteur Gilberte Caye pour leurs conseils. Un grand merci à Fabrice Javel et à Laurent Gilletta pour m’avoir tant de fois accompagnée en plongées ainsi que pour leurs conseils et leur aide indispensable. Merci à Jean-Michel Cottalorda de m’avoir accompagnée sous l’eau ainsi que pour ses photographies. Merci aux Docteurs Anne Capiomont et Dounia Soltan ainsi qu’à Pascaline Bodilis de m’avoir accompagnée sous l’eau. Merci à Pierre Sorrais et à Sébastien Schopps pour leur aide au sein du LEML dès mon arrivée en DEA et pour leur sympathie. Merci beaucoup au Docteur Nicole Mayer-Gostan, au Professeur Patrick Mayzaud et à Marc Boutoute ainsi qu’au Docteur José Zambonino et à Hervé Le Delliou d’avoir eu la gentillesse de m’accueillir et de m’encadrer au sein de leurs laboratoires respectifs. Un très grand merci au Docteur Jean-Yves Bodiou pour sa gentillesse et ses précieux conseils dans la détermination des copépodes. Merci également au Docteurs Thierry Lepage et Stéphane Gasparini de m’avoir fait partagé leurs connaissances. Merci au Professeur Vincent Sbirazzuoli pour son accueil au sein de son laboratoire et les essais de calorimétrie. Merci au Docteur Patricia Pierson pour ses conseils et sa correction minutieuse. Merci à Ornella Passafiume, Nicolas Alexandre, Karen Favier, Frédérique Torrens, Catherine Seytre, Séverine Turck, Parvati Gamas, Elodie Corderos, Virginie Raybaud, Julie Baillet pour leur motivation, leur patience et leur aide dans la réalisation des divers études de cette thèse. Merci à Mat Vanderklikt pour son aide précieuse dans la traduction de mon premier article. Merci à Patrick Chang pour son amabilité et son aide dans la rédaction de mon autre article. Merci à Marie-Lucie Susini pour sa compagnie dans notre bureau commun. Merci à Céline Bongiovanni et à Morgane Bougeard pour cette rencontre… Merci beaucoup à Janick Pichard pour ses prédictions météo mais surtout pour son amitié et sa gentillesse. Merci à Evelyne de m’avoir soutenue dans cette voie et surtout d’avoir toujours été là, à toute ma famille pour son affection. A Gérald…pour tout ce que des mots ne peuvent exprimer… Ce travail a été rendu possible grâce aux soutiens financiers de la Région Provence-Alpes-Côtes d’azur et de l’Agence de l’Eau ainsi que par l’appui du Comité local des Pêches des Alpes-Maritimes dont je remercie le président Alexandre Plusquellec. 2 A Toi… 3 Sommaire I. INTRODUCTION 6 II. SYNTHESE BIBLIOGRAPHIQUE sur CAULERPA TAXIFOLIA 10 II.1. Caractéristiques morphologiques, biologiques et physiologiques de C. taxifolia 11 II.1.1. Evolution saisonnière d’une prairie de C. taxifolia 12 II.1.2. Ubiquité 12 II.1.3. Gestion de l’expansion de C. taxifolia 13 II.2. Toxicité de C. taxifolia 15 II.2.1. Toxines 15 II.2.2. Toxicité 16 Annexe du chapitre II.2. 17 II.3. Impacts écologiques de C. taxifolia 20 II.3.1. Sédiments 20 II.3.2. Flore 20 II.3.3. Invertébrés 21 II.3.4. Poissons 23 III. MATERIEL ET METHODES 29 III.1. Espèces modèles pour l’étude 30 III.1.1. Choix des espèces modèles 30 III.1.2. Biologie des espèces modèles 32 Annexe du chapitre III.1. 43 III.2. Localisation géographique de l’étude : le littoral des Alpes-Maritimes (France) 45 III.2.1. Sites principaux de l’étude 45 III.2.2. Autres sites de l’étude 47 IV. IMPACT DE CAULERPA TAXIFOLIA SUR LES POISSONS 48 IV.1. Evolution pluriannuelle des densités de poissons 49 IV.1.1. Evolution des densités de poissons en relation avec la colonisation de C. taxifolia : nouvelle analyse des données de Harmelin-Vivien et al. (2001) 49 IV.1.2. Evolution des densités de poissons en relation avec la régression de C. taxifolia sur notre site d’étude 53 IV.1.3. Parallèle entre les évolutions des densités de poissons sur les stations rocheuses et sableuse 58 IV.1.4. Synthèse sur l’influence de C. taxifolia sur les densités de poissons sur trois types d’habitats 62 Annexe du chapitre IV.1. 67 4 IV.2. Devenir des jeunes poissons après le recrutement 72 IV.3. Influence de C. taxifolia sur la prédation des jeunes poissons 82 IV.4. Microhabitat 98 IV.5. Activités trophiques 115 IV.5.1. Activités trophiques chez Symphodus ocellatus 115 IV.5.2. Activités trophiques chez Mullus surmuletus 130 IV.5.3. Discussion 156 IV.6. Croissance et composition corporelles 157 IV.6.1. Croissance pondérale 157 IV.6.2. Etat physiologique 166 IV.6.3. Lipides corporels 168 Annexe du chapitre IV.6. 184 IV.7.
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