L'approche ecosystemique des peches necessite de repenser la gestion a des echelles integrees. Les connaissances sur la pression de peche a l'echelle des communautes restent limitees alors que la pression est le levier par lequel s'opere la gestion. Ce travail de these vise a caracteriser de maniere detaillee les captures, soit indirectement la pression, et a lier ses composantes a dierents objectifs de gestion. Pour ce faire, ce travail repose sur des analyses des donnees d'observation a la mer. Dans une perspective ecologique, la diversite des captures est comparee entre dierents engins de p^eche deployes sur deux sites du Sud Gascogne. Elle diere principalement par la taille moyenne et la proportion de piscivores. L'etat de ces deux communautes diere ; cependant les donnees ne permettent pas d'expliquer cette dierence par les dierences entre les captures qui en sont extraites. A l'echelle de la Manche, la combinaison des captures des dierents engins deployes par les p^echeurs anglais et francais constitue une etape supplementaire vers la caracterisation des pressions totales exercees sur ces communautes. Dans une perspective d'exploitation, les captures sont caracterisees par l'estimation des fractions rejetees, l'etude des causes de rejet et de l'ecacite de capture de l'espece cible. Une forte heterogeneite est mise en evidence a tous les niveaux. Ce travail contribue a l'approche ecosystemique avec des metriques et des methodes pour decrire les caracteristiques des captures dans des perspectives ecologique et d'exploitation. Il apporte des elements de discussion sur la repartition des captures et de leur utilisation parmi les composantes de la communaute et sur les principaux facteurs qui les inuencent. Les limites et potentiels des donnees d'observation a la mer pour l'etude de ces questions sont egalement discutes.

The ecosystem approach of sheries requires to rethink sheries management at integrated scales. Knowledge on shing pressure at the community level remains limited while pressure is the lever by which management operates. This work aims at characterizing in detail catches, that is indirectly shing pressure, and to link its components to different management objectives. For doing so, this work relies on analyses of at-sea observer data. The ecosystem approach of sheries requires to rethink sheries management at integrated scales. Knowledge on shing pressure at the community level remains limited while pressure is the lever by which management operates. This work aims at characterizing in detail catches, that is indirectly shing pressure, and to link its components to dierent management objectives. For doing so, this work relies on analyses of at-sea observer data. In an ecological perspective, catch diversity is compared between dierent shing gears deployed on two sites in the Southern Bay of Biscay. It mainly diers by the mean length and piscivore proportion. The state of those two communities diers ; however dierences in community states cannot be explained by the dierences in catches that were extracted from them. At the scale of the English Channel, the combination of catches from dierent gears deployed by English and French shers constitutes another step towards the chacterization of the total pressures exerted on those communities. In an exploitation perspective, catches are characterized by estimating discarded fractions, studying reasons for discarding and eciency of shers to catch their target species. A strong heterogeneity is evidenced at all levels. This work contributes to the ecosystem approach of sheries with metrics and methods to describe catch characteristics in both ecological and exploitation perspectives. It provides inputs for discussion about the distribution of the catch and its utilization among marine community components, and the main factors inuencing them. Limits and potentials of observer data to study those questions are also discussed.