Understanding the structure and dynamics of pelagic ecosystems is critical to develop ecosystem approach to fisheries management. It allows assessing the impacts of fishing activities and environmental factors not only on target stocks but also on all other species including by-catch and protected species belonging to this ecosystem. The structure of the ecosystem is based on prey-predator relationships that are the most important interactions between species. Studying the trophic dynamic leads to the development of ecosystem models which provide the basis to test ecosystem-based management options and provide management and monitoring advices to the WCPFC and Pacific SIDS.