The text is intended as supplementary reading for fisheries workers, especially in developing countries, who do not always have ready access to current literature on applied marine ecology. An attempt is made to develop a wide range of concepts in a form that will hopefully encourage their incorporation into a practical, decision-making context. The food web and associated trophic interactions form the principal theme, in an approach that gives equal emphasis to qualitative, as well as the less easily measured quantitative considerations. An attempt is made to illustrate the consequences of the aggregated nature of much of marine production, as well as the subsequent dispersal of production in space and time, and how these processes affect the potential for economic harvest of commercial components of the ecosystem. Separate sections touch on environmental influences on production, relevant spatial and temporal scales for ecosystem analysis, life history strategies, diversity and stability, the concepts of the ecological niche, the community and the assemblage, and outline some first steps towards quantifiying production in marine ecosystems. Different approaches to representing trophic and other interactions are discussed, with examples from the literature. Reference is made to several ecological subsystems, in order to illustrate the main concepts presented. These include the mangrove ecosystem, the arcto-boreal macrophyte community, a mediterranean demersal fish assemblage, and the oceanic ecosystem associated with high seas tuna stocks. In practical terms, it is concluded that the first and simplest approach to multispecies resource management is not necessarily the manipulation of individual food web components, but the identification, mapping and conservation of critical habitats, especially centres of local production, and their associated ecological dissipation structures.