The role of small pelagic fish in the ecosystem 25 January 2007 Lynne Shannon, Marta Coll, Sergio Neira, Philippe Cury and Jean-Paul Roux ========================== Abstract Small pelagic fish are termed “wasp-waist” species as they dominate mid-trophic levels, comprise relatively few species but attain large abundances that can vary drastically in size. They have been shown to exert top-down control on their prey species and bottom-up control on their predators and, in this way, appear to induce unsuspected ecosystem dynamics. This chapter explores these effects and associated dynamics, not only to reveal the importance of small pelagic fish in structuring marine ecosystems but also to examine the consistency of the role of small pelagic fish across various upwelling systems. The Northern and Southern Benguela, Southern Humboldt, South Catalan Sea and North and Central Adriatic Sea ecosystems are compared in terms of the importance and role of small pelagic fish using information gained from landings: i) Trophic level of the catch, ii) the Fishing-in-Balance (FiB) index and iii) the ratio of pelagic:demersal fish. Indicators derived from trophic models of these ecosystems are selected to assist in defining the effects of small pelagic fisheries, and include: i) Catch/Biomass ratios for small pelagic fish, ii) Exploitation rate E=F/Z, where F is fishing mortality and Z is total mortality (natural and fishing mortality combined) for the main commercial fish species in each system, iii) Sum of all flows to detritus. Models of the Southern Benguela, Southern Humboldt and South Catalan Sea are used to perform two simulations: 1) Closure of