Small pelagics in upwelling systems: patterns of interaction and structural changes in "wasp-waist" ecosystems

In upwelling ecosystems, there is often a crucial intermediate trophic leve l, occupied by small, plankton-feeding pelagic fish dominated by one or a f ew schooling species. Their massive populations may vary radically in size under intensive exploitation. We have used decadal-scale time series to exp lore patterns of interactions between these fish, their prey, and their pre dators so as to quantify functional roles of small pelagic fish in those ec osystems. Top-down control of zooplankton is detected off South Africa. Gha na, Japan, and in the Black Sea. Conversely, bottom-up control of predators , such as predatory fish and marine birds, is observed in the Benguela, Gui nea, and Humboldt currents. Thus small pelagic fish exert a major control o n the trophic dynamics of upwelling ecosystems and constitute midtrophic-le vel "wasp-waist" populations. Ecosystem effects of fishing are addressed by considering potential structural changes at different scales of observatio n, ranging from individuals, via school dynamics, to food webs. The overall impact is explored using a trophic model (Ecosim) given different types of food web control. Ecosystem dynamics can be entirely different depending o n how the food web is controlled (bottom-up, top-down or wasp-waist). The t hreat of eroding intra-specific diversity is emphasized because it may lead to a long-term decline in the productivity of the pelagic fish resources. School composition is shown to reflect the relative species abundance withi n the pelagic community and functions as a "school trap" that could maintai n a collapsed population in a depleted state for lengthy periods, and affec t spatial dynamics such as migrations. As a result, overfishing can alter t he abundance, composition, and distribution in pelagic communities, and may induce drastic changes of state. Predation on zooplankton by the jack mack erel (Trachurus symmetricus murphyi) population ill the South Pacific provi des an example of the alteration of matter fluxes in trophic webs caused by fishery removals. In conclusion, it is doubtful that pelagic fisheries wil l continue to increase without major disruptions to ecosystems. (C) 2000 In ternational Council for the Exploration of the Sea.