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In upwelling ecosystems, there is often a crucial intermediate trophic level, occupied by small, plankton-feeding pelagic fish dominated by one or a few schooling species. Their massive populations may vary radically in size under intensive exploitation. We have used decadal-scale time series to explore patterns of interactions between these fish, their prey, and their predators so as to quantify functional roles of small pelagic fish in those ecosystems. Top-down control of zooplankton is detected off South Africa, Ghana, Japan, and in the Black Sea. Conversely, bottom-up control of predators, such as predatory fish and marine birds, is observed in the Benguela, Guinea, and Humboldt currents. Thus small pelagic fish exert a major control on the trophic dynamics of upwelling ecosystems and constitute midtrophic-level “wasp-waist” populations. Ecosystem effects of fishing are addressed by considering potential structural changes at different scales of observation, 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 on how the food web is controlled (bottom-up, top-down or wasp-waist). The threat 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 within the pelagic community and functions as a “school trap” that could maintain a collapsed population in a depleted state for lengthy periods, and affect spatial dynamics such as migrations. As a result, overfishing can alter the abundance, composition, and distribution in pelagic communities, and may induce drastic changes of state. Predation on zooplankton by the jack mackerel (Trachurus symmetricus murphyi) population in the South Pacific provides an example of the alteration of matter fluxes in trophic webs caused by fishery removals. In conclusion, it is doubtful that pelagic fisheries will continue to increase without major disruptions to ecosystems.

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