Summary

Small pelagic fish are termed “wasp-waist” species as they dominate mid trophic levels and comprise relatively few species but attain large abundances that can vary drastically in size. They have been found 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. Largely based on model results, this chapter explores these effects and associated dynamics, not only illustrating the importance of small pelagic fish in structuring marine ecosystems, but also revealing the consistency of the role of small pelagic fish across various upwelling systems in which they play key roles. 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 and ecological models. Trophic level of the catch, the Fishing-in-Balance (FiB) index and the ratio of pelagic:demersal fish are calculated from reported landings. Sums of all flows to detritus are compared across modelled ecosystems. Models of the Southern Benguela, Southern Humboldt and South Catalan Sea are used to perform two simulations: (1) closure of fisheries on small pelagic fish and (2) collapse of small pelagic fish stocks, to further explore the roles of small pelagic fish in the dynamics of these ecosystems.

Tracking pelagic:demersal fish catch and biomass ratios over time is a means of detecting collapses in the small pelagic fish stocks, and comparing these ratios across ecosystems highlights the greater importance of small pelagic fish in the Humboldt compared to other ecosystems.