The Benguela, California, Humboldt and Canary currents and the coastal waters of Japan support large stocks of sardine or pilchard Sardinops spp. and Sardina sp., anchovy Engraulis spp., horse mackerel Trachurus spp. and chub mackerel Scomber japonicus, with hake Merluccius spp. also abundant in each of the systems except off Japan. In these systems many of the more numerous fish, birds and mammals are opportunistic feeders having catholic diets. As a result species often have overlapping diets, and when a particular food item is plentiful it may be consumed by a wide variety of organisms at higher trophic levels. Frequently the performances of predator populations have been related to the availability of forage species, but predators have not always been influenced similarly by gross changes in the systems. Predators with the most generalized feeding strategies appear best buffered against collapses of particular prey resources. Although seabirds are generally opportunistic feeders, limitations in their foraging range may make them more susceptible than most other predators to system perturbations. Opportunistic feeding by species at lower trophic levels suggests that they could be advantaged by an increased food supply, and it may not be unreasonable to interpret species replacements in catches as reflecting shifts in the dominance of species. Overall catches from the systems have usually been more stable than the catches of individual species, which have shown wide fluctuations. Shifts in the dominance of species in the catches have often been sustained over a number of years. Generalized feeding creates the possibility that more than one species may benefit from energy made available by the collapse of an abundant resource, and there is evidence that species more abundant in former years have in some instances been partially replaced by a number of organisms. The likelihood of a particular organism becoming the main replacing species will probably be influenced by the degree to which it is utilized during and after the collapse of the originally dominant resource.