Since the mid-1970s, three pelagic fish species-herring (Clupea harengus), mackerel (Scomber scombrus), and sand eel (Ammodytes spp.) - have undergone significant changes in biomass. The herring and mackerel stocks that declined from dominant to subordinate positions in the 1970s recovered to a present combined biomass exceeding 5 million metric tons. Sand eels, whose population levels exploded in the late 1970s and early 1980s, are now in decline. The effects of changes in biomass of several million metric tons among these three species of zooplanktivores are examined in relation to possible density-dependent variability in zooplankton and pelagic fish within the Northeast Shelf ecosystem.