The sequential megafaunal collapse (SMC) hypothesis proposes that the depletion of populations of large whales by commercial whaling in the 1960s and 1970s in the North Pacific (primarily the Bering Sea and Aleutian Islands region) deprived mammal-eating killer whales of an important prey resource and thus forced them to switch from preying on fin and sperm whales to preying on sea otters and populations of pinnipeds (a suborder of carnivorous mammals that includes seals and sea lions), precipitating sequential declines in those populations. This hypothesis was developed based on the timing and spatial pattern of large whale removal in the North Pacific and the timing of observed declines in four marine mammal populations: harbor seals, northern fur seals, Steller sea lions, and sea otters.

However, the SMC hypothesis is not supported by a number of recent studies. Wade et al reviewed a broad range of data and concluded that this hypothesis about prey-switching rests upon a poorly supported assumption regarding the importance of adult large whales as primary prey items of killer whales. Mizroch and Rice reviewed the historical information on stomach contents of sampled killer whales in the North Pacific and found that less than 3 percent of the mammal-eating killer whales examined prior to 1968 contained large prey items. Mizroch and Rice also concluded that the primary pulse of commercial whaling in the vicinity of the Aleutian Islands and the Gulf of Alaska (i.e.,north of 50 degrees north latitude) was between 1963 and 1965 and primarily targeted fin and sperm whales (by order of biomass removed in the 1950s and 1960s). For the most part, legal commercial whaling in this area ended in 1967, after which most catches were made south of the Aleutian Islands (i.e., south of 50 degrees north latitude).

Killer whales are known to prey sometimes on the calves of large whales, but evidence from studies of whale scarring and from direct observations of predation attempts strongly suggests that most such predation occurs before calves reach the high-latitude waters that are the focus of the SMC hypothesis. Thus, any idea that, prior to commercial whaling, killer whales were sustained primarily by a diet of whale calves in high latitudes is not supported by the available evidence. It is also worth noting that while attacks on sperm whales have been documented on rare occasions, fin whales do not appear to be targeted as prey by killer whales, as evidenced by the lack of reported attacks and very low rates of scarring.