We discuss the potential for commercial fisheries to adversely impact ≥1 population of marine mammal by the end of the 21st century. To a large degree, patterns over the last 50 years regarding human population growth, success and failure in marine-fisheries management, and life history and status information on marine mammals are the basis for 6 predictions. First, annual worldwide landings of fish and shellfish by the end of the 21st century will be less than 80 million tons. Second, virtually all of the predictions regarding species composition and energy flow within a marine community, based on models developed to date with incomplete information on species abundance, food habits, genetic effects of fishing, and variability of predator food habits, will prove wrong on a decadal or longer time scale. Third, the most common type of competitive interaction between marine mammals and commercial fisheries will be that in which commercial fisheries adversely affect a marine-mammal population by depleting localized food resources without necessarily overfishing the target species of fish (or shellfish). Because of this, the number of extant populations and species richness of marine mammals will be reduced by the end of the 21st century, and coastal populations and species will be affected more negatively than will noncoastal species. Fifth, predator control programs designed to reduce local populations of marine mammals will be common without changes in existing forms of fishery management. Finally, protein from marine mammals will become a more important component of the human diet than it currently is.