- 1In systems where marine mammals are perceived to compete with fisheries, the priorities of conservationists and wildlife managers can come into conflict. Resolution of these conflicts requires quantitative information on the intensity of competition, i.e. the expected reduction in resource acquisition by one competitor as a result of a given increase in resource acquisition by the other.
- 2The intensity of competition will depend on the extent to which marine mammals and fisheries overlap in space and time with their shared resource, and the way in which individual competitors' preferences for particular prey or target species vary in response to variations in the population composition of these prey and the abundance of other components of the system.
- 3We use two examples of marine mammals, Steller sea lions Eumetopias jubatus in Alaska and grey seals Halichoerus grypus in the UK, to illustrate these points and to challenge some intuitive notions about the nature of marine mammal–fisheries interactions.
- 4We conclude that the four main sources of ecological complexity (spatial heterogeneity, individual variation, multi-species interactions and long-term dynamics) can have a substantial effect on scientific predictions of the likely outcome of different management actions.
- 5We argue that a pragmatic approach, which considers the effect of ecological complexity while avoiding the indiscriminate addition of detail to mathematical models of competition, is the best way to provide scientific advice on issues that require urgent action.