Damage to marine environments attracts increasing attention and alarm, instilling in many observers a sense of urgency in promoting preventive and restorative measures (see for example Pauly & Maclean 2003; Roberts 2003; RCEP [Royal Commission on Environmental Pollution] 2004; MEA [Millennium Ecosystem Assessment] 2005). Such concerns have had effect. As noted in a recent British policy document, ‘countries all over the world are reviewing the way they manage their marine environment’ (Defra [Department for Environment, Food and Rural Affairs] 2007, p. 1). In much that has been said and written on this subject it is a basic premise that marine ecosystems and biodiversity ought to be protected, at least to some degree. This implies a moral injunction: that it is wrong to allow these systems to be degraded or destroyed, and right to limit human activities that generate harmful impacts. The interesting and fundamental question is why. What Norton (1982, p. 319) calls an ‘intuitive ethic’ for conservation might be invoked, but in a world of conflicting priorities it will often be necessary to support intuition with evidence and argument. Answers to the question, ‘why conserve?’ are important because the reasons offered for defending marine environments have a bearing on the extent to which conservation can be justified, and on the kinds of policies that might ensue. Although different premises will sometimes point to broadly similar actions, there are circumstances in which the underlying rationales for conservation will have divergent policy implications.