Protecting or conserving the animal resources – be they fish or mammals – that live in the ocean is among the most difficult of global environmental issues. Large portions of the oceans are international space, owned by no one and accessible by many. Fisheries are common pool resources, meaning that it is difficult, either practically or legally, to exclude others from access to them, and that use by some diminishes the ability of others to use the resource. Uncertainty is rampant, about the condition of the resource and about the behavior of those who make use of it (and, sometimes, about the relationship between the two) (see Chapter 18).