The role of a marine sanctuary, where commercial fishing might be prohibited, is evaluated in two models; one where net biological growth is deterministic, and the other where net biological growth is stochastic. There is diffusion (migration) between the sanctuary and the fishing grounds based on the ratios of current stock size to carrying capacity in each area. Fishing is managed under a regime of regulated open access. In the deterministic model, it is possible to determine the steady-state equilibrium and to assess its local stability. In the stochastic model a steady state does not exist, but a stable joint distribution for the fish stock on the grounds and in the sanctuary is possible. The creation of a no-fishing marine sanctuary leads to higher population levels on the grounds and in the sanctuary, and appears to reduce the variation of the population in both areas. The higher population levels and reduced variation has an opportunity cost; foregone harvest from the sanctuary.