In this paper, we are concerned with designing surveys for detecting patches of some exploitable resource, such as a shellfish stock. We take as our objective that, if there is one or more circular patches above some specified size, then the probability of detecting at least one patch should be high. We show how the required sampling intensity can be modified in the light of information about the likely number and size of patches. The results are applied to two surveys for detecting exploitable patches of cockles ( Cerastoderma edule ).