Living marine resources are difficult to observe. They are also potentially highly variable. Most plants, and most animals that feed directly on them, are extremely small and short lived. Most marine ecosystems therefore lack the stability provided by the trees and long-lived animals and plants on land. Variations of a hundredfold in abundance, from year to year or over periods of decades, are not uncommon. This variability makes prediction of natural changes important, and also makes prediction and evaluation of the effects of human intervention difficult. These uncertainties raise difficulties in the way scientific advice on the use of marine resources is offered and how the advice is used in formulating national policy, both in developing new fisheries and in controlling over-exploitation. The technical problems will be briefly reviewed, and the need for modifications to the ways scientists and policy makers interact will be discussed.