Various kinds of no-take areas (refuges, reserves) are gaining attention as
conservation tools. The efficacy of reserves can be considered from the pe
rspectives of providing baseline data sets, protecting the stock, maximizin
g yield to the fishery, or some combination of these. Regardless of the mea
sure of effectiveness of a reserve, practical application requires the deve
lopment of techniques for settling operational and policy questions such as
how large a reserve should be. A simple model, involving population growth
and harvest, is used to explore how the fraction of habitat assigned to a
reserve affects the sustainability of a take and to frame the trade-off bet
ween control of harvest outside of the reserve and the size of the reserve.
This exploration also leads to the discovery of a robust conservation inva
riant for reserves.