1. The most broadly applied generalizations of bioeconomics derive from simple, non-spatial models. We develop a simple continuous spatial model in which harvesting costs are broken down into travel costs to a location and capture costs at that location within a harvested region. 2. This is used to determine the conditions under which the spatial behaviour of harvesters is important and its implications, particularly for the assessment of maximum achievable sustainable yield (MASY) and the optimal management of exploited animal populations. 3. The model suggests that, as a rule of thumb for harvested systems where density dependence is essentially a local process and harvesters act as independent agents, spatial structure will cause a significant impact on the dynamics and reduction in productivity of the system where the ratio of maximum travel cost to minimum capture cost is around 5 or more, and a very large impact where that ratio is around 10 or more. 4. Also, in a spatial harvesting system, secure ownership rights will not result in socially efficient harvesting if the owner only exercises control over overall offtake levels. Efficient management requires control of the spatial pattern of harvesting as well. 5. Synthesis and applications. These findings apply in cases where the spatial scale over which density dependence acts on the exploited population is limited in relation to the extent of the potential harvesting area, and where harvesters act as independent agents, choosing individually where to harvest. In most systems these conditions will not apply perfectly, and in many cases they may be seriously violated, but the rule of thumb above may be used quickly to establish whether spatial structure should warrant further investigation. In managed systems where harvesting decisions are regulated, the model may still be used to understand the gains produced by management.
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