THE LIMITS OF EXPLOITATION: A PRECAUTIONARY APPROACH

As is well known, the biological limit to the exploitation rate of a fish stock is determined by the maximum per capita reproductive rate and the age selectivity of the fishery. Customarily, the selectivity is treated as fixed when the biological limit to harvesting is estimated. From a precautionary perspective, one should control selectivity. A simple model is used to demonstrate the safety benefits of prohibiting the harvesting of juvenile fish. The model leads to a natural rescaling of the maximum per capita recruitment rate, and it is shown that if this quantity is >1, then a spawn-at-least-once policy will prevent a collapse of the stock if fishing mortality targets are exceeded. (A spawn-at-least-once policy requires that fish become vulnerable to commercial gear only after having spawned once.)